New Evidence Questions Dark Energy’s ‘Constant’ Nature

IRA FLATOW: This is Science Friday. I’m Ira Flatow. One of the more head scratching things about the universe is the concept of dark energy. We know that the universe is expanding. But what’s more, that expansion is speeding up. Conventional wisdom used to be that it should be slowing down, but no more. And that’s been attributed to this mysterious force called dark energy.

The energy itself has been represented by a number, a constant called the cosmological constant. But what if that cosmological constant isn’t really a constant? What if the force varies over time? Recent data presented by a group called DESI, the Dark Energy Spectroscopic Instrument, says that possibly dark energy may, and we emphasize may, may be evolving over time, and that would be a big deal. Joining me now to talk about that is Dillon Brout, Assistant Professor of Astronomy at Boston University and part of the DESI collaboration. Welcome to Science Friday, Dr. Brout.

DILLON BROUT: A pleasure to be here. Thanks for having me.

IRA FLATOW: Did I get that right? Tell us what dark energy is, what it’s doing, and how it works.

DILLON BROUT: Right, so OK, let’s start with the story of the universe, which really starts with the Big Bang itself. And this Big Bang is this initial kick of energy sending all of the matter and energy of the universe outwards, and it causes this initial expansion.

For many years, it was believed that the forces of gravity would eventually take over and cause this expansion to slow down. The universe is full of matter. It’s also full of this stuff called dark matter. And all of that interacts in the universe through the force of gravity. And gravity is this attractive force. So we would expect the universe to slow down and perhaps even collapse on itself in the future.

Until 1998, astronomers come along using supernova, these special kinds of exploding stars that we use to map out the universe. And what they find is that not only is this expansion of the universe not slowing down, but it is speeding up. It is accelerating. And not only does that mean that galaxies are expanding from each other at a faster and faster rate, but you need some additional force in the universe to explain that. And that force is colloquially called dark energy.

IRA FLATOW: Was that force always present in the universe and we just didn’t discover it or know about it?

DILLON BROUT: Excellent question. So it was always present, as far as we believe, but it has become more and more impactful as time has gone on, because we believe, or our simplest understanding of what dark energy could be, is that it is a property of space itself. The energy of the vacuum, it’s sometimes called. This is the same idea of the cosmological constant that you just described. And if it is a property of space itself, then if the universe is expanding and we’re getting more space, over time, we’re getting more dark energy. And this is what leads to this snowball effect or accelerating universe.

IRA FLATOW: And so we’ve always thought that it was a constant, right? But now new research seems to possibly throw a little monkey wrench into that idea.

DILLON BROUT: Right. So when we say constant, Einstein’s cosmological constant, what we mean is that it is a constant of space and time. So what we try to search for is deviations in space and time from this constant energy. And what perhaps the DESI collaboration is corroborating and what we’ve been seeing for a couple of years, some subtle hints of this being true. It’s not fully statistically confirmed yet, but it is getting very exciting, certainly, which is that it could be varying with time.

IRA FLATOW: If it is varying with time, how would that affect what we think about the universe?

DILLON BROUT: It really would turn everything upside down as we know it in a lot of good ways and in some not so good ways from the perspective of a cosmologist. The simplest and best theory we have for what dark energy is, is the cosmological constant. So if it’s not that, we really have to start from square one in terms of our understanding for what it could be. And there are many theories popping up every single day on the archive is what it’s called, papers coming out, about what it could be. And we’re going to have to get lots more data in order to verify these alternative possible theories.

IRA FLATOW: Let’s talk about this DESI thing. What is it physically? What’s it looking for? Is it a telescope? Describe physically what we’re seeing here.

DILLON BROUT: DESI is an incredible instrument. It’s a next generation telescope. It’s come online just about three years ago, and the collaboration has finished collecting and analyzing the first year of data. That data set is measuring the velocities of galaxies. What they’ve done is measured 5 million galaxies.

And this really is an incredible feat, because 5 million galaxies is three times more than what had been collected historically over decades. And they collected that in just a single year. And DESI will be going on. It’s funded for five years, and we hope for even longer, which means it will be a truly revolutionary instrument. And this is just the beginning of the data that’s going to be coming from it.

IRA FLATOW: Yeah, and it’s this first data that you’re saying shows that the constant may not be so constant, but you need a lot more data.

DILLON BROUT: We need a lot more data in a number of aspects. So the DESI data is one data set that is used. One of the reasons why we are calling this a hint and not a discovery or anything like that is because we require using external data sets that are complementary to this DESI, Dark Energy Spectroscopic Instrument, data set. So we also use data sets from supernovae, just like was used in 1998 to discover the accelerating universe.

And only in combination with these external data sets does the evidence for this evolving dark energy become more real. What’s nice is that all the data sets do tend to agree with each other. We always like to see agreement between complementary data sets. But really, only it becomes significant when you combine them. So we want more data from DESI, we want more supernova, and we want to corroborate things yet again.

IRA FLATOW: If this is true and if it’s not a constant as we think it is, what does that tell us about how much we really know about the universe?

DILLON BROUT: That’s a great question, and it’s a hot topic right now in cosmology. It really turns things upside down. We think that roughly 70% of the energy budget– remember that Einstein in his famous equations E equals mc squared, tells us that energy and matter are, for lack of a better word, interchangeable. And so we can summarize the energy of the universe and roughly 70% of the energy of the universe is encompassed in this dark energy. Roughly 25% of the energy in the universe is encompassed by dark matter, which has this attractive component through gravity.

And only about 5% of the energy density of the universe can be attributed to ordinary things like you and I, stars, galactic dust, and all this other stuff we can see. So the vast majority of what makes up the universe, we can only see indirectly. And we really have a lot of thinking to do in terms of what can explain all of this indirect evidence. And maybe this gives us new opportunity to rethink things from square one.

IRA FLATOW: Yeah, well, do we know anything more now then about the ultimate fate of the universe?

DILLON BROUT: If anything, we know less, because our estimates of what the ultimate fate of the universe were relying on the fact that dark energy was this really fundamental property of space, that cosmological constant that we’ve talked about. And if it’s not that, all bets are off.

IRA FLATOW: Yeah, yeah. You talked about the other dark stuff, the dark matter. Is dark matter somehow connected to all of this?

DILLON BROUT: It certainly could be. I think if you ask cosmologists, you’d be hard pressed to find many that would bet on them being connected. But certainly how they impact the universe is related. You can think of the expansion of the universe as a tug of war. And this tug of war is between the forces of gravity and the outward pressure coming from this dark energy. And there were times in the universe’s past when dark matter was the most dominant component because the universe was smaller.

And when we say smaller, we mean there was less dark energy. There was less space. But as the universe has expanded, the dark energy has become the most dominant component. And we think today, dark energy is winning. Even in light of the findings of DESI, dark energy is still winning. And we do think that the universe will likely expand forever and continue to be accelerating. The findings are not that the universe is not accelerating. It’s just accelerating less vigorously than we expected.

IRA FLATOW: So don’t we then have to find what the dark matter and the dark energy is made out of? I mean, should there not be particles or something in the quantum world?

DILLON BROUT: Yes. So there are certainly many experiments that are looking for a particle origin of dark matter itself. And those terrestrial experiments on Earth have thus far come up empty. They really placed upper limits on the mass of such particles. We have evidence from telescopes that suggest that this dark matter is likely of some kind of particle origin and not some misinterpretation of the fundamental theory of gravity itself.

But thus far, such searches have come up empty for what dark matter itself could be. And as these DESI findings are showing, dark energy is also potentially we don’t really have a good handle of what it is. So we’re really in the dark, so to speak. And that’s oftentimes why we call these energy and matter dark energy and dark matter, because it really encompasses how little we know.

IRA FLATOW: You say that dark energy is a property of space. And as you get more space, I mean, we’re expanding, you get more energy. But doesn’t that energy have to come from somewhere?

DILLON BROUT: So in Einstein’s equations, which he came up with this initial idea of the energy of empty space. It’s simply introduced by a single number in his equations. It’s really elegant. It’s really beautiful. He included it in his equations for reasons that are different from the reasons that we include them in our equations today. We include them today because we know that the universe is accelerating.

Back when Einstein was alive and when he was coming up with his theories of general and special relativity, he did not know the universe was accelerating. But he had this prevailing idea that the universe was static and eternal, and he needed some outward energy to prevent the universe from collapsing on itself. And that can be represented in the same exact way.

Now, in physics, there’s nothing prohibiting energy of space itself. There isn’t necessarily a conservation of energy is what we call it. If you’re not looking at a closed system and the universe, which we believe is infinite in scope, in space, is not this closed system within which we can calculate the total energy. We can only calculate the energy in any one area of space. And Einstein gave us that value.

Related to your question, what’s also interesting is that quantum mechanics does also come up with a prediction for the energy of empty space coming from these quantum fluctuations. But there’s a problem. And it’s that the prediction from quantum mechanics for this level of energy of empty space is 120 orders of magnitude off the observed value that we find from, for example, we found in DESI or from the supernova themselves. And 120 orders of magnitude is a lot. It’s sometimes called the largest discrepancy between theory and observations in all of science.

IRA FLATOW: So if I understand what you’re saying is that there should be, according to theory, a whole lot more of dark energy than we have found.

DILLON BROUT: Yes, that’s correct. Although many people don’t necessarily accept that as a correct calculation. They think there might be an issue in those calculations in quantum mechanics, because it’s so large that you wouldn’t actually even predict the universe to have survived.

IRA FLATOW: Wow.

DILLON BROUT: So that’s perhaps–

IRA FLATOW: That’s crazy.

DILLON BROUT: Yes. It’s perhaps an issue isolated to the theory in quantum mechanics and not necessarily representative of our measurements with the telescopes that we’re using.

IRA FLATOW: You talked about the universe being a sort of balance, a tug of war between the dark matter and the dark energy. Is there a chance that our estimates of dark matter are wrong and that could be throwing off the numbers?

DILLON BROUT: Absolutely, yeah. Wow, you’re hitting on the kinds of things that we’re looking into now. Can the problem be simplified along different dimensions? It’s important that we allow for all of our uncertainty in what dark matter could be and dark energy could be while we fit the data itself. And so, yes, these results make a statement about dark energy, but it’s also making a statement about dark matter. And a lot of the effort going now in the latest literature that’s coming out since the DESI result is to understand the differences on the dark matter side.

IRA FLATOW: So where do you think this is all headed? How many more years of study do you think– and do you think we’ll ever solve this question?

DILLON BROUT: The good news is that we have a lot more data coming soon. DESI has five total years of data. It’s an incredible data set that alone will shed light on whether or not this signal that we’re seeing is true, first of all. Then there is another telescope coming online hopefully next year. This telescope is called a legacy survey of space and time. It’s located in the Southern hemisphere.

And it’s going to be observing supernova. And we’re going to go from what right now we have about 1,000 or so supernova to maybe as much as a million. And really, the data sets that are coming online are just absolutely phenomenal. And we’ll get a good handle on whether or not these hints are, in fact, true. We’ll have better constraints that will allow us to constrain better, different models of what dark energy could be. Now, whether or not we will have a good understanding and grasp of what dark energy and dark matter in our lifetimes, I couldn’t tell you.

IRA FLATOW: Is part of that problem the fact that we may need new physics to explain this?

DILLON BROUT: Yes, absolutely. We may need new physics and we need the theorists to work hard on developing those new physics ideas. And that is much easier said than done.

IRA FLATOW: Does that mean new theory or new particles or what when we say new physics?

DILLON BROUT: In the case of dark matter, it could be new particles. Really, what the challenge that theorists have on their hands is to explain all of the different signals that we are seeing with the telescopes. So we see this potential evolving dark energy. We see the existence of dark matter.

We have other hints of the possibility of new physics that we haven’t even talked about that have been in the news lately. This is called the Hubble constant tension. And it becomes a real challenge to come up with new theories or particles, whether in the very early baby universe or in the late universe, to explain everything that we see. And really what we want to get is a theory that explains everything.

IRA FLATOW: Thank you very much, Dr. Brout, for taking time to be with us.

DILLON BROUT: Thank you for having me.

IRA FLATOW: Dr. Dillon Brout, Assistant Professor of Astronomy at Boston University and part of the DESI collaboration.

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