Show Notes

• Multiverses – AstroScience
• If We Live in a Mulitverse, How Many are There? — Universe Today
• Does the Multiverse Provide a Theory of Everything? – Suite 101
• Video with Dr. Michio Kaku on the Multiverse
• Parallel Universe — Universe Today
• Thinking About Time Before the Big Bang — Universe Today
• Videos on String Theory and Multiple Universes with physicists  Brian Cox and Leonard Suskind
• Anthropic Principle — Wiki
• String Theory -- The Official String Theory Website
• M-Theory — Wiki
• M-Theory — Caltech
• P-Brane — Solar Physics
• D-Brane – Caltech
• M-Theory and the Multiverse – Discussion on Physics Forums
• Multiverse Explorations — Discussion on the BAUT Forum
• Visual Quantum Mechanics (intro to quantum physics) — KSU
• Quantum Mechanics – Wiki
• Quantum Foam – Astronomy Cafe
• Webmage books by Kelly McCullough

Transcript

Coming Soon!

Shownotes

• 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

Transcript: Large Scale Structure of the Universe

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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.

Shownotes

• 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

Transcript: X-Ray Astronomy

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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.