Ep. 295: The Observable Universe

We understand our place in the Universe because of our direct observations. We can see the light that travelled billions of light years across space to reach us. This sphere of space is the observable universe; everything we can detect. But it’s really just a fraction of the larger, unobservable universe. Today, we’ll talk about both.

Transcript: The Observable Universe

Fraser: Astronomy Cast episode 295 for Monday, February 25, 2013 – The Observable Universe
Welcome to Astronomy Cast, our weekly facts based journey through the cosmos. Where we help you understand not only what we know, but how we know what we know.
My name is Fraser Cain, I’m the publisher of Universe Today. With me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville and the director of CosmoQuest.
Hi Pamela, how are you doing?
Pamela: I’m doing well how are you doing Fraser?
Fraser: Good. Do we have any announcements or things to talk about?
Pamela: We do. We are continuing to advertise for the classes that are going to be starting soon through CosmoQuest. You can get them by going to cosmoquest.org/classes and we currently have two upcoming classes. One of them is on the sun, stars and stellar evolution and is taught by Ray Sanders who goes by DearAstronomer on Twitter. The other class is cosmology by Dr M.R. Francis Matthew. He shows up as Dr Mr Francis on Twitter which amuses me to no end.
Fraser: (Laughs)
Pamela: Both of these classes are starting shortly in April so you can go to cosmoquest.org/classes, check out the dates and times and hopefully
you’ll sign up through vent bright. Nicole Gugliucci will both be making random appearances in these classes and contributing where we can. This is one of those ways that we help employ people who are into astronomy so we do ask you to please pay. This is part of how these two individuals make their living. Sorry that we can’t do everything for free, I know we would if we could. We’re hoping that you’ll sign up. Each is eight hours long and jam packed with information and classes are limited to eight people each.
Fraser: Okay, well lets get on with the show.
Fraser: We understand our place in the Universe because of or direct observations. We can see the light that traveled billions of light years across space to reach us. This sphere of space is the observable universe; everything we can detect. But it’s really just a fraction of the larger, unobservable universe. Today, we’ll talk about both. When we talk about the observable universe, what is it?
Pamela: We often say the universe because we’re lazy.
Fraser: We need to make that distinction right? The universe is the observable and the unobservable theoretical universe.
Pamela: Yes
Fraser: So lets start then with the universe then we can further define our terms. What is the universe?
Pamela: The universe is the entirety of the space time continuum which we occupy that is some sort of unknown geometry that many of us, myself included, think is an unbound finite size which is how you define the surface of a sphere. So you can start anywhere you want on the surface of a sphere and as long as you stay on that surface, you’re never going to hit an edge. That is currently what we believe.
Fraser: I think we all imagine it as a sphere right?
Pamela: A more accurate way of looking at it: we know parallel lines stay parallel to one another so what you have is a doughnut. So you can start anywhere you want on the surface of a doughnut and you’ll never hit a boundary there either.
Fraser: So the universe is a doughnut?
Pamela: It’s technically more like a four dimensional hypertoroid but doughnut works.
Fraser: (Laughs) Wait a second, so we went from the universe being a circle, no the universe is a sphere, no the universe is a doughnut… no actually the universe is a four dimensional hypertuh… this is not helping.
Pamela: We’re just going to stick with Homers doughnut for the rest of this episode and just realize the actual universe uses more syllables and it has a higher scrabble value.
Fraser: (Laughs) Higher scrabble value… we’re a little silly today. The issue being is with a Taurus you get parallel lines moving in any direction. You can move around the ring of the doughnut and your lines stay parallel. You can move around the doughy goodness of the doughnut and your rings stay parallel in both directions so the universe acts like a Taurus
Pamela: This isn’t actually doughnut but we can pretend because it has the side of a doughnut so you can imagine as things race around, the two lines stay parallel to one another. As they race around the top they stay parallel to one another. If this were a doughnut with a better hole in the center you could have lines going through the center.
Fraser: Which leads to the question: Where’s the hole?
Pamela: That’s the thing; if you’re confined to the surface then you can’t figure that out. We’re trapped on the four dimensional version of this doughnut.
Fraser: So imagine a doughnut with a bunch of galaxies inside of it.
Pamela: On the surface of it.
Fraser: Only on the surface of it, not on the inside of it and there is no hole… that’s the whole universe? As we did in the episode “What is the Universe Expanding Into” the universe is everything that there is. If you wonder what’s outside the universe well that’s a thing, that’s part of the universe which makes it still inside the universe. There is no…
Pamela: Science can’t get there from here.
Fraser: Right. Anything you can include that you might think “Well maybe the universe is expanding into that” but that thing is still a part of the universe.
Pamela: If you want to discuss what’s outside of the realm of the universe you have three directions that you can go. Philosophy, religion, or multiverse. Pick one but none of them have scientific evidence at the moment.
Fraser: Right and the fact that if it is, it’s part of the universe. The universe is everything and that way if it’s something it’s part of everything.
Pamela: This is where the space-time continuum part comes in. Our universe has its own specific timey-wimey-ness. It has it’s own cosmological constant, it has it’s own value for the hyperfine structure, it has it’s own value for the gravitational constant and if our universe is one of a multitude of multiverses, some of those other universes that are outside of our universe, according to this not scientifically testable concept, they could each have their own timey-wimey-ness. They could each have their own value…
Fraser: Don’t forget the wibbly-wobbily.
Pamela: Their own wibbly-wobbily..
Fraser: As we understand it this universe came into experience with the Big Bang and has been expanding for 13.8 billion years and now is of a certain size.
Pamela: Yes
Fraser: So that is the “The Universe” so as a subset of that universe is the observable universe, what is that?
Pamela: The observable universe is everything that we can see until we hit what is called the surface of last scattering. This is the cosmic microwave background. There is universe beyond that but we can’t see it because of this opaque wall of microwave background radiation.
Fraser: Is that also a doughnut? It’s a sphere right?
Pamela: Here’s where it gets confusing. We’re attached to the surface of this four dimensional hyper-doughnut thing. Our observable universe is a sphere within that surface because it’s four dimensional. We’re a three dimensional sphere wrapped to the surface of a four dimensional object. I didn’t say I wasn’t going to hurt you with geometry.
Fraser: I love that I was even able to get part of the way there; I feel really proud of myself that I would be able to think of it as a three dimensional sphere. I just went one dimension too high.
Pamela: (Laughing)
Fraser: So you have this three dimensional sphere wrapped to this four dimensional… but as we understand it we look in all directions out from the earth and see in 13.8 billion light years in every direction.
Pamela: Minus 400,000.
Fraser: Planks new number 13.8.
Pamela: Right, minus 400,000 for
Fraser: For the scattering?
Pamela: Yes
Fraser: Okay so can you explain what is that first scattering.
Pamela: So what happened was that our universe formed and everything was compacted down to a small point and then that small point expanded into this doughnut. There is no center to this. Suck all of the air out of a doughnut to a microscopic point and then blow air into the doughnut. There is no center to the surface of that doughnut at any point in time. Never a center.
Fraser: Not the center of the doughnut, the center of the surface of the doughnut.
Pamela: Yes, and at no point was there a center to that surface. Initially everything was so compacted that it was pure energy and as it expanded out that expansion caused it to cool and caused it to lose density. Eventually we were able to have protons, electrons and neutrons start to form… not necessarily in that order. As these things formed they stayed in this constant scattering with the photons and everything is constantly interacting and because of this constant interaction we say that it was all equilibrium together. Another way to think of it is a photon as it went couldn’t go far enough to actually escape, it was just constantly getting absorbed and re emitted in all directions so it was also opaque. Then at a given special moment, everything reached a low enough density that a photon as it tried to travel didn’t necessarily get reabsorbed and that meant it was able to fly free. That moment when the photons are able to fly free from the point where I am, the point you are and from the point we observe, 13.8 minus 400,000 years ago, we’re able to see that shell of objects, photons, that were released at that moment when the universe suddenly was big enough that they could fly free without getting reabsorbed and that’s just a shell. At that time that corresponds to that distance, because light travels at a finite speed, all those photons have finally just now had time to reach us, so we see them.
Fraser: I think that’s just again mind-bending that in every direction you look you are seeing 400,000 years after the beginning of the universe. That’s the wall so wherever you look out in space, yes you are looking at essentially the beginning of the universe. It’s crazy.
Pamela: What’s cool is no matter where you go in our universe, when you look out you will see a different sphere but it’s always coming from that same time and it’s always the same size sphere around you. It’s nothing more than a light travel time distance. The physics is different and conceptually it’s very similar and I’ve used this analogy before to having a globe of light underwater and you can only see this one sphere around you if you’re in very deep lightless water. As you swim around you carry your sphere of illumination with you.
Fraser: If you were see a fish and you were to move over to that fish, you might be centered with that fish but you’ll still be seeing this sphere of water illuminated around you. It’s an amazing idea but it gets more weird because if I was able to travel out to the edge of the observable universe instantly I would now be there but it would now be present day for that position and then I would be seeing the universe again here as the beginning of the universe. It’s that extra time machine part that I think
Pamela: It makes you wish you could be Q or Dr Who
Fraser: It’s that part that you’re looking out and looking back at time at the same time that makes it a little extra a bit of a head-scratch. Also, and I think this is part of why we’re talking about this show is it gives us this insight into the universe without the time machine aspect of this observation. We wouldn’t know anything about the true nature of the universe because we’d be seeing everything now.
Pamela: One of the things that bothers a lot of the scientists is that we live in a somewhat unique time when the universe has been expanding long enough that life can exist… that’s useful. It hasn’t been expanding for so long though that the distant observable parts of the universe have been carried back away from us so that we can no longer see them again. We can see the cosmic microwave background and we can see the first galaxies and we can see even the first stars that were forming. In the future that is not going to be true and that’s kind of annoying.
Fraser: If we’re looking in this direction we’re looking 13.8 billion light years, if we’re looking in this other direction it’s 13.8 billion light years: How big is the observable universe?
Pamela: Well that all kind of depends on what you’re asking. This is where it gets really confusing because it turns out that the light that we’re seeing from the most observable objects… not the most observable, the most distant objects, that light has traveled from an object that was 7 billion light years away but it’s now 47 billion light years away.
Fraser: Okay so hold on a second let me see if I can get this straight. So when that object omitted that light, the light has traveled for 7 billion years from the light perspective. The photons are timeless and they don’t really understand time. If we could watch that photon moving along it would have moved for 7 billion years from it’s perspective yet the universe has been expanding so it will have traveled a total distance of the 13.8 billion light years from our perspective but actually if you could look at its current location now it has moved a total of 47 billion light years. I’m going to do some quick math here…
Pamela: So it’s moved from an object that was 7 billion light years away to us over 13.8 billion light years and the object is now 47 billion light years away.
Fraser: Right so then the size of the observable universe is 94 billion light years across.
Pamela: Yes. So here we are, light from the right has come from objects that we can’t see, but are now 47 billion light years away. To the right and to the left, add those two numbers together and we end up with the diameter of where things are now of 94 billion light years. The light we see was released when it was much closer.
Fraser: But that’s the observable universe. The question then is how big is the unobservable universe?
Pamela: We don’t know, this is one of those awful things. The one thing we do know is we’ve searched the sky for those moments where the light that is released going in two different directions; it goes forward and it goes back. In theory you should see the exact same thing if the universe is small enough, coming at you from two sides. This is the idea of if I shine a laser beam out my face and as it zips around this doughnut shaped universe, given the fullness of time, it will come back and hit where my head was except I’ll kind of be dead and dust by then. The thing is though, when we look for these places where the light has been emitted in all directions, we should see the beam going forward and going back, coming together at two different parts of the sky but we don’t see that. That means from our best understanding of modern physics, our universe is so much larger than what we can see that what we can actually see is probably only a few percent of the greatness of the actual universe.
Fraser: We’ve talked about this in previous shows. The universe could be infinite or it could be finite but it’s just so big that we’re not seeing this curvature; we’re not seeing this mirroring of the light in our observations. I know that scientists working with the Plank mission recently released the most accurate map of the cosmic microwave radiation ever done. Much more sensitive than was done by W. Mapper before and so that must have pushed things back even further. If people maybe haven’t heard about this, what happened with the Plank mission?
Pamela: The Plank mission is our newest, most sensitive and most high resolution mission for observing this wall of microwave background radiation. Prior to it we had initially discovered the cosmic microwave from the ground but the light doesn’t get through our atmosphere that well so we couldn’t understand it very well. There were balloon based missions to try and look at it from high enough in the atmosphere. Eventually the Kobe satellite did an all-sky study that was followed up with the Wilkinson microwave anisotropy probe W map which allowed us to figure out that the geometry of space is flat. It allowed us to get our initial measurements that our universe is 13.7 plus or minus .2, billion years old. Along comes the newest mission the European space agency’s Plank and it continued to look at the sky and continued to help us better understand what the expansion rate is, how fast we accelerate in our expansion, and what was the mass density of our universe. Part of that was also looking for these places where you see the two different points on the sky reflecting the same point in space where light was released. We haven’t seen that but we do have a better age for the universe. Turns out that the age is determined using Plank data is within the error bars of the old measurement but it’s now a million years older than it used to be so the universe was apparently hiding it’s again.
Fraser: So we used to say 13.7 and now we get to say 13.8 billion light years. So if the observable universe is 94 billion light years but it’s only a couple percent of the entire universe itself, based on this Plank data, the math is now failing me but we’re looking at a universe that is hundreds of billions… trillions?
Pamela: Trillions is still too small of a number so if a few percent is 94 billion then you’re looking at tens and tens of trillions maybe even hundreds of trillions of light years across in this four dimensional crazy space. So these start to become numbers that we can’t even really as humans begin to understand.
Fraser: It always makes me sad you know? There is all of this universe that not only can we not discover and see but there is no possible way, that it is outside of the laws of physics that we could never reach this stuff right?
Pamela: Well things are going to get worse.
Fraser: Oh great.
Pamela: One things that frustrates me is that things that are 8 billion light years away are already moving away from us at the speed of light. At some point the amount of light that we’re able to see from other objects is going to get to the point- everything is running away at such a high velocity- that we’re pretty much only going to be able to see the galactic super cluster that we eventually fall into. It could be worse, we’re actually pretty lucky to see all of the large scale structure that we’re able to see today.
Fraser: Right so I think this is one of the last concepts that we wanted to tackle. When you bring time into the concept of the observable universe, every year that goes by the observable universe increases by one light years worth of size right?
Pamela: Yes
Fraser: And that’s good news because more of the universe is coming into view. However the bad news is that the expansion of the universe is accelerating and so in fact, objects at the very edges are now starting to drop off the cosmic horizon.
Pamela: What’s happening is if you take a mega parsec of space, every second this mega parsec of space is expanding by one kilometer so it’s a kilometer per second per mega parsec because the way we string together all those units. It’s not just one kilometer it’s actually 70 kilometers plus or minus 2.2. Every mega parsec of space, every second, is expanding by 70 plus or minus 2.2 kilometers. As you look at more and more mega parsecs, each one of those mega parsecs is growing at that same 70 plus or minus 2.2. You have two mega parsecs you have increased every second by 140 plus or minus 4.4. That adds up. This is one of those things we define by redshift because everything else we have to observationally figure out. Once something is at a redshift of 1.718, which we think currently corresponds to a light travel time, the time light has been traveling to get to us of 9.8 billion years. Everything at that distance and further away is being carried by the expansion of space at the speed of light or faster as you move further away.
Fraser: Wow and people are going to say “Hold on Pamela, nothing can travel faster than the speed of light”.
Pamela: It’s not that they’re not traveling faster than the speed of light, it’s that the space that they’re sitting on is getting expanded. So they’re sitting there on the moving walkway of space time and they’re getting carried with its expansion… that’s allowed.
Fraser: So as it comes back to the conversation of the observable universe you can imagine in the current times the observable universe is growing in that there are things to observe in the observable universe. We will reach this time in the distant future that although the observable universe will still be expanding of course, thanks to the speed of light, there will be nothing to see out there. Eventually it will start to shrink and so you’ll actually get this shrinking of the observable universe because the number of things that we can observe within the age of the universe has gone down.
Pamela: Yeah. One of the neat things to think about is every time we look at the cosmic microwave background, we’re actually seeing something slightly different because we’re seeing light that has traveled from a slightly different place. Because it was so consistent, because we’re looking at cosmological distances not human distances, we don’t actually see a movie of a changing background of irregularities in the microwave background. Again the fullness of time the cosmic microwave background will be seemed to change. I don’t think humanity is going to last that long but it’s just neat to think about how everything is changing and all we’re seeing is a snapshot of our universe.
Fraser: Lawrence Krauss actually came up with a really great… I don’t know if it was a paper or he just had written an essay about it. It talks about the future of cosmology and observational astronomy and how eventually a lot of the stuff that we now know in our current stage, like the age of the universe, is going to start to disappear and that knowledge will be lost to future astronomers because all of the stuff is just going to drop off the cosmic horizon.
Pamela: It’s only lost if we lose our documentation and we fail to spread to other worlds as our sun dies. Think of it more as it’s going to become the Dodo bird. It’s going to become something where we have data tapes… I doubt it will be data tapes in the future… we have recorded memory crystals that contain the Sloan digital sky survey and the data from the large synoptic survey and all the information is going to be gathered in the future. It’s like going into the natural history museum and seeing all of the bones of the dinosaurs. We can’t see the dinosaurs but at least we have data so it will still be a science.
Fraser: That’s a really interesting thought that we have maybe a few trillion years to get our astronomy in order and do the most detailed all-sky survey that we can possibly do before that data starts to drop off of the sky.
Pamela: It’s probably not even that long, it’s probably more in the order of tens of billions of years.
Fraser: Which kind of gets back to the fact that just like the sun and the moon are happened to appear as the same size in the sky so we get really cool eclipses that we happen to live at a neat time in cosmological history where this information is available that if we appeared any time later on, it wouldn’t be available to us.
Pamela: This is where there are so many good… Larry Susskind has written some excellent works on this. It’s the concept of the anthropic principle and the multiverse. There are very few ways to explain the fine tuning of our universe.
Fraser: Have we done a show on the anthropic principle?
Pamela: I have no idea, we have done 295…
Fraser: I should have these at the ready and go “Listen to episode on Anthropic Principle”. The gist being that we’re here to observe the universe so if the universe couldn’t support life then we wouldn’t be here to observe it. Because we can it does and it’s connected. Well cool thank you very much Pamela and we’ll talk to you next week.
Pamela: Sounds great Fraser, talk to you later.
This transcript is not an exact match to the audio file. It has been edited for clarity.

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