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

This episode delves into the enigmatic concept of dark energy, exploring its role in the accelerating expansion of the universe.

1. Dark energy is a mysterious force that accelerates the expansion of the universe.
2. It makes up a significant portion of the universe's composition, yet its exact nature remains unknown.
3. Historical scientific theories have evolved from a static to an expanding universe, now acknowledging acceleration.
4. Modern techniques use cosmic probes like supernovae and galaxy clustering to study dark energy.
5. The episode underscores the importance of continuous scientific inquiry and funding to unravel these cosmic mysteries.

1. Introduction to Dark Energy

An overview of dark energy and its implications on the universe's expansion. Regina Barber: "So what is dark energy? What is this thing that is negative pressure?"

2. Historical Context and Scientific Evolution

Explains the shift from Einstein's static universe theory to the current understanding of an expanding universe. Brian Nord: "And then in 1999, we found out that that term should be in there."

3. Modern Research and Techniques

Discusses how astronomers use cosmic probes to study and understand the universe's expansion. Brian Nord: "So what we have to do is we have to use every reasonable, viable cosmic probe."

1. Stay Curious: Engage with the latest scientific literature and media to understand ongoing research about our universe.
2. Educational Outreach: Participate in or support educational programs that focus on space and cosmology.
3. Science Communication: Share knowledge about cosmic phenomena like dark energy in community discussions to spread awareness.
4. Support Research: Advocate for funding in scientific research to aid crucial studies on cosmic acceleration and other phenomena.
5. Explore Scientific Methods: Experiment with understanding basic scientific methods that astronomers use, like observing cosmic phenomena.

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.

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Regina Barber, Brian Nord

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Regina Barber
You're listening to short wave from NPR.

Hey, short waivers, 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 relativity, that wasnt the case.

Brian Nord
The prevailing theory was actually that the universe was not expanding. However, Einsteins equations in the original form in which he derived them, predicted that the universe was expanding.

Regina Barber
Some versions of this history say that once Einstein wrote these equations, he freaked out at at their implications.

Brian Nord
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.

Regina Barber
And he called this fudge factor his greatest blunder.

Brian Nord
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.

Regina Barber
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.

Brian Nord
Cosmetology. Yeah, yeah, I've definitely got that in random airport conversations, like, oh, could you do my hair?

Regina Barber
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.

Brian Nord
Dark energy is simply the stuff that causes the accelerated expansion of spacetime. We don't know what that stuff is necessarily, but that's what we're calling it.

Regina Barber
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?

Youre listening to shortwave, the science podcast from NPR.

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Regina Barber
Company and affiliates okay, Brian, just to, 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.

Brian Nord
Yes. And it's an energy because we don't believe 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 bakes my noodle a little bit. So the idea is that the vacuum of spacetime 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 thats why it can keep pushing.

Regina Barber
On itself over time, because its everywhere is constant.

Brian Nord
It's because its 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.

Regina Barber
So what do you mean by vacuum? Like, a vacuum in normal physical world is like a lack of air molecules, a smaller density of air versus the density outside. So you have this like push because of the density.

There's a density difference. There's a pressure difference.

Brian Nord
That's right. There's a pressure difference. And yeah, dark energy has a negative pressure.

Regina Barber
What?

Brian Nord
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.

Regina Barber
Okay, so this is the question, then, what is it? Then what is this dark energy? What is this thing that is negative pressure?

Brian Nord
It literally might be the vacuum energy of the universe.

Regina Barber
Okay, how do we test that? How do we know?

Brian Nord
Yeah. And I'm also trying to think about another way to describe that.

Again, if we think of the fabric of spacetime 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 nineties, 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 spacetime. And so they observed as many of these supernovae 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 spacetime, or are they moving because spacetime is itself 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 supernovae are moving away from us faster and faster.

Regina Barber
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, like, a lot of supernova to be able to prove to anything about, like, cosmic acceleration.

Brian Nord
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 background is another one, galaxy clusters. So all these things, in their own way, 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.

Regina Barber
So that actually brings us to. You're saying you have all these probes. Like, we can look at supernova, we can look at large 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?

Brian Nord
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.

Regina Barber
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 are there that are out there that maybe are just as good? Or would you say that this is like the top one?

Brian Nord
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.

Regina Barber
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.

Brian Nord
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 epoch.

Regina Barber
Recently, there's been studies talking about how the rate of that acceleration might have changed over time 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? Future of dark energy?

Brian Nord
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'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 a particular cosmic probe called galaxy clustering, or the two point correlation function. And so 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.

Regina Barber
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?

Brian Nord
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 its constant.

If you do the calculations, you find that in about 50 billion years, every galaxy, Orlando, 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 spacetime. That are pulling them away from each other. They will be so far apart, and spacetime will be moving, quote unquote, fast enough. That light will never be able to get from one of these mega galaxies to the other.

Regina Barber
So it's almost like it will create these isolated universes within a universe.

Brian Nord
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. Yeah.

Regina Barber
So what's the other scenario then? Then where are we going?

Brian Nord
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.

Regina Barber
It still blows my mind that 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.

Brian Nord
And even if we answer. 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 nother 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 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, will we have a chance?

Regina Barber
Well, thank you so much, Brian, for talking to us. I've had a great time.

Brian Nord
Yeah, thank you. Yeah, this was fun.

Regina Barber
Before we head out, a reminder that we'll be back tomorrow with our regular short wave 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.

Wendy Lawrence
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 time.

Don't forget to strap yourself in while you sleep. And good luck going to the bathroom. You'll need it.

Regina Barber
This episode was produced and fact checked by Hannah Chin. It was edited by our showrunner Rebecca Ramirez, and it was engineered by Kwesi 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 US Space and Rocket center, home of Space camp.

I'm Regina Barbour. Thank you for listening to our summer space camp series from NPR.

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