Short Wave - This Mysterious Energy Is Everywhere. Scientists Still Don't Know What It Is

Episode Date: July 16, 2024

The universe — everything in existence — is expanding every second! It's only been about a hundred years that humanity has known this, too — that most galaxies are traveling away from us and the universe is expanding. Just a few decades ago, in the late 1990s, scientists started to notice another peculiar thing: The expansion of the universe is speeding up over time. It's like an explosion where the debris gets faster instead of slowing down. The mysterious force pushing the universe outward faster and faster was named dark energy. Cosmologist Brian Nord joins host Regina G. Barber in a conversation that talks about what dark energy could be and what it implies about the end of our universe. Check out more of our series on space at https://www.npr.org/spacecamp.Curious about other happenings in our universe? Email us at [e-mail] pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hey, short wavers, it's Regina Barber. Today, it's widely accepted that our universe is expanding. This is what I taught in Astronomy 101 for many, many years. But 100 years ago, when Albert Einstein was figuring out general and special activity, that wasn't the case. The prevailing theory was actually that the universe was not expanding. However, Einstein's equations in the original form in which he derived them, predicted that the universe, was expanding. Some versions of this history say that once Einstein wrote these equations,

he freaked out at their implications. And so he added a fudge factor. It was meant to counteract the expansion so that you would get this static universe. And then later he took it out because he was like, no, I guess the universe is expanding. And he called this fudge factor his greatest blunder. And then in 1999, we found out that that term should be in there. You know, it's a shame. If he had gotten this right in the first place he might have been famous. That's Brian Nord. He's a computational cosmologist. And okay, that seems like a lot, which is probably why he says sometimes people get confused.

They think he works in makeup. Cosmetology. Yeah, yeah. I definitely got that in random airport conversations. Like, oh, could you do my hair? And you're like, okay, I guess. But it's cosmology. The study of the cosmos and how the universe started.

And today, Brian's here to tell us about that fudge factor that Einstein made up. Because it does exist. These days, most scientists use it to explain an accelerating, expanding universe, caused by dark energy. Dark energy is simply the stuff that causes the accelerated expansion of space time. We don't know what that stuff is necessarily, but that's what we're calling it. According to NASA, approximately 63 to 70% of our entire universe is made up of dark energy. But like Brian said, what it is is a mystery. So today on the show, we continue our space camp series by peering at dark energy or cosmic acceleration.

How do we know the universe is expanding faster and faster? How do scientists study it? And lastly, how could it help us predict the future of our universe? You're listening to Shortwave, The Science. Science podcast from NPR. Okay, Brian, just to recap, first we thought the universe wasn't moving at all. It was static. And then we found out it was expanding.

And then we realized it was accelerating as it was expanding. And we were like, what could possibly be pushing that universe outward and accelerating? Like, why is it speeding up? And that's how we came up with the term dark energy. Like, do I have this all right? Yes. And it's an energy because we don't believe that there are. that there are particles like atoms.

Our best guess currently is that it is the energy of the vacuum of space time. The reason, and this, this bakes my noodle a little bit. So the idea is that the vacuum of space time has an energy. Not only does it have an energy, its density is constant. So if that is a kind of density, this is a vacuum density, which is pushing pieces of space away from themselves, That means that you have more and more space of the same density. And that's why it can keep pushing on itself over time.

Because it's everywhere? It's because it's everywhere and because its density is constant. The matter density of the universe has to go down over time because there's a finite amount of matter. But that's not the case here. Because space time, it has a vacuum energy. And so as you're adding in more and more space time, you're adding in more and more vacuum.

So what do you mean by vacuum? A vacuum in normal physical world is like a lack of air molecule. A smaller density of air versus the density outside. So you have this like push because of the density difference. There's a density difference. There's a pressure difference. That's right. There's a pressure pressure difference.

And yeah, dark energy has a negative pressure. What? Okay. Okay. Yeah, if dark energy is due to vacuum energy, then it must have a negative pressure. And that negative pressure is what pushes things away from each other. Okay, so this is the question then. What is it then?

What is this dark energy? What is this thing that is negative pressure? It literally might be the vacuum energy of the universe. Okay. How do we test that? No. Yeah, and I'm also trying to think about another way to describe that. If we, again, if we think of the fabric of space time as some large body of water,

one way to figure out how that fabric or body of water is changing is to look at buoys. And so in the late 90s, two competing teams of cosmologists were looking at supernovae, often referred to as exploding stars. And because of their internal properties, they can be really reliable buoys in the space time. And so they observed as many of these supernova as they could at distances away from Earth to see, oh, how are these buoys moving?

Are they moving toward us? Are they moving away from us? And are they moving because, are they moving for some reason in space time? Or are they moving because space time is its, self-changing. And that's indeed what they found, that the best, the best fit to the data at the time, and which is still very close to the best fit to the data, is that these, these supernovae are moving away from us faster and faster. Okay, gotcha. So, like, basically, because we can't

measure cosmic acceleration itself, that, that's the water, like, we measure the movement of supernova, that's the buoys. But just like you'd need a lot of buoys to measure, like, the whole sea, it seems that we need a lot of supernova to be able to prove anything about cosmic acceleration. Yeah. So what we have to do is we have to look out to the universe and find as many more of these

buoys in space time as we can. And there are a lot of these things. We call them cosmic probes. Supernovae are cosmic probes. Galaxies or the distribution of galaxies is a cosmic probe. Cosmic wave backgrounds, another one. Galaxy clusters.

So all these things, in their own way, they give us a little, they give us different hints about how the space time is expanding. And so what we have to do is we have to use every reasonable, viable cosmic probe and try to measure parameters related to dark energy. And then they all have to be self-consistent. So like that actually brings us to, you're saying you have all these probes. Like we can look at supernova. We can look at large. clumps clusters of galaxies. We can look at the beginning of time, which is like the cosmic microwave background. But we don't actually know what dark energy is. Would you say that

that's true? We have ideas, but they are, in my view, we're still at the beginning of trying to understand this and falsify or prove these ideas. How many hypotheses are there? There's the one that you're telling me about, which is like the vacuum energy of the universe. What other hypotheses that are there that are out there that maybe are just as good or would you say that this is like the top one? So we have one very, very important question to ask that differentiates two major sets of hypotheses. One is, as I was saying earlier, if the vacuum energy, which is constant, that would be dark energy is not changing in time. Then there's the other class where dark energy is changing in time. Because we,

don't even know if it changes in time or not. That's where we're at. We don't know if that push outward, that force that, well, that energy that makes the universe accelerate, if that acceleration has been constant since the beginning. That's what you're saying. Right. And so what we want to do is look at these cosmic probes, not just at one point in cosmic time, but we want to look at them at many steps in cosmic time. So if I look at things that are a certain distance away, it means they are also a certain amount of time in the past. And so that means just by looking farther away

and by looking at, say, a set of galaxies so many billion years ago and then another few billion, another few billion, then I can see how the pattern in the galaxies is changing over time to reflect how much dark energy might be pulling things apart at any given cosmic epic. Recently, there's been studies talking about how the rate of that acceleration might have changed overtime or might not be what we thought it was. What is the state of this field now? And what can that tell us about the future of dark energy?

Yeah. So the farther back we can look in time and thus the farther away we're looking, the more cosmic probes we can. measure at every slice of cosmic time and what we're doing now and what we're all what we've always been doing is looking as far back as we can and most recently there's a new study from the dark energy spectroscopic instrument that has looked the farthest back for that for a particular cosmic probe called galaxy clustering or the two-point correlation function and so there their their recent measurements indicate that dark energy appears to be consistent with that vacuum energy model.

And when they combine that with some of these other cosmic probes, they're also seeing that it's consistent with this vacuum energy model. However, there are still several different ways to look at these models and put this data together. And one way of putting it together indicates that perhaps dark energy, the power or strength of dark energy is decreasing in time. That is also possible. And since once they analyze their data that they have and once they get new data, could this give clues on how the universe could end? Oh, yeah.

There are a couple of different major scenarios. Let's say that it is just the vacuum energy. To me, there is kind of a spectacular ending to the universe. if dark energy is the vacuum energy and it's constant, if you do the calculations, you find that in about 50 billion years, every galaxy or many galaxies will have merged into clumps of mega galaxies. And in 50 billion years, because they're being pushed away,

because they're sliding on these, they're sitting on these conveyor belts of space time that are pulling them away from each other, They will be so far apart and space time will be moving, quote unquote, fast enough that light will never be able to get from one of these megag galaxies to the other. So it's almost like it will create these isolated universes within a universe. Yeah, I mean, I hesitate to say universe within a universe is because the technical meaning of universe is everything. But it'll be these islands that you might think is your own universe, that you're alone in it. So what's the other scenario then?

Then where are we going? Well, so if it's getting weaker over time, then you might say that we've lucked out into a kinder scenario in a sense where maybe these island universes or maybe it'll take them longer to form or maybe they won't ever get that far apart. It still blows my mind that, you know, within my not adult lifetime. but my sentient lifetime, scientists have figured out that the universe was accelerating and now we're like even that is, like, what is happening with that acceleration? I mean, there's new stuff happening all the time. And even if we answer this preliminary question, after we answer this preliminary question,

if the strength of dark energy is changing in time, that still doesn't necessarily say what it is. That's a how. And so there's a whole other why to ask. So we're just getting started. Who knows what we'll keep finding out in this weird space that we live in. But, you know, when I try to make the case that this funding is important, it's because, hey, in 50 billion years, we might not be able to do this science.

So if you don't fund us now, we'll we have a chance. Well, thank you so much, Brian, for talking to us. I've had a great time. Yeah, thank you. Yeah, this was fun. Before we head out, a reminder that we'll be back tomorrow with our regular shortwave and back Tuesday with our next installment of the space camp series as the Starship Shortwave, that's you and me, continues on our journey through the universe.

And I have a sneak preview from one of our experts. Hi, Shortwave Space Cadets. It's Wendy Lawrence, former NASA astronaut and current captain of weightlessness. I hear you're going to share what it's like to live in space. Let me tell you. It's wonderful and wild at the same. same time. Don't forget to strap yourself in while you sleep and good luck going to the bathroom. You'll need it. This episode was produced and fact-checked by Hannah Chin. It was edited by our showrunner

Rebecca Ramirez and it was engineered by Quasi Lee. Julia Carney is our space camp project manager. Beth Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy. Special thanks to our friends at the U.S. Space and Rocket Center home of space camp. I'm Regina Barber. listening to our summer space camp series from NPR.