Deep field image by Hubble

What if our universe was just one in an infinite number of parallel universes; a possible outcome from the specific predictions of quantum mechanics. The idea of multiple universes is common in science fiction, but is there any actual science to back this theory up?

Evolution of the large scale structure of the Universe. Image credit: NASA

This week we’re going to think big. Bigger than big. We’re going to consider the biggest things in the Universe. If you could pull way back, and examine regions of space billions of light-years across, what would you see? How is the Universe arranged at the largest scale? And more importantly… why?

Ep. 137: Large Scale Structure of the Universe

• Local Group of Galaxies — Atlas of the Universe
• Triangulum Galaxy — APOD
• Andromeda Galaxy — NOAO
• Groups, Clusters and Superclusters — UTK
• Virgo Supercluster – Atlas of the Universe
• The biggest void in space is 1 billion lightyears across — New Scientist
• Sound waves/ compression waves in the early Universe – WMAP
• Sloan Digital Sky Survey
• Ram Pressure Stripping – Swinburne Astronomy
• Tidal Tails — Internet Encyclopedia of Science
• Elliptical galaxy M87 — SEDS
• Astronomers Observe Formation of Largest Bound Structures in the Universe — Universe Today
• Planck Telescope
• WMAP (Wilkinson Microwave Anisotropy Probe
• James Webb Space Telescope (JWST)
• Large Synoptic Survey Telescope (LSST)
• Galaxy Zoo

Videos:

• Videos from Josh Barnes of the University of Hawaii — Galaxy Transformations
• Movies from the Sloan Digital Sky Survey and WMAP

Papers:

• Large Scale Structures from Galaxy and Cluster Surveys
• Analyzing Large Scale Structure
• Gamma Rays from the Large Scale Structures of the Universe
• Large Scale Structure in the SDSS Survey

Download the transcript

Transcript: Large Scale Structure of the Universe

Fraser Cain: Quantum Mechanics.

Dr. Pamela Gay: Yup the subject that puts dread in the hearts of many, many an undergraduate.

Fraser: Isn’t that like there’s only three people in the world who understand quantum mechanics? Or is it no anyone who tells me they understand quantum mechanics doesn’t understand quantum mechanics?

Pamela: I think string theory there’s only three people who can do the math. With quantum mechanics there are lots of people that can work the equations but in terms of being able to completely internalize it and have their stomach do it.

It’s like kinematics. Your stomach can to kinematics. Your stomach knows you drop a ball. It’s going to land on the ground. Quantum mechanics, every molecule in your body is going un-uh don’t believe it. [Laughter]

Fraser: Alright but you’re getting ahead of us here. So let me do my official introduction and we can go from there. Quantum mechanics is the study of the very tiny; the nature of the reality of the smallest scale. It is a science that defies common sense and delivers no helpful analogies. Yet it delivers the goods making scientific predictions with incredible accuracy.

Let’s look into the history of quantum theory and then struggle to comprehend its connection to the universe. There, so if I was [laughter] to go and find some physicists and corner them and say – oh, I’ve got one right now – and say what’s quantum mechanics? [Laughter] What kind of answer would I get?

Pamela: I think the most straightforward description of it is: a way to using equations that work, describe just about everything as simultaneously being a wave and a particle while making your head really hurt.

Fraser: Okay, depending on how big their ego is and how well they think they understand it it’s a way mathematically. What kind of things can you mathematically describe with quantum theory?

Pamela: The entire spectrum of an atom can be described in very detailed ways using quantum mechanics. I can go out and use a telescope to observe hydrogen in many different states all across the universe. I can understand the 21 centimeter line as a very rare flipping of an electron.

I can understand the different emission lines that come from hydrogen gas that’s heated to different temperatures as a function of the electrons jumping from one energy level to another. I can understand how light gets absorbed by gases and scattered in different directions and how lasers and masers work.

All of that comes out of quantum mechanics. I can very precisely come up with all sorts of crazy numbers that describe how molecules vibrate. It all works. That’s the amazing crazy part about this is it all works.

Fraser: Right, if you plug in the numbers you get an accurate description of reality even though what’s actually going on is defying logic at every turn. How did quantum mechanics come about?

Pamela: It basically came about from I think this was the original case of someone doing an experiment, having something really crazy come out of it and sitting in their chair and letting out a string of expletives.

There were things like if you shoot an incredibly intense beam of red light at a metal plate. The metal plate goes yeah, so, okay and does nothing. But if you shoot higher energy blue light at that same plate you can get electrons getting shot off. The thing is you don’t even have to use as intense a blue light. The intensity of the light seemed to have nothing to do with what was going on. It had to do with the color of the light.

This was finally explained by Einstein with what we call the photoelectric effect where suddenly the light was no longer this continuous intense sea of energy but rather it was discrete packets where the amount of energy in each individual packet was related to the color. That was a revolutionary idea. It forced us to change how we think of things.

All of a sudden it was no longer light spreading out in this thinning sea as it radiates away from the sun. Rather it is a bunch of individual photons where the space in-between the photons increases as you get further and further away and the light is forced to spread itself out over a greater and greater area. All of this was new. Most fundamentally, the idea that light could only have certain energies. That was something completely expected that turned out to again be completely true.

Fraser: How did the first early physicists wrestle with this?

Pamela: There were lots of different things that they had to figure out. The first thing that probably got sorted out is that electrons can be bent with electromagnetic forces have very small masses. When they hit fluorescent screens they can give off light. A cathode ray tube was one of the first experiments that forced us to start thinking about electrons as discreet little packets and starting to understand how they worked.

Fraser: What’s going on in the CRT?

Pamela: Basically you accelerate an electron and we didn’t know that this is what we were doing the first time scientists started doing this. You create as complete a vacuum as you can. Then you do various things that cause electrons to be created. You accelerate them through charged plates, through capacitors.

Then by adjusting magnetic fields you can change where on a fluorescent fluorescing plate that electron hits. By adjusting magnetic fields in the electric fields that you’re using in this system you can actually measure the mass of an electron.

When we first measured it and realized that it was fractions, thousandths of the mass of an atom we realized, wow electrons are these discreet little tiny particles that are extremely small but look at the things we can do with them.

Fraser: But where does the quantum mechanics part of it come into that story?

Pamela: Once we started thinking of things as little tiny here we have an electron, here we have photons, we started probing what is the structure of the atom. It was originally thought that atoms were basically – it was the plum pudding model where you had the protons and the electrons all kind of randomly globbed together.

There were experiments done where beams of electrons were sent into a gold foil. Had the structure of the atom been a plum pudding basically, where you have these randomized plums – the electrons and protons – randomly scattered through the atom you would have gotten one set of the way the electrons get reflected as they go through the gold foil.

Instead what we saw was a scattering that could only be explained if the protons and neutrons were concentrated in a very small nuclei surrounded by a huge swarm of an electron cloud. This started making us think about what is it the electrons are doing if they’re orbiting? There were problems with if the electrons are orbiting why aren’t they constantly radiating energy?

They’re constantly accelerating because they’re moving in a circle. This started to become very problematical. It was in trying to sort out that problem and in looking at the allowed wavelengths of light where we also ran into problems there of as the wavelength gets shorter and shorter the amount of energy should go to infinity. All of a sudden we have warm objects giving off infinite amounts of light.

This is the ultraviolet catastrophe. It’s the way it was referred to back in the days before we really knew about x-rays and gamma rays. In trying to sort out all these different problems people eventually settled on the well we can solve this if we start looking at energy as being quantized, of there being certain limited values allowed where you can’t have an infinitely small wavelength.

Eventually you just run out and things go to zero in the ultraviolet. So the energy goes to zero and the wavelength becomes infinitely small. We were able to solve these problems by doing something extremely unsatisfying and saying energy is discreet and can only have certain given values.

Fraser: When you say energy is discreet and can only have certain given values this is the quantum part of quantum mechanics, right that energy comes in packets of a very specific amount. That’s a quanta, right?

Pamela: Yes and this is where we say energy is equal to a constant times the frequency of the light. It’s that constant that’s always in there and never goes to zero that gives us the: you’re only allowed certain values.

Fraser: Then the smallest theoretical amount of light you can have is one packet, right?

Pamela: Right.

Fraser: So you get zero and then you get one but you can’t have fractions between. That’s good because if you had fractions you would have an infinite amount of energy pouring out.

Pamela: Yes and we don’t so we’re fine. Then we also started having all these other weird things. De Broglie was one of the ones who noticed that electrons have wavelengths too. We were not happy but could almost cope with the concept that light is both a particle and a wave. It has no mass, it acts in weird ways, it hurts but okay we can move on from this.

Then we started realizing as a field – and I wasn’t born yet – that electrons which have mass also can have wavelengths and can interfere with each other. One of the things we knew is you can take beams of light and send them through slits and get amazing patterns on the wall, interference and diffraction patterns. This has to do with the wave nature of light.

The same way waves going through rocks going towards the shore you can end up with a completely straight wave hitting these rocks turning into perfect sections of a circle radiating away from the slit through the rocks.

Fraser: We did a whole show on this with our wave particle duality so I know we’ve got sort of all the details on that if people want to listen to that show.

Pamela: People started doing experiments, oh dear electrons interfere as well.

Fraser: What would be the apparatus? What would be the experiment to see an electron interfere with itself?

Pamela: Here’s the cool way that you do it. If you have thin enough slits and you send beams of electrons, one electron at a time through the slit you instead of getting two perfect images of the slit on some screen in the distance – which is what your stomach would say should happen, you shine electrons on a slit they go through the slit, they hit a detector on the back, and you get an image of the slits.

Fraser: Right they either get bounced because they hit the sides of the slits or they go right through the slit and you get them hitting the detector.

Pamela: Yes, no big deal. The reality is that when the electrons one at a time, not having anything else to deal with, but one at a time pass through these slits it’s like they know that there’s other electrons coming.

They go through and over time they’ll build up this interference pattern on the detector just as though there were a whole series of waves interfering with one another.

Fraser: You’ve got an electron going through both slits at once?

Pamela: And interfering with itself.

Fraser: And interfering with itself to create this pattern.

Pamela: It’s a probability function. This is the really weird thing. It’s so cool and we’ll see if we can find an animation to put up on our website. It’s so cool to watch this happen.

You’ll see on your detector a flash of electron hit over to the left. Electron hit directly in line from the slit. Electron hit over from the right; another electron over on the slit.

They build up over time and it is one electron at a time that they build up this interference pattern. They build it up with a perfect distribution of the majority of the electrons landing where you have the highest probability.

Fraser: This is the same pattern that you see with photons but it is quite astonishing you see it with electrons, right?

Pamela: And they did. This was when we started to realize that the actions of light and particles are dictated by probability. Electrons are just like light, capable of somehow knowing hey I’m a wave I’m going through slits. I should interfere with something and that something may be coming later. [Laughter] That’s just cool.

Fraser: Yeah, now you can keep going, right? You don’t just stop with electrons.

Pamela: No. What ends up happening is as you get to larger and larger physical objects their wavelengths start being smaller than they are. The wavelength of a human being is way smaller than a human being.

You can’t send me through two slits and have me interfere with myself on the other end. At a certain point you stop being able to run these experiments because the wavelength and the physical size just doesn’t quite allow that to happen.

Fraser: I think they’ve gone as far as like Buckyballs, right? They’ve actually got atoms from molecules with like 60 carbon atoms and have been able to send them through and have them interfere. It’s crazy. [Laughter]

Pamela: Yeah it doesn’t make any sense. Then there are other consequences of quantum mechanics that we’re still rather uncomfortable with. One of these is the Heisenberg uncertainty principle. The idea behind this one is waves carry energy in them. They carry momentum. As a wave is traveling through space if it hits something it can impart that momentum and do work basically.

If I want to measure the rate at which that wavelength is traveling through space and I want to measure that wavelength very, very precisely I’m kind of stuck. I’m only able to either know its momentum very precisely or its position in space very precisely. I’m not actually allowed to know both of these very precisely at the exact same time.

The way to think about it is if I concentrate its position into a point then I no longer know anything about its wavelength. All I know is all of the light is right at this point right now that’s all I know. Suddenly the wavelength information is completely gone.

But if I very carefully am able to determine its wavelength somehow what I’m looking at is the period oscillations of the electromagnetic wave. That doesn’t have a central point. That doesn’t have the specific position. This means basically I can either know exactly where the wave is or I can know exactly its wavelength but don’t ask me to tell you both at the same time.

Fraser: The same thing goes with an electron going around a proton?

Pamela: Exactly.

Fraser: You can know its momentum or its position?

Pamela: Yes and you can’t know both. This applies to lots of different ways of looking at these particles. We have the most common way of looking at it is looking at it in terms of position momentum. We can also look at in terms of energy in position. Energy is another way of looking at wavelength.

We’re trying to sort out what’s going on and we keep coming up against the well you’re not allowed to know something. We also have once we start looking at things in terms of relativity we start running into energy and time uncertainties. All of these things tie together and it leads to a place where we can only know half the information at any one point.

Anyone who is deeply in love with kinematics and the notion that if you know the position and velocity of everything at any one moment you can predict to the complete unfurling of the rest of the universe. The uncertainty principle says I can’t know that.

Fraser: Right, it’s not just issues of your instruments aren’t good enough. No instruments could ever be made good enough to know those two pieces of information at the same time.

Pamela: Yes, it’s just the universe doesn’t allow it. It’s the way it’s wired. There are different ways of cheating the system. There was a really neat experiment done – the understanding of the experiment is still being debated – but the experiment itself is really cool.

If you have one of these double slits and you shine light through it you get this wonderful interference pattern on a wall. You can very carefully arrange thin wires between the double slits and the wall where those wires fall in the shadows of the fringe pattern. Putting them there doesn’t affect what you see in any way.

If you put a lens between the slits and the wall the lens treats the light as though it was a bunch of particles. As far as a lens is concerned light is a particle. It will actually take that pretty diffraction pattern and get rid of it and make two images of the slits on the wall. You go from pretty diffraction pattern to instead slit and slit nicely glowing like your stomach expected you to see originally.

If you do this experiment putting the lens in after putting these wires in you still won’t see the wires in the pretty slits. If you cover up one of those slits so that the interference is no longer happening suddenly in the image of that slit you didn’t cover up you can see the shadows of the wires. So the light knows it’s no longer interfering.

Fraser: Right and this is after it’s already gone through the slit and so you’re blocking it between the slit and the detector – the slit in the wall.

Pamela: No, you actually do block it before it goes into the slit.

Fraser: Okay you block it before it goes through the slit.

Pamela: Then suddenly the interference pattern is no longer there. It’s kind of creepy that you can have the light in terms of the wires knows that it’s supposed to be interfering. Then that interfering light hits the lens and suddenly goes oops I’m going to be a particle now.

You’re able to basically flip it from behaving like a wave when it is interfering with the shadows and the wire to acting like a particle as it passes through the lens. That’s just cool. You can mess with your own head while messing with light.

Fraser: I know that quantum mechanics is sort of beautiful and works really well and is very well supported by mathematics. Yet it’s a real thorn in the side of the folks trying to unite the forces of the universe, right? It won’t behave and if you try to bring gravity down to the realm of quantum mechanics the two don’t get along.

Pamela: Yeah, one of the problems that we run into is quantum mechanics is able to deal with predicting how energy is partitioned between thermal energy, between kinetic energy. It allows us to understand how gases behave. It allows us to understand transitions of electrons and atoms, the vibrations of molecules.

As you increase the density of the system it allows us to continue understanding what’s going on through quantum mechanics and thermodynamics getting brought together where you can start to understand that if you can pack a system dense enough like in a white dwarf star suddenly all of the electrons go okay, poly-exclusion principle.

Poly-exclusion principle says electrons have to be oriented in two different orientations as the orbit in the same energy of an atom. We call this spin-up and spin-down just because the electrons aren’t actually spinning that we know of. It’s just a convenient way of thinking of the electrons. You can imagine it one north pole up one north pole down as they rotate. They aren’t doing that, it’s just a way of envisioning it.

If you have to pack electrons tight enough suddenly it’s not just the electrons in energy levels around this atom, atoms electrons in orbits around those atoms over there. Suddenly it’s all the electrons basically forming one lattice-work of energy levels carefully obeying the poly-exclusion principle as they form what we call a degenerate gas. All of this works with quantum mechanics.

As we continue to crank up the densities making neutron stars now suddenly it has realized oh shoot can’t have electrons and proton this close together, must combine and they combine to form neutrons. We can explain how the energy, how the mass is all conserved and what gets shot out during the process. We can explain neutron stars.

When you crank things up further suddenly quantum mechanics can’t explain what happens when neutrons are no longer able to exist. When the densities get so high that even the neutrons kind of give up the ghost and say okay I’m going to be something different now so that I take up less space. What that different thing the neutrons become to take up less space and form black holes, quantum mechanics can’t go there. Quantum mechanics has no way of addressing gravitational effects.

This is one of the things that I’m hoping gets solved in my lifetime because there’s a way of viewing the universe where gravity is geometry. There’s a way of viewing the universe where gravity is still part of particle physics where it is conducted by the flow of gravitons. We don’t know which view is fully true or if both are true.

We have geometry and particles the same way we have the particles and wavelengths. I don’t know how once we’ve discovered the Higgs-Boson and once we’ve discovered the graviton if we discover those things we’re going to describe to some future generation of physicists the way the universe is shaped.

Fraser: I guess at the same front you can’t take quantum mechanics and scale it up beyond a few collections of atoms?

Pamela: That’s where we start having to deal with what we call classical mechanics where we start describing things in bulk motions. Suddenly things behave in ways our stomachs understand.

Fraser: Right, there’s no uncertainty about the orbit of Jupiter.

Pamela: Exactly.

Fraser: You can predict it in that situation it really does just come down to the quality of your instruments.

Pamela: We have this weird universe where when things start moving too fast or their masses get too large suddenly we have to turn to Einstein.

Then when you go the other direction and you start dealing with things that are extremely small you have to start using quantum mechanics.

In that middle land you can use Newtonian mechanics and our stomachs understand the planet Earth and most of the physics we have to deal with day to day.

Fraser: The thing that I like about quantum mechanics and I can only assume because I barely understand it is that it is one of those sciences that really defies as you say your stomach understanding.

It won’t let you intuitively progress making analogies in your mind and kind of well it ought to be like this because [laughter] this makes sense to me.

I’ve seen rocks fall and I understand how if I swing something around my head it goes in a nice circle and all that king of stuff. Your intuition is worthless and you have to throw it out.

You have to just say I’m just going to start performing experiments and I’m going to see what the experiments tell me and I’m going to accept the results and that’s that.

I find that whole thing really refreshing because if you listen to this show we don’t have like a single analogy. There’s nothing. Sometimes how your mother is sometimes like a particle….no that [laughter] doesn’t work.

There are just no analogies, nothing that can help you out on this. You have to just listen to nature and listen to the experiments and follow the math. That’s the only kind of progress that you can do.

I think that’s great because I think that so much science – although scientists really try to progress and try to be objective and just listen to their experiments and so on – a lot of that personality experience gets brought into it.

This is one of those situations where nobody’s got any sort of opinion on it. I guess maybe they build up opinions about one theory and another theory but in the end really just the human being has no evolved way to deal with it.

I really like quantum mechanics. We’ve got a few other topics that we wanted to really go into in this area so I think we’re going to spend a couple of shows and talk about some of the really interesting things that have come out of quantum mechanics.

One being entanglement, spooky action at a distance and we have a couple of other things we want to talk about as well, especially how it relates to astronomy so we’ll get on with that in the next couple of shows. Thanks Pamela.

Fraser Cane: Hi Pamela, are you ready to think big?

Dr. Pamela Gay: I hope I am.

Fraser: This week we’re going to think really big. Bigger than big, we’re going to consider the biggest things in the universe.

If you can pull way back and examine regions of space billions of light years across, what would you see? How was the universe arranged at the largest scale? More importantly why?

Let’s think small for starters. We’ve got the Earth going around the sun. The sun is a member of a spiral galaxy called the Milky Way, then what?

Pamela: Then we belong to the local group, which is this little tiny group of galaxies. It’s pretty much us and Andromeda and Triangulum. It’s a little tiny system.

Fraser: This is the only time you get to describe a collection of galaxies [laughter] as tiny. Any other time gignormous I believe would be the word you would use.

Pamela: Right so the thing is on the grand scheme of the entire cosmos our local group, which is mostly made up of dwarf galaxies but has a few other big galaxies, like I said us, Andromeda and Triangulum are tiny compared to what’s out there.

Fraser: Okay let’s get a sense of scale then. How big is our local group? Why do we call those galaxies the local group?

Pamela: Gravitationally we’re all bound together. We’re all part of basically a swarm of systems that are happily orbiting one another. They will happily one day consume one another as we grow into a bigger and bigger pile of mass with fewer and fewer discreet objects within it.

Our small groups of gravitationally bound together galaxies are slowly moving toward other much larger collections of galaxies that we’ll later fall into just because their gravity is that much bigger.

Fraser: Like the Milky Way we’re all kind of collected together into the local group. I know Andromeda is like two and a half million light years away so that’s kind of the scale that we’re looking at here. There are dozens of dwarf galaxies as well.

Pamela: Right.

Fraser: What are we moving towards then?

Pamela: We’re on the outskirts of what is called the Virgo supercluster, which is our local supercluster. It is the big city that we are basically currently a suburb of.

As city sprawl takes over, that city is going to slowly consume us except in this case instead of the city growing bigger the city is actually gravitationally sucking us in.

We’re sort of falling downhill gravity-wise into this nearby supercluster.

Fraser: Let’s get a sense of scale then. We have say 50 galaxies in our local group, how big is the Virgo supercluster?

Pamela: The Virgo supercluster is actually big enough that if you scroll a telescope across that section of the sky you will see about a hundred galaxy groups and clusters that are part of this super cluster.

It is us, the Virgo cluster itself. There are other things like the Earth’s major group that are falling into it. There is the M 101 group. All of us together make up the Virgo supercluster.

Fraser: Can I grab a telescope and see it?

Pamela: Yes and that’s one of the cool parts. You can actually go outside and see lots of superclusters. The Virgo supercluster is probably one of the most dramatic with the Virgo cluster within it.

You can also go out and you can easily see the Coma cluster. Galaxy clusters are big enough that they can appear as easily recognized high numbers of galaxies in a concentrated region on the sky in photos and just by sweeping your telescope across the sky.

Just like the Hyades cluster appears as a definitely much denser region of stars, the Virgo and Coma clusters appear as obviously much denser regions of galaxies. You just need a bigger telescope to see a cluster of galaxies because they’re faint.

Fraser: Right. So most of the galaxies that we can see are part of the Virgo supercluster?

Pamela: Yes, that’s one of the coolest things is we’re all basically falling together and we’re all part of one large city of galaxies.

Fraser: Right, now is the Virgo supercluster part of something even bigger?

Pamela: This is where we’re trying to figure out how you name things. The Virgo supercluster itself is the corner of where a bunch of different walls come together.

Our universe has evolved over time from being pretty much a solid block. If you look back at what was the structure of the universe back in the days of the cosmic microwave background the density variations back then were very minute. The fluctuations that we see in cosmic microwave background are just one part in thousands.

As we look today we end up seeing these huge voids of basically nothing in these really dense walls, these really dense superclusters where all of the mass has gravitationally been pulled together.

Superclusters are where the different walls come together. The Virgo supercluster is just one place where all of these different walls come together.

Fraser: Okay, I’m going to need some kind of an analogy. I’m thinking walls.

Pamela: Swiss cheese.

Fraser: Swiss cheese? Bubbles, foamy bubbles.

Pamela: Yeah when you go to the grocery store there are all sorts of different cheese with holes. There is cheese that has really tiny holes, the really cheap American Swiss. Then there is the lacy Swiss cheese.

With the lacy Swiss cheese where a bunch of different bubbles are not quite touching at their corners you end up with a lot more cheese.

Then there is the wall between two bubbles and then you end up with a junction of four bubbles. That gives you more cheese at the junction of four bubbles.

Fraser: Okay so the holes are the air pockets, right? So that’s the voids that we have in the universe. Then the cheese [laughter] is the galaxies. How big are those voids then?

Pamela: The voids can be absolutely huge. Just to give you some perspective before jumping in to the size of the voids, our own Milky Way galaxy is about 100,000 light years from edge to edge. That’s its diameter.

Andromeda, our nearest giant neighbor is about 2 1/2 million light years away. The Virgo supercluster is way bigger. It is 110 million light years across. A void, a small void not a giant void, just an average run of the mill local void is 200 million light years in emptiness.

Fraser: Nothing?

Pamela: Nothing. They start looking and they might find like one or two lonely galaxies inside of them but these are giant empty spaces. Two hundred million is a small one. They can be two or three times this size.

We’re finding the supervoids are more like 400 million light years side to side of pretty much absolutely nothing. So we went from a universe that was pretty much one continuous blob of stuff with very slight, slight irregularities in the distribution of matter.

The slight places where there is a little bit more were able to gravitationally grab on to the stuff around them. The places that were a little bit less, they just sort of gave up and let themselves be torn apart gravitationally.

The places with less have gotten less and the places with more have gotten more. In the universe, he who has more gets more. It’s kind of unfair.

Fraser: It’s like the rich get richer, right?

Pamela: Yes.

Fraser: The poor get poorer in terms of space.

Pamela: Gravity doesn’t understand Robin Hood.

Fraser: So let’s see if I understand this. Way back, right at the big bang there were these fluctuations in density of the newborn universe.

Those fluctuations in density have been expanded for billions of years so that now we have voids.

Pamela: It’s just the very slight fluctuations that we see in the cosmic microwave backgrounds that these inhomogenaties at the level of one in 10,000ish.

It’s this slight inhomogenaties that have led to everything we see today.

Fraser: Do we know why there were those differences?

Pamela: This goes back to I think we actually covered this in an earlier show where there was basically sound waves moving through – and sound waves has a very physical meaning. Sound waves are when you have compression waves moving through a medium. There were essentially sound waves moving through the early universe.

The different wave lengths of those waves got frozen out where you ended up with slight over-densities and slight under-densities. It’s much more complicated then that but that’s the simplest way of looking at it.

We had waves traveling through the earlier medium and those waves got frozen out as places with higher densities and lower densities.

Fraser: Right, I can imagine waves going across the ocean if you suddenly froze the ocean and then measured the thickness of the ocean you would have places that were thicker. You would have places that were thinner just because of where the waves were at that moment.

Pamela: In this case it is much more like measuring the density in the air where complex notes are being played by an orchestra. All those different sounds that you’re hearing have different wavelengths and those get frozen out.

Fraser: And then the rich got richer and the poor got poorer for billions of years until here we are today.

Pamela: Exactly.

Fraser: Alright then if I was to sort of pull way back out and just see the whole universe?

Pamela: You can sort of do this locally with the Sloan Digital Sky Survey.

Fraser: Right so how many of these voids would I see? How long would these walls be?

Pamela: What’s neat is we can actually watch the growing of the structure because as we look further and further back we’re able to see further back in time. Light takes a finite amount of time to get from point A to point B.

When we look at nearby things we can measure tens of different voids. We can measure tens of different superclusters. As we look further back in time at objects that are further away and their light is taking longer and longer to get to us we start seeing fewer and fewer superclusters. We start seeing fewer and fewer clusters themselves. The clusters get smaller.

There have always been giant clusters. Some of these giant clusters formed in the first billions of years of the universe but they’re continuing to grow. They’re continuing to build and their numbers are growing over time.

While we see tens of voids the numbers and sizes of the voids drop out as you look back in time. The numbers and sizes of the superclusters drop as you look further back in time. We can actually see our universe evolve.

Fraser: I guess the voids increasing are where you’ve got these walls of galaxies and at some point they snap because of the universe. A galaxy has to make a decision to either go left or right.

It’s either going to stick with these buddies or go off with those buddies and then you get a new void appearing in-between and those voids get expanded.

Pamela: One of the things as we turn the clock forward is this process that you just described is going to make our universe go from simply being this lacy foam to actually being nothing more than isolated islands of galaxies.

They are eventually basically going to turn into isolated galaxy as things cool off with time.

Fraser: That was my next question is how is this all going to turn out? Run the clock forward as you said new voids pop open they become isolated collections of galaxies these superclusters. Then you’re saying they become galaxy?

Pamela: Right. We’re still working on the details of the model. What’s really cool is we can actually watch these systems that are already held together to the point that the expansion of the universe has no bearing on the galaxies involved in superclusters. They’re gravitationally held together.

Their separations are caused strictly by what their orbital size is as they orbit round and round the center of the supercluster. Over time galaxies will sweep past each other and gravitationally glom on to each other or material will get stripped out of galaxies.

There’s this whole language that is rather violent involving galaxies. We talk about galaxy harassment where two galaxies sweep past each other. They’re moving fast enough that they don’t actually merge into one new system but the gravitational pull of the one galaxy on the other is able to grab some stars. It is able to grab out gas and trigger star formation.

Once that star formation is over what might have previously been a nice healthy galaxy is now a dead elliptical system. We have what we talk about as ram-pressure stripping. As a galaxy falls into one of these superclusters it hits all the material that has been stripped out of galaxies that fell in earlier. As it hits this material it is like getting sandblasted except at the gravitational and pressure levels. You have these systems that aren’t gravitationally able to hold on to their gas and dust in the face of getting knocked by the material that it is falling into.

In some cases the knocking triggers star formation again. You have these complex tidal tails star formation going on. It’s quick and when it is over the systems are dead. You don’t find star formation in giant superclusters at the same rate that you find it in smaller systems like our own local group.

Fraser: I guess we can already see the future of some of these superclusters. There are some monstrous galaxies with trillions of times the mass of the Milky Way right at the heart of these galaxy clusters.

Pamela: And maybe seven.

Fraser: Yeah, they’ve already pulled away so many galaxies and torn them apart and added them to the mass. Eventually I guess they all have to have their time to meet [laughter] their final destination with whatever is the big elliptical galaxy at the core of the galaxy cluster.

Is that what astronomers think then? In the end there will just be in each of these superclusters one big galaxy?

Pamela: The question is just how far do you evolve the clock and at what point do you say I have a bunch of black holes or I have a bunch of galaxies. Over time you’re going to have the gas and dust getting used up in star formation getting knocked out into the inner cluster media.

You end up with the cluster itself having this thicker and thicker gas that is orbiting outside of any specific galaxy settling toward the center. It’s so hot that it doesn’t all compact down and form something nice and small and compact in the center. As you have these galaxies harassing each other they shred each other.

Given enough time they merge together and merge together more. At the same time you have the stars are dying in the galaxies. You have the systems themselves the stars are turning off over time. At what point do you say I now have a bunch of dead stars and black holes, this really isn’t a galaxy anymore and I have a system that is filled with more gas than stars and dust.

It’s a matter of what do you name these future entities. I’m not sure anyone has sat down and looked at the models trillions of years in the future and said I’m going to stop calling a supercluster a collection of galaxies and start calling it a collection of dead compact objects.

Fraser: We should come up with a name then. [laughter] Listeners, now is your chance. Hurry, we’ll write a paper. We’ll claim the name.

Pamela: I think mostly astronomers are worried about trying to figure out things that have happened in the past. Our models say, yes everything is falling into superclusters. We haven’t really started to worry about how superclusters die.

Fraser: This is going to be completely out of left field, but what’s a great attractor?

Pamela: If you go outside and you turn on your microwave eyes and then decide that you’re gong to go into orbit because you can see the microwave much better in that direction.

Fraser: That’s easy we do this all the time.

Pamela: Right and things like Planck if it launches successfully tomorrow, and the Wilkinson Microwave Anisotropy Probe in the past make very good measurements of the cosmic microwave background.

When we measure our motion relative to the cosmic microwave background we can go aha we’re going in that direction because of Doppler shifting. We see light in one direction is shifted toward the red. Light in another direction is shifted toward the blue. It’s nice in opposite sides of the sky so we know we’re traveling in the direction that we see the light shifted toward the blue.

When we add up all the motions that we know about, when we add up the motion of the sun around the center of the galaxy, when we add up the motion of the galaxy relative to the local group. When we then start looking at the motion of the local group what we find is we’re headed in a direction that is blocked out by the disc of the Milky Way.

We’re not quite sure what we’re headed toward. We think this is a combination of both giant chunk of mass that direction and giant void of mass in the opposite direction. A lack of mass will cause you to go toward the direction of a perfectly average pile of mass because your forces aren’t balanced. It’s not anti-gravity but it’s a lack of gravity and it kind of has the same effect.

Fraser: Whatever it is that we’re actually being attracted towards, we can’t see it. It is like driving down the highway with your front windshield blacked out. The disc of the galaxy is blocking in the sky. Do astronomers have any idea?

Pamela: We think that part of this motion is probably caused by what is called the Shapley supercluster. This is a giant, giant collection of galaxies that is off in the direction of the Centaurus constellation. We think that that’s part of the cause.

There is also a void that was identified a couple of years ago in the opposite direction that we think is another part of the cause. There’s probably still more mass waiting to be discovered that’s hiding behind the dust and gas that makes up the plane of the Milky Way. We just can’t see what galaxies are beyond the area our Milky Way is obscuring.

Fraser: I guess it’s only been in the last few years that astronomers have had a lot of the technology to be able to peer right through the Milky Way, right? I mean x-rays.

Pamela: X-rays help.

Fraser: Yeah and I know that astronomers have been putting together surveys of galaxies and clusters and stuff on the other side of the Milky Way using Chandra and that kind of thing.

Pamela: One of the problems is Chandra only allows us to see the biggest clusters of galaxies and the nearest clusters of galaxies. Those are the only ones that have enough mass to be generating enough hot gas that it emits x-ray light.

We’re also looking in the infrared. Infrared can also pierce through the dust. We’re looking in the radio. It’s by combining all of this different data. Now of course there is stuff that gives off light in the infrared in the disc. There is stuff that gives off light in the radio in the disc.

No matter what happens it’s always going to be hard. We have no completely transparent band that allows us to look through all of the stuff in the disc of the Milky Way.

Fraser: There isn’t any need for a kind of scary pseudoscience description of what the great attractor is?

Pamela: No it’s simply that there is something big over there and we’re falling toward it. That’s okay, we understand how that works.

Fraser: This is one of those situations where our position is not in our favor.

Pamela: Right.

Fraser: That’s it.

Pamela: Sometimes chance alignments happen.

Fraser: Are there any missions out there that you think will really help us understand the large scale structure of the universe?

Pamela: The James Webb Space Telescope is definitely going to help us understand the evolution of the large scale structure. The Sloan Digital Sky Survey and the future Large Synaptic Survey Telescope – the LSST are both going to play major roles as well.

What’s happening right now with the Sloan Digital Sky Surveys, there is a robotic telescope out in New Mexico that is systematically looking at the sky both taking regular images that hopefully lots of you have looked at in Galaxy Zoo. If you haven’t, go to galaxyzoo.org. They’re measuring as well as what does the galaxy look like through photographs, they’re also taking spectra and measuring the red shifts.

This allows us to figure out the distance to these galaxies. By combining all of this different information they’re making amazing 3-dimensional pictures of what the nearest billionish and beyond that but with decreasing numbers of sources. You start looking at quasars to measure the density of the universe.

They’re working to build detailed 3-dimensional maps of first our Milky Way using stars, then the nearest part using all the galaxies and then the more distant parts using quasars.

Sloan Digital Sky Survey is going to keep going deeper. They’re going to keep looking at different types of details with different surveys. We have in the future the Large Synaptic Survey Telescope coming onboard which is going to take image after image of the sky. Over time you can add those together to get deeper and deeper images.

James Webb space telescope is not a survey telescope but it’s going to allow us to over time take the equivalent of the Hubble deep field but in the infrared giving us an even deeper view of our universe, a narrow cone of understanding.

Fraser: It’s the time machine idea where James Webb is going to be looking right back to hundreds of millions of years.

Pamela: Exactly, the very first objects.

Fraser: Yeah, after the universe formed and really watch as those differences in density started to turn into the big galaxy clusters that we see today.

Pamela: You can go out and see both sides of this in both real data and in models. There’s a fellow down in the University of Hawaii, Josh Barnes who has done some amazing movies of how do galaxies merge. If you want to see how galaxies self-destruct he has models that do that.

There are all sorts of different videos on how does large scale structure evolve over time. There are Sloan Digital Sky Survey fly-through movies that allow you to go from right there on the mountain peak in New Mexico zooming out to the Milky Way, zooming out to the local group, and zooming out to the quasars that are at the edge of the survey.

You can fly through our understanding of the universe.

Fraser: Awesome. I think that wraps up the biggest there is. Thanks Pamela.

]]> http://www.astronomycast.com/astronomy/cosmology/ep-137-large-scale-structure-of-the-universe/feed/ 6 http://www.astronomycast.com/astronomy/ep-135-x-ray-astronomy/ http://www.astronomycast.com/astronomy/ep-135-x-ray-astronomy/#comments Thu, 07 May 2009 17:40:36 +0000 Astronomy Cast http://www.astronomycast.com/?p=724

Fraser Cain: We’re moving along in the x-ray spectrum. If you’ve ever broken a bone x-rays are a part of the electromagnetic spectrum that you’ve experienced personally. Doctors use x-rays to study the human body but astronomers use x-rays to study some of the hottest most energetic places in the universe.

So, let’s put on our x-ray specs and see what we can see. Alright so landscape time I think we were down to 7 nanometers for ultraviolet astronomy. We’ve gone from meters in the radio all the way down to 7 nanometers. Where do we find x-rays?

Dr. Pamela Gay: This is where we actually get into people debating because there are those out there who argue that the ultraviolet goes down as short as 7 nanometers.

Then there are those that say that x-rays go as long as 10 nanometers. Somewhere between 10 and 7 nanometers is where the x-rays pick up.

Fraser: How can that even be an argument?

Pamela: Different people decide they’re going to look in different books and get different definitions. Some people like to round things to the nearest 10^th because it is convenient.

It’s just light that we can’t see. So there is really no clean reason to define it any particular way.

Fraser: Is there like some kind of physical characteristic that divides the line between ultraviolet and x-ray?

Pamela: No. [Laughter]

Fraser: Okay, and how small does it get?

Pamela: It gets pretty short. With x-rays we’re also looking at things that get down to about a hundredth of a nanometer. So we go from roughly 10 nanometers down to 0.01 nanometers.

Fraser: What are those, picometers?

Pamela: Well it’s not quite a picometers. It is actually ten picometers. We’re starting to get down to the point where without thinking really hard most of us don’t know what are the correct abbreviations we should be using. It’s just small.

This is where we’re starting to get down into really high energies. These things when they hit you, if they happen to hit a piece of DNA quite happily destroy it.

Fraser: Really.

Pamela: Yeah it’s one of the kinds of frustrating pieces with the really hard x-rays is they carry enough energy that they can start to blow things apart. It is cool but dangerous.

Luckily most of the x-rays we deal with day to day in real life are the softer ones that are a lot less harmful. You don’t want to get a lot of them but getting a dental x-ray, getting a leg x-ray now and then is not going to kill you.

Fraser: Those are soft x-rays. Those are the larger wavelengths?

Pamela: The larger wavelengths in the lower energies.

Fraser: The lower energies and then the harder x-rays are the shorter wavelengths and the higher energies.

Pamela: Exactly.

Fraser: Then I guess we’ve got a little protection from x-rays though living here on the surface of the Earth?

Pamela: This is actually one of the really good things. We know that x-rays can go through air. Otherwise you’d have a lot of problems in the hospital because the x-rays wouldn’t make it from the machine to your leg. X-rays in general can only travel a couple of meters through air before they get absorbed.

While x-rays start to enter our atmosphere they don’t make it very far before all of their energy is absorbed into the atmosphere. So we’re actually kept safe from all the background of x-ray that exists out there in outer space.

Fraser: Then x-ray astronomy from the surface of the Earth completely worthless.

Pamela: Yeah it’s just not going to happen.

Fraser: It’s not going to happen and you might as well be looking at nothing, it’s completely black.

Pamela: But balloons work and that’s kind of cool.

Fraser: Let’s talk a bit about again the detectors and the equipment. In my experience with dental x-rays is there some great big kind of gun like thing [Laughter] stuck next to my mouth and then I have to chew on this film in my mouth.

Then I hear this bzzzt for a second and then I spit out the little pieces of film and the dentist runs away and comes back and shows me my teeth. Clearly we’ve got an x-ray source and maybe my dentist would replace that with I don’t know a black hole.

Then we’ve got a detector [Laughter] which in this case is the film. How does that compare with what astronomers are doing?

Pamela: Clearly we don’t have friendly little dental hygienists running back and forth from the Chandra X-Ray Observatory with film to develop in that back mysterious room of your dentist’s office.

Fraser: Right but I mean with visible and even with ultraviolet we’ve got a big dish, right? We’re using the same kind of equipment, the same kind of set up?

Pamela: No here x-rays are annoyingly difficult to try and focus. They would much rather get absorbed or go through the side of your telescope than to get focused down in a nice friendly way onto a detector.

What we have to basically do is cause them to very carefully through grazing angles get bounced onto a detector. We can’t focus them the way that you can focus a nice normal ray of optical light or even infrared light.

Take a big round thing in outer space and line the inside of it with shelves of aluminum foil. They don’t actually use like cooking aluminum foil. Line the inside with foil such that if something was trying to come in at an almost parallel line to the foil it would just barely hit it at a grazing angle and get reflected off.

If it hits enough of these pieces of foil at the slight grazing angle we can actually very carefully and very slightly bend the x-ray light. We can’t bend it enough to get it into a nice clean focus. When we try and take images of x-rays what they actually do is rely on shadows. This is kind of weird to think about.

They actually have these really complicated plates in many x-ray detectors that have a series of holes in them. The x-rays are able to go through the holes and then get blocked by other parts of this plate.

They are able to reconstruct the image by where they do and don’t see light. If you have light coming in from the left you’ll get one set of shadows. If you have light coming in from the right you’ll get a different set of shadows. By looking at the shadows and the light they can figure out yes there was light from the left; yes there was light from the right and it was brighter.

It is a lot of complicated math to get an image but these images allow us to take an entirely new view on the universe and they’re some of the most spectacular ones that we have.

If you’ve never gone out and looked at what supernova remnants for instance look like in x-ray the Chandra Space Telescope has an amazing galaxy. My favorite is this object called 1572 that looks like there is a forest growing out of basically a purple jellyfish.

Fraser: Right and of course these aren’t the true colors. They are coming up on computer and they are associating one kind of color let’s say with hard x-rays and another kind of color with the softer x-rays or different temperatures and trying to recreate what’s going on.

Of course you couldn’t actually see this with your eyeballs. That’s the way Chandra works? If you look at Chandra it is long and skinny and kind of looks like a space telescope.

Pamela: It’s actually got these grazing angles, these foils that very carefully deflect the light just slightly to get it down to the detector. We have detectors that are essentially able to detect each and every single photon that hits the detector and translate that into a signal.

Fraser: I guess with a telescope, with a great big mirror you don’t have to get every single photon you just have to get a lot of them but you can focus a lot of them in to concentrate it. In the case of an x-ray one, it’s very hard to get all of the photons.

You’re not going to get much more photons than the size of your detector itself. As you said it can narrow a few down but that’s it. Every photon that hits is hitting with such high energy that they’re probably easier to catch in terms of the detector.

It is easier to detect a photon that hits with x-ray energy as opposed to say a photon that hits with I don’t know, visible light energy, radio wave [Laughter] right?

Pamela: With the optical telescope all of the star’s light that’s hitting the mirror gets focused down to a very few pixels on the CCD detector. Whereas with x-ray telescopes we have those photons going through and then we’re purposely blocking some of them. The ones that do hit the detector are scattered all over the place.

We have to catch each and every single one of those. We’re able to do that with things like micro-calorimeters and with Teslas. We’re able to finally after a lot of originally all we could say was yes there’s an object – no there’s no object. We’re finally able to build amazing images of objects using x-ray light.

Fraser: I know that Chandra for example does very, very long exposures. They’ll point Chandra at a galaxy cluster or a black hole for a very long time to build up these it’s almost like you can think of it as like drops of rain falling – photon, photon, photon – and then it just builds up the image over time.

Pamela: It’s not a fast process. One of the things that really amazes me about this is each of these photons is so much more powerful. While it may take longer to build up a few hundred of them those few hundred if you focused all of them on to one happy little amoeba that would normally be bathing in sunlight on slide, you would toast the poor innocent amoeba. You’re carrying huge amounts of energies in each and every one of these photons.

Fraser: Another interesting thing is that they leave Chandra sometimes open can actually get science done just by the in-betweens. Chandra is moving from one location to another location and they’ll still record everything it sees as it is sloughing to a new spot. Astronomers have made use of all of that in-between data as well. It’s pretty cool.

Pamela: One of the cool things that we’re finding and Chandra is not solely responsible for this but the entire sky glows in the x-ray. There’s this diffuse background of x-ray radiation just like there’s a diffused flow of microwave radiation.

But we know where the microwave comes from and we’re still trying to figure out where all this x-ray radiation is coming from.

Fraser: Are there any theories?

Pamela: Lots but we don’t know for certain. For instance we know early in the universe there was wild star formation. There were lots of quasars and lots of active galaxies.

It could just be that all of those added up might, maybe but it is a lot of energy to come up with. We’re still trying to sort this one out.

Fraser: Right it would have to be red shifted as well because it would be so far away.

Pamela: If you start off in the gamma ray you can land yourself happily in the x-ray.

Fraser: Right but that just means that yeah there’s some really – and I guess gamma rays that’s next week. What kinds of things – we’ve kind of hinted at it – what kinds of things are we going to want to look at and study in the x-ray?

Pamela: One of the coolest things to actually look at in the x-ray is our own moon.

Fraser: What, our high energy moon? [Laughter]

Pamela: It’s not exactly that our moon itself is radiating away x-rays but our sun does give off x-rays. At least it normally does, right now much to the annoyance of every lunar scientist it’s adamantly not having any sunspots and not having any coronal mass ejections with their associated x-rays.

When you have a normal active moon when a blast of x-ray radiation from the sun hits the moon it can interact with different minerals and fluoresce. Anytime you see a mineral fluoresce this is for instance what you get when you shine ultraviolet light on a white shirt, when you shine ultraviolet light onto a rock that contains fluoride. The high energy light gets absorbed.

Some of the energy makes the molecule vibrate madly. Then some of that light gets re-radiated with a longer wavelength and we can see it. With the x-ray fluorescence some of these high energy particles has some of this high energy light gets absorbed by the minerals on the moon and then re-emitted as softer x-rays.

We can actually see the glowing x-ray moon against a background of much, much fainter x-ray emission from the whole universe. Then on the dark side of the moon – the side of the moon that’s not getting lit up with the sunlight – you see darkness.

You can end up with these really neat images of bright x-ray fluorescence from the moon background x-rays and then the dark part of the moon just sitting there going I’m dark.

Fraser: Right because it’s not getting any light from the sun. Okay, the moon, what else, the sun?

Pamela: The sun and here it’s not that our sun is hot enough to just haphazardly be emitting in the x-rays. X-ray corresponds to temperatures far greater than our sun. When you take a magnetic field line and rearrange it, this is what happens when we get coronal mass ejections. There is a huge amount of energy in the magnetic field lines.

When you release all of that energy into space some of it gets released in the form of x-ray. Some of it gets released in the form of high velocity particles. It’s those x-rays from the sun that we see getting reflected off of the moon. Those are all pretty cool, they are all pretty nearby and pretty easy to observe.

The easiest thing to observe is actually the center of our galaxy. When you look toward the center of the Milky Way you see a large bright x-ray source. This is actually just background from our own black hole flickering away in the center of the Milky Way.

Fraser: I know that the center of the Milky Way is blocked by gas and dust. In the visible light and ultraviolet we can see it in the infrared. We are able to see it in x-rays?

Pamela: We’re able to see it in x-rays as well.

Fraser: Isn’t that kind of amazing? X-rays are blocked by a few meters of our atmosphere and yet they can pierce the shrouds of gas and dust from the center of the Milky Way. That’s quite amazing.

Pamela: That itself still isn’t one of the brightest sources. That’s just sitting there flickering periodically giving off x-ray light. The brightest things of all are things that we have to randomly go searching for. These are x-ray binary systems.

This is where you take a couple of high mass objects put them in orbit around one another and let one of them steal mass off of the other. The brightest x-ray source known for a long time was Scorpius X-1. It is an object about 9,000 light years away. It gives off more light in just the x-ray than the sun gives off in all of its wavelengths combined.

In the system we have what we think is just a nice happy normal neutron star orbiting another star and sucking material out of its atmosphere. As that material spirals in it radiates away energy because it is going really fast. In order to slow down and orbit correctly it has to radiate some of that kinetic energy away as thermal energy. That energy is getting radiated away in the x-ray.

Fraser: If you could actually see the object, what would it look like? Would it be sort of like have an ecretion disc around it and then be the neutron star spinning at the middle?

Pamela: It is sufficiently far away that it is just a boring point source. It is just this blob of x-ray emission.

Fraser: Of course but if we could get close and see?

Pamela: If we were able to get close and see we’d probably see bright x-rays coming from the ecretion disc itself and then also perhaps flickering at the point where material is getting torn off and were perhaps interfering with the magnetic field of the companion star.

You’d also see bright x-rays associated with the magnetic field of the neutron star itself. I have to admit I don’t know enough about the details of these systems to know what would be brightest, what would be faintest but you would have a lot of different places would be giving off x-ray emissions at varying amounts.

Fraser: Do the x-rays come off in jets or the just kind of come off in all directions equally?

Pamela: With this, it is actually thermal radiation. The same way you get thermal radiation from a hot rock.

Fraser: That’s all directions?

Pamela: It is all directions.

Fraser: There are objects out there that do give off jets that can be seen in the x-ray.

Pamela: This is one of the cool things. It’s not actually the jets that are doing the x-ray emitting. It is the way they shock the material around them that leads to the x-rays.

One of the really cool things that happens when you start looking at giant active galaxies especially when they’re in clusters of galaxies is you have jets that are emitting in the radio predominantly. These are electrons flying through space spiraling around magnetic field lines. As they spiral they’re giving off light in the radio. As these accelerated particles that are accelerated by the magnetic field associated with an ecretion disc in an active black hole get accelerated down the jets eventually they start colliding with the interstellar medium, the intergalactic medium depending on if it is a super massive black hole in a galaxy.

You can also get this with stellar mass black holes. In either case these jets end up hitting the gas and dust that pretty much is everywhere in space. Space is full of gas and dust. As it decelerates against this gas and dust it shocks that gas and dust. It is the shocking, the sudden deceleration that ends up leading to the x-ray emission.

Fraser: Wow. You’re getting these kinds of spots of very high temperature. I know that some of the greatest pictures taken by Chandra are these gigantic galaxy clusters colliding together with gas clouds of millions of degrees just because of all of the energies involved. One of the questions that I have is how can you have these gigantic gas clouds of millions of degrees, are we getting stars there?

Pamela: The amazing thing about these clouds is well as astronomers refer to them as dense they’re not dense and their high temperature doesn’t come from the high pressure or lots of collisions taking place. They’re actually such a high temperature because you have the particles, the atoms getting accelerated into these giant massive, massive objects.

When they do occasionally collide they’ve been accelerated by gravity. When they collide they radiate away all of that kinetic energy in the form of x-ray emission. In this case it is the velocity is equivalent to the temperature. You end up with this really high temperature because you have a lot of gravity.

Fraser: Oh, and we’re seeing the collisions.

Pamela: We’re seeing the collisions. You don’t have to have a lot of them to have a lot of the x-rays.

Fraser: If you actually were in your spaceship in those hot gas clouds

Pamela: You wouldn’t notice.

Fraser: You wouldn’t even notice.

Pamela: You’re like vacuum of space, I’m fine not a big deal.

Fraser: Yeah, million degree gas? That’s not so bad. What else would we want to look at with x-rays?

Pamela: The coolest objects of all of course are supernova remnants. This is the case where you have an exploded star. It’s not the whole exploding part that gives off the really cool x-rays although you do get cool x-rays with the explosion part as well. The really cool stuff comes when the shed atmosphere of the supernova, when the part of the star that gets flung out into space starts hitting the gas and dust that’s between the stars it rapidly decelerates.

In the process it can give off just the most amazing x-ray emission. Here you can end up with all sorts of wonderful structures. We don’t know what causes all of the structures. In many cases it is just well that’s the shape the gas and dust was in and when you shocked it and it ended up with this new shape.

It’s kind of like what happens when you blow on a really dusty bookshelf. You get a clear space and then how the dust around the clear space is shaped depends on how long it has been since you cleaned your shelf, how much friction there is, all sorts of stuff.

Trying to sort out supernova remnant is sort of like trying to sort out the structure of the clean spot on the shelf you’ve just blown on. It is hard and we’re not entirely sure what all is there to take into consideration yet. A planet can get in the way. Asteroid belts can get in the way. All of these things can affect the final shape of a supernova remnant.

Fraser: What do you think are some of the big questions out there – and I guess you alluded to one of them – that would be answered by x-ray astronomy? What I’m mentioning is what this background x-ray radiation that we see everywhere is?

Pamela: One of the coolest things that we can do with x-rays is we can trace out how is it that galaxies go through their life cycle. How is it that when they’re young they’re rich in star formation and have active galactic nuclei? How often do these super massive black holes go through feeding phases?

As we look back to further and further in the past we’re able to see galaxies at all of their different stages of life. We’re able to see ah; when we look here there are a lot of quasars. When we look here there aren’t but now we see spirals merging together and their central super massive black holes turning back on.

We can see this turning on and turning off of the super massive black holes in the form of basically the x-rays they split out while they’re in the process of feeding. We can study how is it that we get to this wonderful galaxy that we live in and we get to the diversity of galaxies all around us over the course of the universe’s evolution.

Fraser: That’s one of the big questions that I guess with the launch of the James Webb telescope and some of the other observatories coming out. We’re probably going to get a pretty good answer to within the next couple of decades which is how did we get from the big bang and just a spray of hydrogen and helium to smaller galaxies, to bigger galaxies, to the big grand spirals that we have today. What’s the process? Did they all come together quickly? Did they come together in bits and pieces?

I think that’s one of the questions that I see as I work on articles that keep coming up again and again. Astronomers just don’t really know the exact process. Obviously they have some ideas. The x-ray is just another tool in the toolkit to do that.

Pamela: It is a long journey and we have many more telescopes to go before it is done.

Fraser: Well I think that wraps up x-rays for this week. I think we’ve only got one left.

Pamela: I think we have gamma rays another one of those that doesn’t have an edge. It and the radio define the two ends of the spectrum.

Fraser: Next week we learn how to make The Hulk.

Supernova remnant W49B, seen in X-rays and visible light.

We continue our journey through the electromagnetic spectrum with X-rays.  If you've ever broken a bone, you probably know how X-rays are most commonly used.  While doctors use X-rays to study the human body, and astronomers use X-rays to study some of the hottest places in the Universe. So let's put on our X-ray specs, and see what we can see.

Ep. 135: X-Ray Astronomy

• X-rays — NASA
• Chandra Field Guide to X-ray Astronomy
• Hard X-rays (shorter wavelengths, higher energy)
  
• Soft X-rays (longer wavelengths, lower energy)
• X-ray detectors – NASA
• NASA's Scientific Ballooning Program (which includes X-ray)
• Supernova Remnant 1572 (Tycho's Remnant) by Chandra
• False colors associated with X-ray — Chandra
• The Diffuse X-ray Background
• The X-ray Moon – APOD
• The Sun as an X-ray Source – NASA
• Astronomers Resolve Milky Way's Mysterious X-ray Glow – Universe Today
• X-ray Binary stars
• Scorpius X-1, one of  the first extrasolar X-ray sources
• Chandra's Study of X-ray jets
• Interstellar Medium
• Intergalactic Medium
• Groups and Clusters of Galaxies from Chandra's Field Guide
• The Life Cycle of Galaxies — Cornell U

Download the transcript

Transcript: X-Ray Astronomy

M81 in ultraviolet. Image credit: NASA

Our next visit in this tour through the electromagnetic spectrum is the ultraviolet. You can't see it, but anyone who's spent a day out in the hot sun without sunblock has sure experienced its effects. Ultraviolet radiation is associated with the birth of stars and some of the hottest places in the Universe.

Ep. 134: Ultraviolet Astronomy

• Ultraviolet Astronomy
• Ultraviolet Astronomy — Encarta
• The Ultraviolet Universe – JPL Webcast by Dr. Christopher Martin, GALEX
  
• About UVA, UVB and health risks — Healthlink
• Black Body Spectrum — Wolfram Research
• Paper:  Far- UV Spectroscopy of Star Forming Regions — arXiv
• Deepest Ultraviolet Image of the Universe — ESO
• Best Ultraviolet Image of the Andromeda Galaxy – Universe Today
  
• M 33 in Ultraviolet — Universe Today
• The Big Bang and deuterium – Berkeley
• FUSE deuterium explanation — Universe Today
• Mice Galaxies- HubbleSite
• Antennae Galaxies — HubbleSite
• Pinwheel Galaxy by GALEX — Universe Today
• Hubble's Ultraviolet Views of Nearby Galaxies Yield Clues to Early Universe — HubbleSite
• Lyman-Alpha systems
  
• Lyman-Alpha Forest — UCLA
  
• FUSE image of Mira Galaxy — Bad Astronomy
  

Ultraviolet Telescopes:

• Astron-1
• Astrosat
• Astronomical Netherlands Satellite
• Extreme Ultraviolet Imaging Telescope
• FUSE
• GALEX
• Hubble
  
• International Ultraviolet Explorer
• Orbiting Astronomic Observatory
• Swift Telescope

Download the transcript

Transcript: Ultraviolet Astronomy

Fraser Cain: Hi Pamela how was your conference?

Dr. Pamela Gay: It was amazing. The folks at NEAF are a really great group and the stuff that they had there from all sorts of different brand new telescope designs from all the different manufacturers.

They had solar observing set; they had all sorts of workshops throughout the weekend. It was really an impressive conference and I highly advise anyone out on the east coast to go to it next year.

Fraser: You did a live question show?

Pamela: I did a live question show.

Fraser: Our next visit in the tour through the electromagnetic spectrum is in the ultraviolet. You can’t see it but anyone who spends a day out in the hot sun without sun block has sure experienced its effects. That’s sunburn.

Ultraviolet radiation is associated with the birth of stars and some of the hottest places in the universe. So once again I guess we want to get our bearings. Ultraviolet is shorter wavelengths than the visible light that we can see with our eyeballs. Where does it range?

Pamela: It in general starts right where the atmosphere starts blocking it at about 320 nanometers and then it goes all the way down to about 7 nanometers for the most extreme ultraviolet.

Fraser: [Laughter] Seven nanometers. You just mentioned that it is blocked by the atmosphere.

Pamela: This is probably a good thing. Once we start getting ultraviolet we’re dealing with damaging radiation from the sun. Luckily we have ozone in our atmosphere so up in the highest levels of our atmosphere the ozone just happily absorbs all of the radiation.

It heats up and as the planet rotates it dissipates its heat back out and we’re able to be protected. While some UV still gets through at the longest of the wavelengths and still causes sunburn for the unprotected, in general we’re kept safe by the ozone in the atmosphere.

Fraser: We look at our sun block it says that it is like UVA and UVB. How does that compare to the wavelengths?

Pamela: The UVA is from about 320 nanometers longward to 400 nanometers. This is where stuff is getting through the atmosphere at a fairly good rate and the UVB is from 320 nanometers down to 290 nanometers at which point the atmosphere is pretty much opaque.

We are starting to deplete the ozone in our atmosphere periodically thanks to fluorocarbons and other nasty pollutants so we do have to a fair bit about the UVA getting through but less about the UVB. But, it is the UVB that is more dangerous when it hits us.

Fraser: Then but we’re definitely blocking out the stuff, the 7 nanometer stuff.

Pamela: [Laughter] Yeah that has no hope of getting through our atmosphere.

Fraser: What is the source of ultraviolet? What creates ultraviolet rays?

Pamela: In general anything that is hot enough can generate ultraviolet. Our own sun doesn’t give off the majority of its light in these really high wavelengths but because it is giving off a curve of light it still has some emission that is coming out in the ultraviolet just like it has some emission that is coming off out in the radio.

It is these high energy particles from our sun that we worry about the most. We also get ultraviolet light from different atomic transition lines. For instance in hydrogen one of the fundamental lines Lyman alpha is in the ultraviolet. We also get it from different atomic transitions and things like carbon and nitrogen.

Fraser: When you say transitions, this is where an atom jumps from one energy level to another energy level and releases a photon?

Pamela: That’s exactly what’s happening. You excite the atoms to a high energy state and the electrons jump up. But just like people can’t stay excited for a long time electrons don’t stay excited.

Eventually they collapse back down to lower energy levels and in the process give off photons. For elements like carbon, nitrogen, oxygen, magnesium even iron to a certain point, many of these transitions are occurring in the ultraviolet.

Fraser: That allows you to be a bit of a detective. You can look at some object, you can see in this case ultraviolet rays coming off of it at a very specific wavelength and that can help you figure out what process is going on with that object?

Pamela: What’s even cooler about this is this particular set of transitions is very sensitive to temperature. We can look at the different strengths of these different transitions and know exactly how hot or how cool a given star is.

We can use these transitions to measure temperatures as well as to measure the abundance which helps us learn: what was the chemical process that got to build that object.

Fraser: Right and so you said that hot objects are responsible. Our sun is going from the radio all the way up to ultraviolet and beyond.

More specifically if you’re going to want to do some ultraviolet astronomy what kinds of things are you going to be looking for?

Pamela: In addition to seeing these high temperature, end of the black body spectrum we also were very sensitive to things like the transition lines in gas that is heated from 10,000 to about 100,000 degrees.

This is where you have hot gas surrounding hot young stars. We start to be able to see star forming regions very well in the ultraviolet.

Fraser: What about the dust? Does the dust block the ultraviolet?

Pamela: Yeah dust is a problem but it is only a problem for part of the ultraviolet. This is one of the kinds of cool weird things about this. You do get a fairly significant amount of dust observed where right around 217 nanometers you have a peak in the absorption from the local dust.

As you get to shorter and shorter wavelengths, the dust is able to just sort of blast its way right through the dust and so at the longer parts of the ultraviolet, we can’t see anything through the dust. As we look at the shortest part of the ultraviolet we start to be able to see through the dust.

Fraser: Why are newly forming stars so hot?

Pamela: It is not that all the newly forming stars are hot but it is that the star forming regions themselves are filled with young, young stars. Some stars in particular the hottest stars only live for a short period of time.

If you want to see hot stars that only live for maybe at most a few million years you have to look for stellar nurseries. It is one of the tragedies in a way that these really amazing systems that are blaring away with massive amounts of mass loss giving off huge amounts of ultraviolet light live for very short times. They’re very disruptive to the other stars around them that will live more staid existences going on for billions of years. They blast away at the gas and dust and then often explode violently as supernovae.

To see these young stars in action, these short-lived stars in action, we have to look at the stellar nurseries. Where we see ultraviolet light, that’s where these hot stars are and that is also where we’ll find star forming for the stars that will live much longer lives as well.

Fraser: Right, okay so what else?

Pamela: One of the other really cool things about ultraviolet is we start to be able to test our theories for the big bang. We talk a lot about how during the first three minutes there was basically the entire universe acted like the inside of a star.

There were a lot of different nuclear reactions going on. It gave us helium, trace amounts of lithium and beryllium. One of the things that we don’t talk about very much is it also gave us deuterium. It gave us a heavy form of hydrogen that has both a proton and a neutron in it.

In general during these nuclear reactions, deuterium is just an intermediary product. It is just something that happens at a halfway through step and then it quickly gets turned into something else. When the entire universe pretty much stopped having nuclear reactions all at once some of the reactions weren’t seen through to completion.

All of this deuterium that was getting formed as an intermediate step was left behind when the nuclear reaction was turned off. That deuterium is still there and over time within stars, the deuterium is getting destroyed. We can look through and we can see the atomic transition lines from deuterium separate from hydrogen in the ultraviolet.

Their wavelengths are slightly different because their masses are slightly different. We’re able to look back and measure the ratio of deuterium from hydrogen as it is decreased over time and through the evolution of the universe.

Fraser: That’s just another independent line of evidence that we can trace back to the big bang.

Pamela: We have our predictions on how much deuterium should have been there. We’re able to go back and determine that’s exactly what we see.

Fraser: Wow. I’m guessing that it is great for around super massive black holes, right? There’s so much energy, so much temperature there.

Pamela: This is where we start to see things like what is referred to as the blue hump in the light from active galactic nuclei from galaxies that have all sorts of feeding processes going on with their super massive black hole.

All of the energy is causing the inner parts of the ecretion disc to give off vast amounts of ultraviolet radiation. It is really cool to look at galaxies in these really hot wavelengths because you see the core activity.

You see the noddiness of the star forming in the arms. It is as though you’re seeing the galaxy in just its most interesting parts in some ways, its most energetic parts.

Fraser: Right if you looked at a galaxy in visible light it might look kind of bluish or more reddish. If you look at a full galaxy in the ultraviolet, if it is undergoing a lot of star formation, it is completely different from a more regular galaxy.

I guess a lot of the times because there is a lot of star formation happening that indicates that something recently happened, there is some kind of trauma to the galaxy?

Pamela: When you see unusually large amounts of star forming. Some galaxies are just naturally still undergoing star formation. Our own Milky Way still is naturally undergoing star formation. When you look at things like the Mice, like the Antennae, these colliding systems in the ultraviolet you see massive amounts of star formation.

What’s neat is as you shorten the wavelength that you’re looking at these galaxies in what you see is they go from these big dusty red things and then you strip away the red light and the arms get a little bit smaller.

Then as you start to go into the ultraviolet you’re left just looking at narrow ridge lines of the arms where the star formation is at the most active. You see just these pockets of star formation tracing out the structure of the galaxy. It is as though

Fraser: That’s like the bones of the galaxy.

Pamela: Yeah, it’s very much of a skeletal system.

Fraser: What about planetary astronomy? Is that any good here in our solar system? I suppose the sun – there is a lot of research done on the sun.

Pamela: We look at the sun, we look at the reflections that we get in the ultraviolet off of the other planets and that helps us understand by how they’re reflecting different things what that tells us about the surface chemistry. Different things emit at different wavelengths.

In general, ultraviolet isn’t the most interesting of things for looking at planets. It is mostly something that we use to try and understand the hydrogen, to understand the distribution of different atoms in distant galaxies.

In fact with galaxies we desperately have to use the ultraviolet to be able to find the most distant systems.

Fraser: Let’s talk about gear then. What does a telescope designed to capture ultraviolet radiation look like?

Pamela: It is again like the infrared where we’re that different from visible light so we do use very similar equipment. We use

Fraser: A dish.

Pamela: Yeah, we use photoconductors that are like CCDs but we have to be very careful to block out what is called red leak. This is where you start getting the visible spectrum light leaking in. Detectors are very, very sensitive to red light, to visible light which is all red compared to the ultraviolet. You have to very carefully reject all of this light from entering the detectors.

One of the problems that we deal with is that we’re very good at building photocathodes that are sensitive to the ultraviolet. Even though they’re sensitive they aren’t nearly as efficient at capturing that light as the detectors we use in the optical.

In the optical we’re able to get what we call quantum efficiency, the number of photons that get turned into a signal that makes a pretty picture. In the visible light we’re often able to up to 90% of quantum efficiency. When we’re using photocathodes in the ultraviolet we’re only getting 20-40% efficiency. We’re losing a whole lot of light.

Detectors in the ultraviolet are also much more sensitive to dirt and irregularities in the surface. We have to build much cleaner, much more everything; fewer surfaces for instances. In the visible you can knock your light around 2 or 3 times and you’re not going to sweat it too much.

But in the ultraviolet you want to use as few surfaces as possible. In every reflection you end up losing some of those photons and every photon is precious in the ultraviolet.

Fraser: Right and this is the big problem. We’re here underneath the Earth’s atmosphere. That’s great because we don’t get lots of really terrible sunburns or horribly irradiated [Laughter] but it is really bad because you think that the atmosphere is bad for a regular telescope, it is awful for ultraviolet.

Could you just take the standard telescope, a big research telescope and pull off the CCD cameras that are used for regular visible light astronomy and bolt in the UV equipment and it would work? The telescope is the same thing?

Pamela: At the most basic level yes. Some surfaces are better than others. Silver vs. aluminum becomes a question but yeah a good reflecting telescope.

Especially if you have your detector up at prime focus so you’re only looking at one reflection that would work. But you have to get above the atmosphere. We’re always launching these suckers into space.

Fraser: The true enemy of the ultraviolet astronomer is the atmosphere.

Pamela: While we’ve had this whole series of really good missions, they was GALAX, FUSE, EUVE – the extreme ultraviolet explorer, Hubble space telescope does amazing work in the ultraviolet.

But all these missions are temporary and we have no plans to replace any of the current missions in the future. We’re reaching a point where we’re not going to have the ability observe in the ultraviolet anymore.

Fraser: What are some examples, you just sort of went through them pretty quickly? GALAX, FUSE and Hubble those are sort of currently operating UV telescopes?

Pamela: FUSE is dead. It died a few years ago. Its last sad little servo finally bit the dust. GALAX only lasted 29 months so it is also a past-tense mission as well. Ultraviolet explorer is a past-tense mission. Hubble is the only one up there that is currently actively taking ultraviolet images.

Assuming all goes well with the upcoming Hubble servicing mission, it has a few more years of life left in it. Once that is past, there is nothing on the books currently to replace it.

Fraser: Nothing, wow.

Pamela: We’re about to lose the ultraviolet and this is frustrating because there is some data like looking at Lyman-break galaxies that we can do a lot of work thanks to red shift.

We can do a lot of work in the visible when things are very far away but as we start to look at nearby systems we’re losing all of that information.

Fraser: I guess that was going to be sort of leading into my next big question: if money were no object and the most wonderful ultraviolet observatory could get launched, what would astronomers want to be looking for? What deep questions would the ultraviolet help us answer?

Pamela: Ultraviolet in addition to helping us trace out deuterium abundances over time also helps us understand where all the hydrogen gas is. Where is all the absorption taking place?

Lyman-break galaxies we generally talk about these things when they’re at red shifts greater than 2.5, when they’re several billion light years away. These are systems where when they start emitting hot light that light gets absorbed at 912 nanometers by hydrogen gas. This is the Lyman alpha transition.

So you see in the light coming off of the galaxy this nice pretty curve, pretty curve and then it just drops. What’s neat is when you look at these things in different wavelengths you see pretty galaxy in red, green, you see nothing in the ultraviolet. By seeing this difference where it is there in the visible, it is there in the visible and then it goes away and in the ultraviolet we’re able to locate very quickly, very easily where distant galaxies are located.

What are also cool are distant objects giving off light, that light passes through clouds of gas between us and those distant galaxies. As that light passes through random isolated clouds of hydrogen gas, clouds that have no stars in them, clouds that never bother to form galaxies or star clusters or anything else, those clouds of gas that are intervening, absorb out light in the Lyman alpha creating what is called the Lyman alpha forest.

We can map out where these pockets of cold gas are located throughout the universe by looking at how they absorbed this ultraviolet light.

Fraser: Then knowing where the cold gas is – the cold hydrogen gas is the stuff that stars eventually get made of – this is the stuff that is left over from the big bang? It is kind of like it is the raw material that has never been used.

Knowing the amounts and knowing the locations and the percentages of this gas compared to a galaxy will tell astronomers how long galaxies can go how fast they’re going to be forming stars, all kinds of things.

Pamela: We don’t get to all of that information just from the ultraviolet but at least it helps us figure out. One of the questions with dark matter is how much of it is just stuff that’s cold?

This allows us to find where all the pockets of cold normal matter and when we find them they don’t add up to the missing dark matter.

Fraser: When we talk about how much of the universe is unknown we talk about the 20-something percent dark matter and the 70-something percent dark energy and of the 4 percent of matter, half of that is missing as well.

[Laughter] So it is regular matter, we just can’t see it. That could be this cold hydrogen.

Pamela: As I said before you use ultraviolet to very carefully measure temperatures, to get at chemical abundances. It allows us to get at things like why is the Veil nebula such a pretty purple color? It is because of the oxygen in it.

We’re able to get at different information about the compositions of nebula, the compositions of stars only because we have access to the ultraviolet.

Fraser: I think that one of the greatest space photos in my opinion is an ultraviolet image. I think it was taken by FUSE of the star Myra which has this huge long glowing tail almost like a comet.

FUSE built up this image of Myra. We’ll be able to link to that in the show notes. Was there anything else on ultraviolet that you thought should be brought up?

Pamela: Nope, it is a fairly dangerous wavelength and we’ve now hit the bad for life part of the electromagnetic spectrum.

Fraser: Yes, so next up x-rays.

Pamela: Yeah, welcome back to death and destruction.

Fraser: Sweet, that’s what we like. We’ll talk to you for the next show.

Hubble Deep Field.

As astronomers discovered that we live in a great big universe, they considered a fundamental question: is the universe the same everywhere? Imagine if gravity was stronger billions of light years away… Or in the past. It sounds like a simple question, but the answer has been tricky to unravel.

Ep. 123: Homogeneity

Homogeneity Show Notes

• Chris Lintott
• Chris's page on Wiki
  
• Galaxy Zoo
• The Cosmological Principal – Wiki
• Abstract:  A measurement of large-scale peculiar velocities of clusters of galaxies: results and cosmological implications — Kashlinsky, A., et al. (2008)
• Why do we assume that the Universe is homogeneic and isotropic for all observers? — Cornell U.
• Homogeneic and Isotropic -- Universe Adventure
• Presentation:  Dark Energy and the Homogeneous Universe — Columbia University
• Galaxy Zoo Results Show the Universe Isn't Lopsided – Universe Today
• "In the Eye of the Beholder" — testing the anti-clockwise bias – Galaxy Zoo Blog
• Intro to the Cosmic Microwave Background – U of Chicago
• WMAP
• Inflation Theory – WMAP site
• Inflation for Beginners – from Cosmology for Beginners
• Sloan Digital Sky Survey
• "Swiss Cheese Universe Challenges Dark Energy" – New Scientist
• What Can Swiss Cheese Teach us About Dark Energy? – NASA
• Smoothness of the CMB -- NASA
• The Lumpy Universe – NASA
• Presentation:  Observations Through a Lumpy Universe – Columbia University
• Astronomers Detect 'Dark Flow' of Matter from Beyond the Visible Universe – Universe Today
• Astronomy Cast episode on Time (117)
• Cesium Fountain Atomic Clock
• Was the speed of light different in the early Universe? — Wiki
• Cosmic Dark Ages — NASA
• The Dark Energy Illusion — Intro article and links to paper by Timothy Clifton, Pedro Ferreira, and Kate Land
• Who Ordered the Dark Energy and Dark Matter? -- Pamela and Fraser's 365 Days of Astronomy podcast
• Are We in the Middle of a Cosmic Void? – Universe Today
• Large Synoptic Survey Telescope

Books:

• The Isotropic Universe by Derek J. Raine
• The Inflationary Universe by Alan Guth
• The Unknown Universe by Richard Hammond

Download the transcript

Transcript: Homogeneity

Dr. Pamela Gay: With me this week while Fraser is on a well-deserved vacation is Dr. Chris Lintott of Oxford Astrophysics. Good afternoon Chris.

Dr. Chris Lintott: Hi, there. How are you?

Pamela: I’m doing well. We survived! We’ve been down here in Long Beach, California for the 213^th meeting of the American Astronomical Society and the launch of the International Year of Astronomy.

Dr. Lintott: Well congratulations you helped to launch the International Year of Astronomy in the U.S. I noticed. The rest of us will catch up next week when that’s the International launch. [Laughter] But it was great fun. We had a movie premier which was excellent – free beer and all sorts of good things.

Pamela: All courtesy of Interstellar Studios and it was the launch of 400 Years of the Telescope. We had Galileo Beer from Sierra Nevada.

Dr. Lintott: And the Second Life Island opened.

Pamela: The Second Life Island opened. We had George Hrab from the Geologic Podcast who does the theme music for 365 Days of Astronomy. Which you all should go and subscribe to at iTunes.

Dr. Lintott: Which I do an episode for so..

Pamela: Yes you do and we already have some AstronomyCast episodes in there. So yeah, we got through. Fraser’s now on vacation and you and I are here to make sure that AstronomyCast keeps going through all these different chaotic wonderful things.

This week we’re going to talk about the Cosmological Principle. As astronomers discovered that we live in a great big Universe they considered a fundamental question. Is the Universe the same everywhere?

Imagine if gravity was stronger billions of light years away, or in the past. It sounds like a simple question but the answer has been tricky to unravel. The answer kind of starts with this crazy thing called the Cosmological Principle.

Dr. Lintott: So now you just want me to explain it. [Laughter] I love the way that you make it sound like it’s got the capital letters it deserves. Before we tell you what it is, let me explain why it is important.

The Cosmological Principle isn’t something we’ve discovered about the Universe. This isn’t like the age of the Universe or the process by which galaxies form or even the fact that most of the matter is in the form of this mysterious dark matter. Those are facts that we’ve established.

The Cosmological Principle is an assumption that we must make in order to say anything about the Universe as a whole. What we have to do is we have to assume that the bit of the Universe we can see and get information from is typical of the whole because if that’s true then we can make local measurements. We can do the equivalent of a table top experiment on our bit of the Universe.

We can measure the density and say this must be the density of the whole Universe. We can look at the conditions around us and say these must be the conditions in the whole Universe. That enables us to do any cosmology.

We can say things about the Universe as a whole. Not just our little bit of the Universe looks like it started 13.7 billion years ago but the whole Universe began 13.7 billion years ago. For these grandiose claims you need to make this assumption. You need to say the Universe is big and we live in a typical small part of it.

Pamela: And so having reached this mediocre state of being nothing special, nowhere special and perhaps as we’ll talk more on this show, in no when special we move on and we look at two big concepts.

The idea that the Universe is homogenaic and that the Universe is isotropic and with isotropic it’s your work with Galaxy Zoo that’s actually helped look at evidentially how can we say isotropic is real?

Dr. Lintott: In the small way. Another way of stating the Cosmological Principle, keep in the back of your head during this episode, all it is, is that we don’t have anywhere special. We have to think about it in technical ways if we want to test it.

One way to say it technically is the Universe is both homogeneous on large scales, and isotropic. Now what isotropic means we take that first is that it looks the same in all directions. So if I look right into the distant Universe I will see a Universe that looks much like the one I see if I look left. Same if I look up and look down, that way it doesn’t matter which direction I choose to look in.

The Universe is the same on large scales. So we’d like to test that. One way to do that is to look at galaxies. You can set up a thought experiment. Let’s look out of the window in the hotel room here. Imagine every galaxy in that direction was rotating clockwise, all in the same direction.

Now imagine an alien astronomycast presenter [Laughter] well, another alien astronomycast presenter, looking back from the other side at those galaxies. We see them clockwise, he, she or it would see them moving anti-clockwise. So, there you have a measurement that depends on which direction you’re looking. In that Universe isotropy is broken.

So we thought with Galaxy Zoo inspired by the work of Michael Longo we should double check this wasn’t the case and to our horror, we found as most of your listeners know by now, more galaxies appear to be going anti-clockwise than clockwise.

The reason this result was so disturbing and controversial and why I had great fun stirring up cosmologists [Laughter] with this is if that had been true then in some sense the Cosmological Principle is broken and you wouldn’t be able to say anything at all about the large-scale Universe.

Pamela: Now what it turns out is, it’s more the human mind is a little bit broken.

Dr. Lintott: Sure, we have a bias we think and that means that actually when we do the test properly, looking at mirror images and so on – on galaxyzoo.org – in case any of your listeners haven’t come and helped us out, then we find consistency with Isoptropy.

Astronomers aren’t satisfied with that they weren’t hanging around waiting for us. What you want to do is test these things on the largest scale possible. So we look at our largest scale observation of the Universe which is the Microwave Background. This is as you know radiation emitted 300,000 years or so after the Big Bang from the point where light suddenly was able to travel across the whole Universe.

Pamela: We have an entire AstronomyCast episode about this so go check it out if you want to learn more about the Cosmic Microwave Background.

Dr. Lintott: And if you’re still here, [Laughter] then all you need to know for the CMB at this point is that it is a picture of the Universe – part of the Universe as it was 300,000 years after the beginning. In fact, we find it works. If you look to the right then look to the left, if you look up and you look down you see a Universe that looks very similar.

For example you can characterize this Universe by temperature. The Universe is the same temperature at that time to better than one part in 10,000 wherever you look. It is incredibly smooth. Actually it is so smooth that it causes a problem.

Pamela: This is actually quite troubling because we’re just starting to see the light – by definition – that’s traveling from the left side and we’re just starting to receive the light that’s traveling from the right side.

Dr. Lintott: So they haven’t had time to cross or to communicate with each other.

Pamela: How do they know what temperature to be? How do they know what distribution of hot and cold spots they should have?

Dr. Lintott: The famous way of looking at this is even if there are monsters in each part of the Universe with heaters and fridges whose job it is to try and match the temperature with their other monster colleagues, there’s no way they could do so. They can’t talk to each other so they can’t coordinate their activities.

Pamela: These parts of the Universe are accelerating themselves apart with the Universe’s expansion.

Dr. Lintott: So they will never talk to each other. The monsters will be left alone [Laughter] with a fridge and a heater and presumably some sort of electricity supply.

It’s a fundamental problem called the Cosmic Conspiracy. One way out of it – monsters aside – is to just say there’s something about the Big Bang that produces smooth Universes.

Pamela: This is where we invoke the idea of inflation.

Dr. Lintott: Sure, inflation is the idea that, take your standard Big Bang, something happened, the Universe has been expanding, and just allow me to tweak it slightly so that in the first tiny fraction of a second there was an incredibly rapid expansion.

Pamela: After those two pieces of the Universe talk to each other?

Dr. Lintott: Yes because then you can have a much smaller Universe to begin with. The monsters can communicate. Light can travel from one side to the other. Even if there were random fluctuations in temperature, then heat will flow from the hot region to the cold region and things will reach equilibrium and then you expand and then you have the Universe we see today.

So, inflation is another route out of this. But we’ve digressed a long way from the Cosmological Principle. The point was we’re halfway there. We have an Isotropic Universe as far as we can see.

Pamela: Then the next thing we have to start worrying about is the idea of homogeneity. Here side scale starts to become very important. You look around our own planet Earth and compare it to Mars – very different worlds.

Look around our solar system. Our solar system isn’t like other solar systems that we’ve found so far. There are probably some out there like us somewhere.

Look at our galaxy. Our galaxy is part of a local group that is a bit different from the stuff beside it. So, on these small scales everything is a little bit different.

Dr. Lintott: To make it even more simple than that, if we want to, we don’t really mind whether it is a planet or galaxy or whatever. Take a box, let’s say a meter cubed. Let’s put that box down somewhere in the Universe and ask how much matter is there within it.

Obviously I’m going to get a different answer if I put it down in the middle of the Earth [Laughter] it will have much more mass than if I put it down a hundred miles up above my head where there will be very little. Yet there will still be more there than if I put it a billion light years away, not in the middle of any galaxy. The odds are there will be nothing at all.

So you see the density on that scale changes. It matters where in the Universe I put the box. If I have a much larger box, a really large one, say a billion light years on the side, then wherever I put that in the Universe the assumption we’re making is that there’ll be roughly the same amount within it. The Universe is homogeneous on that scale.

If you want a local analogy, take cake. It’s always good to take cake. [Laughter] Imagine a sponge cake with raisins in it. It matters if you have a pin prick you put into the cake. It will matter whether you go into the cake or you go into a raisin. So on those scales, the scale of a pin head; you get very different answers to what the cake is as you prod your pin in and out.

But if you look at a box, if you take a slice of cake it doesn’t matter where that slice comes from, as long as you’re not picky about icing on the top [Laughter] or whatever, because you’ll always get raisins and cake in roughly the same constituency. So you want a big enough box that you care about raisins and cake and not about individual things.

Pamela: We see this just within subdivisions here in the United States.

Dr. Lintott: What’s a subdivision?

Pamela: Well, this is where for whatever reasons developers decide everyone is going to live in very similar houses with very similar yards.

Dr. Lintott: In English we say estate, but still.

Pamela: So I’m sitting in my plain Jane back yard on my plain Jane yard and the grass around me is a bit different than the back patio I can see. But, if I expand my box to contain yard and patio I can start to average out. It’s still lumpy, bumpy not very smooth.

If I instead expand my circle out and I take in my house, my yard, my neighbor’s houses and yards, there is still variety. There is always that person with the pool. There is always the person who decided to build an addition over the garage.

Dr. Lintott: But if you take a big enough section there is probably always two pools scattered among them. They’ll be in different in places, attached to different houses but the density of the pool would be the same.

Pamela: Anyone who has tried to find someone living in a subdivision knows that really, everything looks the same. Everywhere, once you’re trapped within all the winding roads.

Dr. Lintott: Sure, so what this boils down to is you’ve got take a big enough box for our Universe to be homogeneous. We need it to be so so that the Cosmological Principle applies and cosmologists can be happy.

Have we had a big enough box to test it? I think the answer is just about no. Obviously our galaxy scale box won’t do. Obviously the size of the local group won’t do. You must have something bigger than the biggest galaxy clusters.

The biggest view of the Universe we’ve got is the Sloan Digital Sky Survey, a picture of a million galaxies. Even on the scale of the Sloan survey, it’s not quite big enough to see homogeneous.

There are very large fluctuations across the Sloan, some very large scale features which may or may not be important. So if we go a bit bigger than that we believe that we’re dealing with a homogeneous Universe. But we’re not sure.

Pamela: We live basically in a Swiss cheese Universe. The problem is well we all know Swiss cheese has holes and well cheesy bits of various different sizes.

One of the problems we also have in trying to understand these issues of scale is the Universe’s structure is evolving over time and we have to correct for that in some ways.

Dr. Lintott: Yes, once you get to very big. With Sloan we’re just about okay. Once we get to the big scales we have to worry about changes with time. So think about it. We talked about the Microwave Background, this very smooth but not quite smooth, tiny fluctuation, one part in 10,000 at early Universe.

And yet we look around us today and we have this galaxy cluster or not galaxy cluster, very lumpy Universe. Both are homogeneous as far as we can tell. But the scale of the fluctuations has changed. That’s entirely due to the beautiful work of gravity which takes the small fluctuations that existed at the beginning and exaggerates them.

You mentioned being in a region in the Microwave Background Universe, 300,000 years after the Big Bang. That has more matter than its surroundings. It will have slightly stronger gravitational pulls so material will tend on average to flow into it and away from the less dense regions.

So the rich get richer, the poor get poorer. These differences exaggerate every time you go from the smooth Universe of the CMB to the fluctuations we see today. It’s a great triumph of modern cosmology that that calculation works out correctly.

But if you start with the observed Microwave Background, apply what we know about gravity and this other force, dark energy that I’m sure we’ll get on to at some point, you produce something that looks much like the web of galaxies that we see today.

Pamela: What’s really neat is there have been some recent studies where they’re trying to figure out the bulk motions of one group of galaxies relative to another.

You start adding up the forces you get from this cluster over here, that cluster over there and you actually have to take into account that lack of gravity coming from these voids in space.

Dr. Lintott: Sure the regions that aren’t pulling you quite as closely as the galaxy cluster on the other side. We actually see some of these motions on very large scales. The work is slightly controversial. There were some press releases and papers out a few months ago.

There’s a suggestion that cross motion of the Sloan is a net motion in one direction. If that’s true, that means we haven’t yet reached the scale on which things are homogeneous. Just outside our field of vision there must be a really big compression of matter pulling us in that direction.

Pamela: One of the things that we have to deal with as we look out and we try and decipher the evolution of the Universe from the not quite but mostly smooth Cosmic Microwave Background to today’s Swiss cheese Universe is we’re assuming a model that has a given amount of mass, a given amount of energy and gravity affecting things in a consistent way.

Dr. Lintott: Ah, okay now this is the hard question, right? So far we’ve dealt with Cosmological Principle as if it depends on space. We’ve talked about this being a special place, but of course that’s not the only problem.

Imagine, let’s be archaeologists looking at the surface, or anthropologists looking at the Earth. You’d want to make the same assumption. If you’re going to take a small region of the Earth’s surface and study the people there you want to know that that is typical in order to draw conclusions about the whole of human population.

However, you’d also want to know what time you were living in and see whether you were living in a special time. If you were in Rome 2000 years ago, you would draw very different conclusions from studying Rome today. For all I know the chariots driving habits were the same. [Laughter] You know what I mean. Time is very important. We have to be careful not to make the same mistake with the Universe.

In particular, we just assume that the laws of physics that we measure in the lab today are those that governed the Universe of 300,000 years after the Big Bang of three seconds after the Big Bang; of three micro-seconds after the Big Bang. And all the way back we assume physics is the same.

Pamela: One of the nice things is that even though we are making this rather radical assumption, we’re able to build mathematically consistent models that allow us to in computers simulate what we’re actually observing.

Dr. Lintott: Right so that’s one way we can be sure that things are working. We make the simple assumption that physics works and so far so good. But we can also test it directly.

Let’s take something simple. Let’s assume that all of physics is the same. Einstein’s still right wherever you are in the Universe and quantum mechanics does whatever quantum mechanics does and all the rest. Let’s change one thing. Let’s change the speed of light.

Pamela: Now, the speed of light is one of those numbers that crops up all over physics. It’s sort of like Pi and Fe but the difference is it actually has meaning. It describes the rate at which photons travel through space.

Dr. Lintott: It has such meaning. It’s the fundamental definition of the second. But what it means to count time depends on the speed of light. We count oscillations in cesium. Is that correct?

Pamela: It’s cesium it’s the resonance.

Dr. Lintott: I’ve got a feeling they may have changed it do a different more…

Pamela: No, it’s still cesium. We did an episode on this recently so if you want to know more about time, go back three or four episodes.

Dr. Lintott: I hope I’m getting paid for every [Laughter] astronomycast episode that I’ve…

Pamela: I’m not getting paid.

Dr. Lintott: Anyway, the point is so the speed of light is something we can physically measure. It’s at the heart of the Theory of Relativity.

Everyone seems to know. They only know two things about Einstein, they know E=mc² probably a topic for a show you’ve already done.

Pamela: Yes.

Dr. Lintott: There you go. And they know that the speed of light is a constant, at least in a vacuum. The true statement is in a vacuum. So, given that everyone knows this, can we test it in the early Universe?

The answer amazingly is yes. We can measure the speed of light not just here but we can measure the speed of light a billion years after the Big Bang.

Pamela: This is because of the fine-structure constant which is one of those quantities that helps describe in quantum mechanics the energy level transitions that cause light to be released at very specific wavelengths in different atoms where each element has its own fingerprint of allowed colors of light that it emits.

Dr. Lintott: So when you look at a street light that distinguishing yellow glow is because sodium always emits in the yellow for example. When we look at distant galaxies we see loads and loads of these lines from all the different elements that are there.

It’s how we know what galaxies are made of. You can take one particular element like hydrogen for example and you see literally hundreds of lines in the spectrum of the galaxy from just hydrogen. Now the gaps between those lines depends on this thing called the fine-structure constant. It doesn’t matter what it is, one over 137 or so in appropriate units.

It’s nicely [Laughter] memorable. But that depends on the speed of light. So if the speed of light changed when we looked at distant galaxies we should see this ladder of lines due to hydrogen changing and we don’t. We see them in exactly the same ratio as they should be to better than one part in a hundred. There’s no evidence for any change whatsoever.

Pamela: That better than one part in a hundred is an error bar. Not saying that it’s changing within that.

Dr. Lintott: No, it’s consistent to zero.

Pamela: Yes as near as we can tell the speed of light is the speed of light is the speed of light. As we build progressively larger and larger telescopes we’re looking back to the very first galaxies that have ever formed. With those first lights from elements being emitted you see this consistent fingerprinting of the elements.

Dr. Lintott: We do but you’ve still got a problem because that only takes us back to alright I’ll concede the earliest galaxies were, so first galaxies maybe half a billion years? Something like that after the Big Bang?

So let’s say we can repeat this experiment from that. We’re not far off being able to do that and yet I’m sure the speed of light would be consistent with being a constant back then.

That’s not enough because lots of the things we’d like to explain happened before that. For example we talked earlier about the smoothness of the early Universe, the isoptropy of it the fact that temperature was the same on one side of the sky as the other.

You, I think or one of us brought up the idea of inflation as a solution to this, this rapid expansion. Which works, it solves this problem and a few others. I always describe it matches this as trading several problems for one crazy idea because that’s what it is.

There is no physical reason to believe in inflation. There’s no theory that tells us that the Universe should suddenly it sped up its like expansion.

Pamela: It’s just mathematically works.

Dr. Lintott: Sort of, it’s not even that but it’s if you allow this to happen by forcing the equations to do it, then you’ve solved lots of your problems. So, physicists are alert for other solutions which might be more elegant and solve the same thing.

One such set of theories is called variable speed of light theory or VSL. It’s been worked on and off for the last 20-30 years by a whole variety of people, particularly by João Magueijo at the Imperial College in London who wrote a book on the subject. The book is called “Faster than the Speed of Light” and it’s possibly the rudest cosmology book [Laughter] I’ve ever read.

It’s worth it for entertainment value alone, you should read the book. His idea is that if the speed of light changed dramatically in those early days before we can measure it, then you can get away with inflation. You can allow different parts of the Universe to talk to each other. You can see how this might happen.

If you speed up the speed of light then the seemingly disparate parts of the Universe can communicate very quickly. It’s like having high speed broadband connection [Laughter] and instead of resorting to dial-up. They can reach the same temperature and then the Universe can go on with its merry way with the speed of light settling down to normal. It’s a beautiful, probably wrong idea.

But it at least seems to fit within the way we’d like scientific discovery to go. Newton wasn’t wrong. You can fly to the moon with Mr. Newton. He was just inaccurate at high speeds or near large masses. So maybe, the suggestion goes, Einstein wasn’t wrong. The speed of light is constant except in the early Universe.

Pamela: This always gets us to the question of testability. The problem is, right now we have fingerprints going back to the formation of the first galaxies. When we try and look back before that, we can’t. It’s the cosmic dark ages when we’re in a Universe where there’s just nothing hanging around emitting new light.

Dr. Lintott: Actually there’s a lot more work to be done on the theory. So, who knows? It may be the theory will make predictions as to how things will change later in the Universe. At the minute it’s still not much more than a fun idea. It’s a toy idea. I don’t mean that as derogatory as it sounded. It’s a description of “let’s break this and see what happens” physicists’ model.

Let me give you another example though. We’ve been playing with breaking the Cosmological Principle in the early Universe. There are reasons to want it to be broken nearby as well. A few radical cosmologists are beginning to advocate. Lots of people have been involved in this.

There was some recent work that got me thinking from my next door neighbor in Oxford, Tim Clifton who has the office next to me and Pedro Ferreira and Kate Land from Galaxy Zoo. They’ve been thinking about how to get rid of this pesky dark energy. Now you must have an episode on dark energy.

Pamela: I think it was the last one we recorded.

Dr. Lintott: There you go. Dark energy is an accelerating force that is apparently driving the expansion of the Universe onward and onward and onward faster and faster. No one ordered it. No one wanted it in their model of the Universe. It doesn’t make any sense and we don’t understand it. We just see its effect.

Most cosmological effort is going into trying to explain it, to pin down its properties and explain what this accelerating force is. Or, you can allow me to do the following thing, saying Tim, Pedro and Kate. Allow us to be somewhere special.

In fact, allow us to be sitting not in a perfectly typical region of the Universe but near the center of a large void. A region of space that has less stuff in it than the surroundings, than the average and that gets rid of dark energy it turns out for a very simple reason.

When we talk about the expansion history of the Universe – accelerations and decelerations and gravity and dark energy – what we’re really talking about is a local measurement at the rate of expansion. We measure that here and we get a certain value.

Pamela: And this is after we’ve corrected for all the local motion.

Dr. Lintott: Yeah and all that astrophysics nonsense [Laughter] as well and understood the stars we’re looking at and so on. That’s just detail.

Imagine being in the center of the void and we look out further and further and we make this measurement by looking at supernovae or Cepheid variables or whatever your standard candle of choice is.

Pamela: And we have a show on this.

Dr. Lintott: Sure, excellent [Laughter] you’re good. Your competition for this week is to string all the AstronomyCast episodes into one sentence and [Laughter] send it to me.

The point was, as you look further and further away and you’re leaving the void so you’re measuring the expansion in a Universe that appears to be getting denser, more gravity, and greater pull back on the expansion so there’s slower expansion. So as we look out of the void the expansion slows down.

That’s the same as looking in the past so we interpret that as an acceleration. Who knows if that’s true? The scale of the void that you need turns out to be very deep. We need to be about two-thirds of the average density of the whole Universe. That’s quite a large discrepancy.

Pamela: But we know that there are huge variations in density across the Universe.

Dr. Lintott: We don’t see anything on that scale on what we can see. So then the question is what scale must this point be? You’ll be amused to know, no doubt, the prediction is that it should be slightly larger than our existing surveys can detect.

In other words if we had something that went twice as deep as the Sloan Digital Sky Survey does then we’d see the edge of the void and we’d see this increase in density. So, it’s a prediction.

If this exists, if we happen to be living not just in a point but near the center of it – we need to be at the center of it – otherwise we’d see the difference if we looked in different directions. But we know we don’t do that. If we live in such a world, we can get rid of dark energy and we can all react.

It will take the advent of the next generation of sky surveys, particularly the Large Synoptic Survey Telescope, in let’s hope 2015 [Laughter] no let’s hope for 2015.

Pamela: I know.

Dr. Lintott: Let’s hope for 2015 to do that. At least it is a prediction. For now we’ll cling to the Cosmological Principle.

Pamela: So we have a theory, we have a test, we have a telescope on the way and when everything goes live and when the science results are back, we’re going to have to have you back on the show Chris. It’s been a pleasure.