Ep. 11: A Universe of Dark Energy

Shownotes

• Discovery of Dark Energy: The hosts recount how observations in the late 1990s revealed that the Universe’s expansion is accelerating, a surprising finding that contradicted previous expectations of a decelerating expansion due to gravitational attraction.​

• Evidence Supporting Dark Energy:

  • Supernova Observations: Distant Type Ia supernovae appear dimmer than anticipated, suggesting they are farther away than their redshifts would indicate, implying an accelerated expansion.​
  • Cosmic Microwave Background (CMB): Measurements of the CMB provide insights into the Universe’s geometry and energy density, supporting the existence of dark energy.​
  • Large Scale Structure: The distribution and evolution of galaxies and galaxy clusters offer additional evidence for an accelerating Universe.​

• Theoretical Explanations:

  • Cosmological Constant (Λ): Einstein’s proposed constant represents a uniform energy density filling space homogeneously.​
  • Quintessence: A dynamic field with energy density that can change over time and space, differing from the static cosmological constant.​
  • Alternative Theories: Modifications to general relativity and other hypotheses attempt to explain the observed acceleration without invoking dark energy.​

Transcript

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Fraser Cain [00:01:06] Astronomycast. Episode 11 for Monday, November 20th, 2006. A universe of dark energy. Welcome to Astronomycast, our weekly facts -based journey through the cosmos. Each week we try to help you understand not only what we know, but how we know what we know. My name is Fraser Cain, and I’m the publisher of Universe Today. And with me is Dr. Pamela Gay, a professor at the University of Southern Illinois, Edwardsville. Hi Pamela. 

Pamela Gay [00:01:29] Hey Fraser, how are you doing? 

Fraser Cain [00:01:31] Very well. Okay, so I’ll get some of this stuff out of the way, and we hit our big topic this week. So remember, you can visit our website at Astronomycast .com. You can send us emails at info at Astronomycast .com. You can join the forum discussion. There’s a link on every show on our website. You can subscribe to this show by pointing your podcatching software at Astronomycast .com slash podcast dot XML. Or you can subscribe directly by going to iTunes. Just search for Astronomycast. Oh, and also some people have been complaining that Astronomycast and other podcasts have been crashing their iPods, so I think this has been happening with the latest version of iTunes. And apparently the fix is to turn off the equalizer setting for your iPod in iTunes. Although I guess if you aren’t able to hear, if it’s crashing your iPod, you can’t even hear to find out the fix. But anyway, if you are able to listen to this. 

Pamela Gay [00:02:21] If they’re listening on their computer because their iPod isn’t working. 

Fraser Cain [00:02:24] Yeah, exactly. That’s the fix. OK, so last Thursday, NASA announced new research from the Hubble Space Telescope. And according to astronomers, it appears that dark energy has been with us here in the universe as far back as nine billion years ago. So this week, we’re going to try and explain more about dark energy because we’ve got so many questions about dark energy. What it is and where it started. So, Pamela, you’re not going to like this, but this is going to be another show where I’m going, what is it? 

Pamela Gay [00:02:54] And it’s going to be another show where I’m going. We’re not quite sure. 

Fraser Cain [00:02:57] All right. What do we know? 

Pamela Gay [00:02:59] Well, what we do know is that when you look out at the universe, you find that there’s gravity trying to pull things together. But there’s this other mystery force, this mystery energy, this dark energy that is causing things to actually go apart from one another. So we have gravity pulling things together and we have this dark energy pushing things apart. And we’re not entirely sure what it is. 

Fraser Cain [00:03:24] OK. So how was it discovered then? 

Pamela Gay [00:03:26] Well, back in 1998, there were two different teams looking to find high redshift supernova. This was the Supernova Cosmology Project led by Saul Perlmutter and the high redshift supernova search team led by Brian Schmidt. And these two teams were trying to find out what the Hubble constant is at large distances. 

Fraser Cain [00:03:49] Sorry, what’s the Hubble constant? 

Pamela Gay [00:03:50] I was about to say, just that. It’s all right. It’s always good when you read my mind. The Hubble constant is an expression of how gravity is changing the rate at which our universe is expanding. So right now, the galaxies nearest that aren’t gravitationally bound to us are moving away from us at a rate of about 70 kilometers per second per megaparsec of distance between us and them. And that number changes as we go back to earlier and earlier times in the universe. And these two different teams were trying to measure the rate at which the universe is expanding at different periods by using supernova as trace particles floating in the expanding universe that we can measure how fast they’re going and assume that the rest of space around them is moving at the same rate. 

Fraser Cain [00:04:43] And this kind of goes back to our show last week where we talked about how to measure distance in the universe because, you know, if you can get a really good fix on the Hubble constant, then you can have a really nice yardstick for measuring how fast various galaxies are moving away from us and get a sense of their distance, right? 

Pamela Gay [00:05:00] That’s exactly what’s going on. And what they expected to find was that gravity was slowing down the expansion rate of the universe. And they didn’t. Much to their surprise, what they found is that there’s something that is causing the universe to increase in the rate at which it’s flying apart. 

Fraser Cain [00:05:22] So I want to dig into a little more detail here. So so they were looking at the supernova and getting a sense of what their distance was. And they were finding that the actual distance that they were seeing the supernova at was not what they were expecting. Is that right? 

Pamela Gay [00:05:38] Well, what they were finding was the distance related to how fast they’re moving was wasn’t quite what they were expecting. Basically, if you think that the expansion rate of the universe is slowing down, then when you look back at things, you expect to see a certain relationship between recession rate and distance of an object, where the recession rate in the past will be a higher number than it would be if this was a constant acceleration. Now, if instead you find that the supernova are going slower than you expected at a given distance, that means that somehow we’re accelerating ourselves apart. So it’s one of these things where you expect supernova at a certain distance to be going a certain speed if the expansion rate stays constant. If the expansion rate is slowing down over time, you expect them to have a different speed. And if the expansion rate is speeding up over time, you expect to look back at that distance and see them at yet a third speed. So they’re doing all sorts of complicated math that is relating how the expansion rate is changing with what distance what object should be at if it has a given speed. And they had a certain set of expectations. And just like the rabbit going through the rabbit hole with Alice following, everything got turned on its head and wasn’t quite what anyone expected. And they found, wait, the universe is accelerating itself apart. And this was a completely revolutionary idea. And the fact that two teams working in competition with each other, both had the same head scratching result, got the entire astronomical community to look up and go, oh, this is new. We need to pay attention to this. This is exciting. It also meant that the way we looked at the universe had to suddenly be changed. Things that we’d been setting to zero for a long time or setting to negative values for a long time in theories suddenly had to change to incorporate these new results. All of our theories for the universe had to suddenly account for a completely unexpected observation. 

Fraser Cain [00:08:02] Right. So it’s expansion would close to a stop and then it might collapse back in on itself like a big crunch. Or if it had enough outward velocity, maybe it would just keep on expanding, slowly closing but never quite stopping. But no, in fact, it’s like someone just turned on the accelerator and, you know, speeding up the expansion. 

Pamela Gay [00:08:22] Imagine that you start pushing a car up a hill and you let go of it. And instead of coming back to roll over you, it decides to accelerate up the hill. It’s just not what anyone expected to see. And it’s what we discovered. Our universe, which we thought was basically expanding up a hill and slowing down, was instead accelerating up the hill. 

Fraser Cain [00:08:47] Now, even though it was discovered in 1998, that’s not the first time people have been thinking about this, though, right? 

Pamela Gay [00:08:55] No. This was the first time we had solid observational evidence. But as far back as when Einstein first came up with his theory of relativity, people have been looking at ways that the universe might be accelerating apart. For Einstein, it was a matter of his worldview told him that the universe was a stationary place where the galaxies were held in place forever. And when he looked at his theories, it showed that gravity was causing them to move and either the universe was accelerating apart or collapsing together, depending on the amount of mass. And that didn’t fit his worldview. But there are different times in mathematics where you’re allowed to just add a constant and it’s completely mathematically legal. And he looked at his math and it was legal for him to add a constant. So he did. And what he said was there’s this cosmological constant that acts in opposition to gravity. So while gravity is trying to collapse things in, it is working to push things apart. And thus, the universe was held in perfect equilibrium. But it wasn’t a stable equilibrium. It was a rather uncomfortable way for the universe to be. It’s sort of like it’s one thing to be hanging from a rope underneath the swing where your rope is actually a piece of solid metal steel. Now, imagine instead that you flip yourself up and you’re balanced on the end of the solid metal swing exactly above the bar of the swing. You might be stable there, but if you sneeze, you’re going to go cascading off of that point of equilibrium. He had the universe held in that unstable equilibrium between his cosmological constant and gravity, and it just wasn’t a comfortable place to be. 

Fraser Cain [00:10:48] And he later called it his greatest mistake. 

Pamela Gay [00:10:51] A few years later, Hubble, who was working out in California, discovered that galaxies are all moving apart from us. Once you get past those that are gravitationally bound to us, the only way you have all the galaxies receding is if you have an expanding universe. And when those results came out, Einstein basically took back his cosmological constant. But once you put an idea out there, theorists love to chase ideas. And so there were theorists who were trying to figure out, well, what if there is a cosmological constant and it’s just small so we don’t see it affecting locally the expansion rate of the universe? What if it is out there? What if there’s some sort of a field? What if there’s some sort of a vacuum energy? And these different possibilities were kicked around. And people came up with beautiful theories for how in a vacuum, any energy you have in the vacuum can condense into particle and antiparticle that exist for a moment and then annihilate one another. And there’s this constant boil in space of particles and antiparticles forming and destroying one another. And the energy of this entire vacuum, particles annihilating one another and coming into existence, is a background energy that works in opposition to gravity. Matter and energy are just two forms of the exact same thing. So just as you can say, gravitational mass draws things together, you can say that the gravitational effects of energy can draw things together or push them apart depending on the sign of the energy. It just works out that the energy has a negative pressure. The energy is working to push things apart. 

Fraser Cain [00:12:41] Now, hasn’t this been detected in the laboratory? 

Pamela Gay [00:12:44] It’s kind of hard to detect. Basically, we’re looking for the energy of 30 proton masses per cubic meter. You need to be looking in really big chunks of space to see the effects of vacuum energy. There are people out there who claim that they’ve detected dark energy in the laboratory. I’m not sure I buy it. I’m not sure I’m also qualified to buy it or not buy it. But it’s not the same hit you on the head with a sledgehammer results that you get when you look at the universe as a whole. 

Fraser Cain [00:13:17] Now, you’ve already kind of beat me to my question, which is, you know, what is it? So the first concept of what could possibly be accelerating the universe like this is this thought of like there’s this vacuum energy bubbling up in every square meter or cubic meter of space in the entire universe. And more and more energy is kind of pouring out of this. But how does that expand the size of the universe? 

Pamela Gay [00:13:45] Well, so this energy has a repulsive characteristic. It has what’s called a negative pressure, and it’s working to push things apart. Well, our universe is expanding. And the more universe expands, the more space there is for this vacuum energy to bubble. The more space we have for this vacuum energy to bubble, the more repulsive force it’s able to put out. So it’s sort of like the bigger space gets, the more vacuum energy we have, the more the universe is pushing itself apart. 

Fraser Cain [00:14:19] Right. 

Pamela Gay [00:14:20] If there was more of it than there actually is, then our universe would have actually torn itself apart in the first moments. Atoms wouldn’t have had a chance to form. Everything would have been completely shredded. But luckily, it’s a small thing. It makes up 70 percent of our universe. But the amount of mass energy contained in the vacuum energy is small enough that our universe was allowed to form and exist and expand without shredding itself. 

Fraser Cain [00:14:51] So what are the other there must be some other theories for what what this could be? 

Pamela Gay [00:14:54] Well, there are other theories. One of them is something called quintessence, which is basically a field theory that says there’s a very light field that tracks through all of space. And the way the field works is it’s causing things to expand. It’s it’s not a thing that currently has a lot of proponents behind it. Current observations are leading people to think more and more in terms of vacuum energy, but we don’t have concrete results. And one of the problems with vacuum energy is the theorists working on it are having trouble getting their numbers and their theories to match what we observe in reality. And it’s a big problem. 

Fraser Cain [00:15:38] And it probably shares the same the same problem with dark matter that it all just depends on us understanding gravity. 

Pamela Gay [00:15:46] It’s even more subtle than that. It depends on us understanding the particle world. It requires us to know what’s out there, what energies different particles that we haven’t even discovered necessarily have. 

Fraser Cain [00:15:57] Sure. But I mean, we were calculating the distance to, you know, or the amount of gravity that objects at large scale should be affecting on one another. 

Pamela Gay [00:16:05] Well, right. 

Fraser Cain [00:16:07] And if we don’t understand gravity at those long, large distances, then. 

Pamela Gay [00:16:10] So that is part of it. There are a group of people who believe that dark energy is just a matter of not knowing how gravity works at really large scales. But with the vacuum energy, we also have to understand the entire subatomic world. And we’re still finding new subatomic particles. There is a new boson type particle, and that’s too much detail. But there is a new particle discovered just last week. And depending on what particles you put into your calculations, you can get results that say that the universe should have dark energy that acts 10 to the 120 times more than it does. That’s a lot.

Fraser Cain [00:16:50] So it’s almost like we’re going to find out more about dark energy with some of the new super colliders than we will with the, you know, looking at with our telescopes. 

Pamela Gay [00:17:01] So with our telescopes, we’re able to say, OK, so we know that there was dark energy at five billion years ago. And at that point, it started to dominate. We started to have the effects of the acceleration being more powerful than the effects of the deceleration from gravity. Now, with the new results, we can look all the way back to nine billion years ago. And our universe is only 13 .7 billion years old. So we can look way back into the early childhood of our universe and say nine billion years ago, it was there and it was affecting things. It just wasn’t a dominant player in how our universe was expanding at that point. 

Fraser Cain [00:17:39] So what does this hold for the future of our universe? 

Pamela Gay [00:17:43] Well, it means that we’re just going to keep expanding forever. It means that eventually our universe is going to die basically of energy death. 

Fraser Cain [00:17:52] Right. So when you say expanding forever, like right now, you know, we can see all these other galaxies. Is there going to be a point where we just won’t be able to see any other galaxies or? 

Pamela Gay [00:18:01] Here the catch is that the longer the universe is around, the more light years away we can see things. But the light that’s being generated by the other galaxies is going to cease to be. Over time, all the stars are going to die. We’re going to be left with black holes with neutron stars that start out hot enough that just by being hot, they’re giving off light. We’re going to have white dwarf stars that start off just by being hot. They give off light. But over time, they’re going to cool off and cool off and they’re going to stop giving off light. And over time, we’re just going to have a universe that’s basically consists of dead chunks of former stars, dead chunks of former galaxies. 

Fraser Cain [00:18:47] Accelerating apart from each other. 

Pamela Gay [00:18:48] Accelerating apart from each other. 

Fraser Cain [00:18:50] Sounds nice. 

Pamela Gay [00:18:51] It’s a kind of bleak future. 

Fraser Cain [00:18:53] I guess until some new great discovery happens. 

Pamela Gay [00:18:56] Right. Until we figure out how to change the physics of the universe using a giant computer. But that’s the story of Isaac Asimov. And it’s hard to figure out how to do that when most of the time I can’t even get my computer to stay stable for more than three months at a time. 

Fraser Cain [00:19:12] So what are people doing now and sort of what’s next to kind of get to the bottom of this? 

Pamela Gay [00:19:20] Well, there’s three different directions. There’s the let’s keep measuring supernova as far back as we can. The more data we have, the more constraints we can put on the theorists. So there are a variety of different ways that people are planning to map out our universe. These are all part of the joint dark energy mission that is part of NASA’s Beyond Einstein program and is supported also by the U .S. Department of Energy. They’re looking for ways to map out supernova at higher and higher redshifts at larger and larger distances. Some of the programs that they’re looking at are the Advanced Dark Energy Physics Telescope, a project led by Charles Bennett at Johns Hopkins University. There’s Destiny, the dark energy space telescope that’s led by Todd Lauer out at the National Optical Astronomy Observatory in Tucson. Again, it’s going to be looking at type 1a supernova like we talked about last week. And then there’s also the Supernova Acceleration Probe, which is led by Saul Perlmutter, one of the people from the original 1998 discoveries. He’s out at Lawrence Berkeley National Laboratory. And again, that’s another program to look at supernova. So by finding supernova at larger and larger distances, we can map how the universe’s expansion rate has been changing over large chunks of the history of the universe. And by carefully seeing how the different values play against each other, we can first of all say, OK, so has the dark energy characteristics changed or is it pretty constant as a function of volume? 

Fraser Cain [00:20:59] Now, what do you mean by has it changed? What could that what would that mean? 

Pamela Gay [00:21:03] So right now, if you look at what we think is vacuum energy, you find that there’s the equivalent of 30 proton masses of energy per cubic meter. Well, what if in the past it was only 10 proton masses of energy? What if it was instead 100 proton masses of energy? And we know that’s probably not true. But different amounts of vacuum energy in a different volume would imply different physics is at play. So we need to figure out, is this just boiling virtual particles? Is this something else? And figuring out exactly what is going on means we need to be able to trace the expansion. Think of it this way. You throw out and I’m going to keep returning to this lousy thing of raisin bread, I think, on a regular basis. You take a thing of raisin bread, throw it on your counter. If your room stays the exact same temperature and moisture the entire time the bread is rising, the yeast will probably behave in a consistent manner and cause the bread to expand in a consistent way. But what if partway through the rising of the loaf of bread your heat goes out and the room plunges down to zero degrees Celsius for some reason? Your bread is probably going to stop rising. What if instead some sort of a plague blighted the yeast halfway through the rising of the bread? That’s also going to affect how the bread rises. Well, we don’t know what factors in the early universe might have affected how dark energy was working to push apart the universe. 

Fraser Cain [00:22:42] So if the amount of this dark energy in each sort of meter of space was actually increasing, then that could have a dramatically different effect on the future universe. 

Speaker 1 [00:22:52] Right. 

Fraser Cain [00:22:52] Maybe even the physics at play. 

Pamela Gay [00:22:54] So imagine if at the very beginning of the universe there was no dark energy and we’re slowly getting more and more and more dark energy over time. 

Fraser Cain [00:23:04] As well as more space. 

Pamela Gay [00:23:05] As well as more space. 

Fraser Cain [00:23:07] Right. 

Pamela Gay [00:23:07] So that’s two different things that are working to accelerate the universe apart. That can have rather dire consequences on our future because in the future the universe could shred itself apart. We don’t see evidence for that right now.

Speaker 1 [00:23:21] Right. 

Fraser Cain [00:23:22] So you could get like so far that the amount of energy is so strong that it’s like pushing galaxies apart and then maybe it’s even able to push planets apart from the stars and then it’s able to push, you know, atoms apart. 

Pamela Gay [00:23:34] In the most extreme circumstances, my desk could tear itself apart. 

Fraser Cain [00:23:40] Right. That’s what they call the big rip, right? 

Pamela Gay [00:23:42] Yeah. We see no evidence for that, but it’s still kind of neat to think about. 

Fraser Cain [00:23:46] So what does this latest evidence from Hubble tell us? 

Pamela Gay [00:23:48] What this latest evidence tells us is dark energy didn’t just mysteriously appear five billion years ago. Over time, how gravity and dark energy have interplayed has changed. It’s sort of like if you start a game of tug of war and initially everyone’s pulling as hard as they can and the rope is holding on structurally, it’s okay. But as the people get more and more tired, they’re pulling less. And imagine it’s an elastic rope. It might be able to start pulling those people together as they get more and more tired. Well, with the interplay of gravity and dark energy, it sort of works in the opposite direction. As objects move further and further apart, as the universe expands, gravity, it doesn’t so much get more tired, but it does get more weak. And gravitational force is inversely proportional to the square of the distance, which basically means if we took the moon and moved it twice as far away, the gravitational force between the Earth and the moon would be four times less. Now, as the gravity gets less and less effective at holding things together, the dark energy, which was always there in the background, is able to exert more of a force on the stuff around it. Its force suddenly matters more. Like with our elastic rope, it was always elastic, but you couldn’t notice as much when people were pulling really hard. So as the gravity gets less effective, the dark energy is able to overcome the gravity and start pushing things apart. That point at which dark energy was able to start pushing things apart occurred about five billion years ago, and it started to dominate about five billion years ago. But if you looked further back in time, we knew that mass, that gravity dominated. Now we know that even though gravity dominated, the dark energy was still there. It was still having an effect. It was just a small effect that it took a lot of results to be able to find. The new results rely on 23 different distant supernova, and that doesn’t sound like a lot, but these things are hard to find. There are some of the faintest things that we’re regularly searching out. We’re looking for a little faint firefly flare -up in a distant galaxy, and it’s impressive that they were able to find and carefully measure 23 different supernova and get these results. 

Fraser Cain [00:26:33] Right. And I think also they’ve got a better handle on that the amount of dark energy per meter doesn’t seem to be changing over time. 

Pamela Gay [00:26:44] It seems to be holding constant right around 30 proton masses worth of energy per cubic meter. 

Fraser Cain [00:26:50] And hopefully with this next series of spacecraft and further research, they’ll be able to kind of tighten those constraints. And maybe more experiments with the supercolliders, with the particle colliders, will help us understand what kinds of particles could be actually interacting with each other. 

Pamela Gay [00:27:09] The better the map, the better you can get from point A to B, and our way of getting from point A to point B is by building a theory. 

Fraser Cain [00:27:17] Well, unfortunately I guess we’ve got no answer for people this week, but hopefully at least they can understand what people are talking about when they talk about dark energy. And I think this is one of those great subjects that we’re going to be able to watch it change in real time and maybe, you know, within our lifetime we’ll see the answer and know what it is. 

Pamela Gay [00:27:37] It’s certainly a possibility with all these neat new missions going up into space. And who knows, maybe we will find an even more effective way of mapping out the distant universe than using supernova. Like we mentioned last week, there are people who are chasing gamma ray bursts, and maybe that’ll work, and maybe we’ll be able to see even further back into the beginnings of time. 

Fraser Cain [00:27:56] Great. All right, well thanks Pamela. We’ll talk to you again next week. 

Pamela Gay [00:28:00] It’s been my pleasure. This has been Astronomy Cast, a weekly fax -based journey through the cosmos. Music provided by Travis Earle. Thanks for listening. 

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