Astronomy
#749: Dark Energy Changing Over Time
We thought Dark Energy was constant with time, but new results from DESI say maybe not, and honestly, if it wasn’t constant the Hubble Tension would be a whole lot easier to solve.
Show Notes- Hubble Tension Definition
- Historical Debate & the current state
- Cosmological Constant Rediscovery
- Dark Energy Implication
- Implications for Dark Energy
- New Physics Possibility
- Dark Energy Spectroscopic Instrument
- Black Hole Growth Model
- Dark Energy Evidence
- DESI’s Contributions
- Dark Energy Evolution
- Exciting Developments in Cosmology
- Upcoming Observatories and Missions
- Scientific Progress and Openness
Fraser Cain: AstronomyCast, Episode 749, is Dark Energy Changing Over Time. Welcome to AstronomyCast, 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.
I’m Fraser Cain, I’m the publisher of Universe Today. With me as always is Dr. Pamela Gay, a Senior Scientist for the Planetary Science Institute and the Director of CosmoQuest. Hey Pamela, how are you doing?
Good.
Dr. Pamela Gay: I am feeling like I wasted my weekend by doing nothing but reading a book, but at the same time that was like the absolute best way I could spend my weekend.
Fraser Cain: Yes.
Dr. Pamela Gay: So folks, there are books out there. Yes. Read them.
I’m currently reading Ancillary Justice and it’s two sequels, I did not know it was a trilogy.
Fraser Cain: That’s on my list.
Dr. Pamela Gay: Yeah, I really want to read that book.
Fraser Cain: Yeah. Okay.
Dr. Pamela Gay: It’s totally worth it.
Fraser Cain: Okay.
Dr. Pamela Gay: Occasionally you have to go back and re-read things because the context changes on you and it’s like, oh.
Fraser Cain: Oh, so you have to concentrate?
Dr. Pamela Gay: Yeah.
Fraser Cain: Oh, that’s a big ask right now, but sure.
Dr. Pamela Gay: Yeah, it’s like that and the giddy and the ninth books require concentration, but they’re all totally worth it. No regrets. So I would like to say to all of you, if you, like me, need to occasionally escape this timeline, I’m not sure how we landed on books.
Fraser Cain: Yes. Read a book. It’s good for you.
The Hubble tension is a vexing problem, with astronomers measuring the expansion of the Universe at different points in its history and getting different results. Errors have mostly been ruled out, which leaves the potential for new physics. Has the strength of dark energy been changing over time?
And we will talk about it in a second, but it’s time for a break. And we’re back. So I guess we should first explain the Hubble tension, the crisis in cosmology, and then we can use that then to continue on this conversation.
So what is the Hubble tension?
Dr. Pamela Gay: All right. So since Edwin Hubble figured out that our Universe is expanding, people have been trying to figure out at what rate it’s been expanding. And there has always been debate.
When I was an undergraduate, they were like, it’s somewhere between 50 and 100, and we’re going to use 100, because that makes the math easier, like literally professors would say that.
Fraser Cain: Yeah. Who doesn’t love easy math?
Dr. Pamela Gay: Yeah, exactly. By the time I got into graduate school, they were like, it’s probably between 60 and 80, and it’s been slowly narrowing down ever since. And the tension at that point in time was not called the Hubble tension, it was called the debate.
And there was always multiple people yelling at each other in the literature, usually calling each other…
Fraser Cain: Politely, academically?
Dr. Pamela Gay: No.
Fraser Cain: No? They’re mean about it?
Dr. Pamela Gay: Yeah, super mean. People didn’t want to be in the same place as each other, levels of yelling at each other. De Vaucouleurs was a small Frenchman with a loud voice who apparently by sheer force of will maintained the debate until he died.
Fraser Cain: Wow.
Dr. Pamela Gay: Yeah. So, finally, De Vaucouleurs died. We thought we were on a path to figuring this out.
The arguments, Alan Sandage was the other person on that debate. Things seemed to be calming down. We got the WMAP data, we had the supernova data trickling in, trickling in.
Sorry, it wasn’t WMAP at this point. We had other microwave data before that. So things were coming in.
It was looking good. And then in 1998, two different supernovae programs. This was the HI-Z Supernova Program and the Supernova Cosmology Project.
I both realized, oh, expletive. If you do a plot of supernova apparent brightness versus distance, and they should all have the same luminosity. So how bright something appears tells you how far away it is.
So if you do brightness versus redshift, that tells you the expansion rate of the universe. And they were expecting a straight line, or they were expecting the expansion of the universe to be slowing with time as gravity slowed it back down. But both surveys found that instead of the universe slowing with time, instead of the universe being constantly expanding with time, our universe has decided it is going to accelerate and expand faster with time, which was not on the menu.
Fraser Cain: Right. Nobel Prize is all around.
Dr. Pamela Gay: Nobel Prize is all around. In 2011, so the paper came out in 98, and 2011 already they got the Nobel Prize, which is pretty darn fast. So this led to the realization that we need to be even more careful in how we make these measurements to try and figure out what’s going on.
Because we name things without understanding them, the name dark energy was given to whatever the thing is that’s pushing the universe apart. When they ran the equations looking at the mass energy distribution of the universe, they figured out 68% of the mass energy distribution is dark energy, 27% is dark matter, and the remaining 5% is the normal baryonic matter like we’re made of.
Fraser Cain: Right. Right. I mean, I think it’s important to distinguish for people that you get this measurement of the expansion rate of the universe, this Hubble constant, and this has been measured nearby using Cepheid variables, and they get one number, 73, was it megaparsecs?
Dr. Pamela Gay: Kilometers per second per megaparsec.
Fraser Cain: Kilometers per second per megaparsec. And then when you do the same measurements in the Cosmic Microwave Background Radiation, you get more like 67, 68.
Dr. Pamela Gay: I thought they were both in the 70s now?
Fraser Cain: No, no.
Dr. Pamela Gay: Sorry, both in the 60s rather. I thought they were both in the 60s.
Fraser Cain: No, no. 73 for the Cepheids and 68-ish, 67 and a half for the CMB. But for the longest time, the error bars overlapped, and so you could say, well, it’s probably somewhere in between.
But most recently, thanks to the Planck mission, you got the most accurate measurement of the CMB version. And thanks to James Webb and Nobel Prize winner Adam Riess continuing his work, he was able to narrow it down. It’s like 73.0. The error bars are almost gone, and the error bars don’t overlap. But I hope people understand, if dark energy is this force that is appearing in the universe over time as there is more universe, then you would expect those numbers to diverge because you’re getting this accelerating expansion in the universe. But this is accounted for, right? This is the expansion subtracting the dark energy.
Dr. Pamela Gay: So the Hubble constant we’re looking at is the current expansion rate of the universe. So when you run the equations, you are supposed to be able to run things backwards to get to the Big Bang. We have various signposts, like when the Cosmic Microwave Background formed, that you can’t really move around in time very much.
And so the idea was that you should be able to look at the hot and cold spots in the microwave background. We know exactly when those should have appeared the moment the universe became neutral. We know exactly how big they should be, based on the mean free path that photons could travel.
And that should allow us to calculate, knowing the geometry of the universe, knowing when that occurred, what the expansion rate is by how big they appear.
Fraser Cain: All right, we’re going to continue this conversation, but it is time for another break. And we’re back. So this all assumes that dark energy is a constant in the universe.
And let’s understand what that means by a constant in the universe.
Dr. Pamela Gay: It’s one of the weirdest things.
Fraser Cain: There’s more dark energy now than there was early on in the universe, right? So how can you have constant dark energy, but now have more dark energy?
Dr. Pamela Gay: It’s constant density. So from looking at all of the initial dark energy measurements that are looking at the past few billion years, it looked like the rate of change in the expansion rate was constant. And it implied that every cubic meter of the universe has about a proton of energy in it, give or take, and that that amount of energy per cubic meter was constant.
And so the more cubic meters you add to the universe, the more dark energy there is, because there’s more universe with more cubic meters.
Fraser Cain: Got it. So the more universe that opens up thanks to the expansion and the acceleration, thanks to dark energy, the more you get more of that pushing force that’s coming from all that additional space that’s been created. And so then, once we learned about the Hubble tension and once we saw that the error bars had overlapped, then astronomers needed to look for new physics as one possibility.
If we know that the measurements are accurate and they’ve been double checked and triple checked, then where do the new physics, what kinds of new physics would explain this?
Dr. Pamela Gay: So we were looking for basically three different potential solutions. The most straightforward and least likely, as near as we can tell, is that we don’t understand gravity, that gravity at the largest scales doesn’t work the way we suspect it does. And we know from the fact that general relativity breaks down in the cores of black holes that there is something lacking in our understanding of gravity.
We know from the fact that we can’t unify gravity with the other forces that there’s something different in how we need to understand it than the way we currently understand it.
Fraser Cain: Right. But our observations of gravity at the largest scales, at the smallest scales, hold consistent.
Dr. Pamela Gay: Yeah. So the only place that gravity so far doesn’t work so well for us is the cores of black holes. So the next place that we start looking is maybe our observations are foobarred.
And Adam Riess has single-handedly been driving the entire field to re-evaluate in excruciatingly painful detail all possible errors in our measurements of all standard candles. So our understanding of what is the period luminosity relationship for Cepheid variables has been re-examined in excruciating detail.
Fraser Cain: With Webb.
Dr. Pamela Gay: With everything over time. He has figured out how to do things with Hubble. Hubble was not designed for to get better photometry out of it.
Right.
Fraser Cain: But every time a new tool appears, he says, let me just check the distance ladder with that tool, please. And has made the most accurate version. He did it for Cepheid variables and he recently completed it for Type 20 supernova.
If something better comes along, he’ll use that too.
Dr. Pamela Gay: Yeah. And so far that hasn’t been a source of error that we can rely on. Which is a really strange statement.
But like we want to find some source of error that is systematic over distance that allows us to know that the supernovae results or something are wrong. So we just haven’t found that. So then the next place to look is, OK, if if we go and we look in even greater detail than what we’re currently doing, if we push our measurements back further in the universe, if we look at earlier times, can we bridge between the cosmological results from the baryon acoustic oscillations, the hot and cold spots in the cosmological background?
Can we bridge that result with our modern results just by starting to fill in more of the data? Right.
Fraser Cain: And there’s like a six billion light year gap, six billion year gap between where the Type 20 supernovae end and where the CMB begins. And there just isn’t anything great in that. People look at quasars, people consider gravitational waves from colliding black holes and these get you part of the way.
But their error bars are too big that they’re not helpful in resolving this issue.
Dr. Pamela Gay: And so we occasionally find one off good things that we can use the timing of gravitationally lens multiple time background galaxies, things like that. But there’s just not enough of this. So the slow and study work we’re doing, because when all else fails, do the slow and study really hard stuff.
This is where the dark energy spectroscopic instrument. This is where the Nancy Grace Roman telescope, all of these instruments are being built to slowly and carefully map out the positions of millions and millions of galaxies going back in time to look at the evolution of the large scale structure of the universe.
Fraser Cain: We just got an image from the Euclid mission, which is also a part of this team where they had done their version of their first version of the deep field, which is kind of like the Hubble deep field. I think Hubble deep field, its original one, gave us 100000 galaxies in this little spot in the sky. You could give us 26 million galaxies in a lot of galaxies in its deep field.
So the capability of the we’ve got the right tools at the right time for the right mystery. And we will continue this conversation in a second, but it’s time for another break and we’re back. All right.
So we’re going to talk about Desi and we’re going to talk about some of the other ones as well. But the question I guess is if dark energy changes over time, then that will beautifully explain this discrepancy between what we saw in the CMB and what we see today. Because the amount of dark matter, dark energy, the amount of dark energy flowing into the universe could have been variable and then done.
You have your explanation.
Dr. Pamela Gay: And this is where researchers, theorists in particular, are working really hard to see was there something that happened around when the cosmic microwave background formed between then and when the universe was a couple of billion years old? Was there something in that window where some force came into being that hadn’t previously existed that can explain this? And one of the things that like I totally went down a rabbit hole one weekend, I messaged you this could be a Nobel Prize.
There’s a team looking at the way that we model black holes in the Robertson-Walker metric and they were looking at how our models weren’t correctly taking into account rotation. There’d been some assumptions made to simplify things and reworking their equations, adding in details instead of assumptions. They were able to come up with a model that basically said the formation of black holes coupled to space-time allows black holes to grow faster than current models would have predicted and that in the process of them growing, this would be counterbalanced with a repulsive dark energy-like force.
And looking at data, they were able to go through and seemingly demonstrate that black holes had grown in ways that weren’t predicted by the morphology of the galaxies that they occupied. But there’s also a whole bunch of papers in the literature saying no, no, they cherry-picked, they cherry-picked, don’t go there. And so we’re now in this point where, yes, there are clearly some black holes that are not what we would have expected, but there’s others that aren’t.
What does this mean? Is this the right road to go down? And maybe, but probably incomplete at this point.
Fraser Cain: And it’s pretty weird to imagine that the bigger the black holes are, there’s some connection between black holes and dark energy. I’ve interviewed people who have been pitching the science and I’m like, okay, so what’s the mechanism? And they’re like, we don’t know.
Dr. Pamela Gay: Yeah, it falls out of the math. That’s all we know is it falls out of the math.
Fraser Cain: Yeah, yeah. All we see is that there’s a correlation. Yeah.
And so I think, you know, bringing the story back around, there’s where DESI, you know, from the dark energy spectroscopic instrument, you get this. This correlation seems to be there. But, but in more general, the, the, you know, we’re, we’ve gotten the first data results from DESI, the first of what will become many years of this there, this is their first crack at it.
And that you are seeing support for the model that dark energy is variable and not constant. And this is, and this is being revealed in the data that is getting released from DESI.
Dr. Pamela Gay: And this is based on the distribution of large scale structure as a function of time, looking at the distribution of galaxies in the DESI sample.
Fraser Cain: Right. And so one thing, and that’s the gold standard.
Dr. Pamela Gay: Yeah.
Fraser Cain: Right. That, that seeing the large, seeing the expansion of the large scale structures in the universe, what began as those baryonic acoustic oscillations, as you mentioned, those hot and cold spots turned into filaments of galaxies at the largest scales that we see today. And as we watch those expand as dark energy is pushing them apart, that is the, that is the gold standard.
That is the one that is the hardest to explain by any other system. The one that, you know, you may say, okay, great. We figured out how dark energy works.
We figured out how dark matter works. You know, maybe it’s just, we don’t understand gravity. We’ll explain those large structures accelerating away from each other in all directions.
So I just wanted to sort of like, what’s exciting about DESI is how this reinforces this idea that dark energy is increasing using the most important observations that tell us that these things are happening.
Dr. Pamela Gay: And they’re not just looking at the bright galaxies and quasars. They’re also looking at the dark Lyman alpha forest lines. So cold gas will absorb the continuum light from background galaxies as it passes through in the Lyman alpha transition.
This is the one to two transition in the hydrogen atom. And so what you have is wherever there is a blob of cold material between us and a distant quasar or light source, basically, that light from the distant source will end up with a forest of lines created at the red shifts of each of these clouds. So this Lyman alpha forest allows us to map out the locations of more than just the bright emission line galaxies that is what we’re normally looking at.
So with DESI, we’re seeing a mapping of the distribution of mass as both the luminous galaxies and the Lyman alpha forest gas clouds.
Fraser Cain: And so then if dark energy was changing over time, what would that sort of, how would that change manifest itself? Like would it have started stronger in the beginning and then been and then been reducing over time?
Dr. Pamela Gay: So what they’re finding is that that is what it seems to be hinting at. And this is like a three to four sigma result. This is not the gold standard six sigma we dream of.
Fraser Cain: To be fair, three sigma is ninety nine point seven percent. Four sigma is ninety nine point nine five percent. And those are pretty good.
Dr. Pamela Gay: The issue is that there’s multiple solutions that fit equally well at that level. So their results, if you only look at the galaxies, are completely fit by the lambda cold dark matter models where we assume a set amount of dark energy. We assume dark matter has a certain temperature distribution.
Let the universe go. That still works. But when you start to combine what they’re seeing with the supernova results, what they’re seeing with other cosmological results, everything together seems to maybe we’re waiting for more data releases, fit better with a dropping over time amount of dark energy.
Fraser Cain: Wow. So like big rip averted?
Dr. Pamela Gay: Maybe, we don’t know. I mean, that’s the crazy thing is, is we’re at a point in time where like the thing we we could use most to get rid of the Hubble tension is something that that births dark energy like Athena from Zeus’s skull sometime after the formation of the cosmic background and then allows it to taper off over time. So if you have dark energy springing into existence and then petering out, that will get us a universe that may avoid a big rip and may also avoid the Hubble tension.
And theorists are grasping at straws. When I had my this set of research papers could lead to a Nobel Prize, false reading of the literature because I cherry picked what I was reading. I tweeted, and as you do, and another researcher was, well, no, I actually, yes, anything that can explain dark energy and come into existence at roughly the one billion year point in the universe, everyone’s going to jump on that.
Anything that suddenly starts to exist about that point is going to get blamed for being dark energy.
Fraser Cain: Yeah. And I think like this is going to sound super weird, but having a constant amount of dark energy is a perfectly understandable outcome you could have for the universe that we know that there are quantum fluctuations that are going on across the universe, that the very nature of space time itself is this bubbling, roiling mass of quantum fluctuations. And so if you have a cubic meter of space, then you’re going to have all of, you know, you think of this, this idea of like virtual particles popping in and out of existence, that this is what space time does.
And then you could then say, well, and that makes sense then that, that these, this roiling quantum fluctuations is an outward force that pushes. We know that it can cause the Casimir effect. So we know that there’s, that this force can exist.
And so you’d be like, yeah, that, you know, that, that makes sense. And yet. But for it to be very, for it to have appeared at a random time and for it to be declining over time, that is hard to explain.
And the theorists, as you say, have got their, their work cut out for them now.
Dr. Pamela Gay: And, and this is where, like you were saying, we’re only on the second data release for the, the DESI team. Roman hasn’t launched yet. Euclid’s just starting to return data.
Fraser Cain: Ruben doesn’t show up till the end of the year.
Dr. Pamela Gay: Yeah.
Fraser Cain: Yeah. SpaceX just launched.
Dr. Pamela Gay: Yeah.
Fraser Cain: So there, there are five, six powerful missions and ground-based observatories that are going to be able to come together and give us the most definitive answer to this question. But we are seeing the early glimpse. We’re seeing the preview.
We’re seeing some of the cameos. And we’re excited already about this show, but we got to just wait for all of those data to finally come in. And we are, but I feel like five years from now, we will have a real, like we will look back on, on past us’s and go, you simple children.
Dr. Pamela Gay: Well, I just love the fact that, that since the Nobel prize came out, while we were already recording this show, we’ve gone from dark energy is constant with time to, and, and that, that change is fabulous. And, and when you gave us the title for, for this week’s show, dark energy changing with time, it was just like, over time, our understanding has changed and over time it may be changing. And this is all just excellent.
And I love every moment of it.
Fraser Cain: And like, again, to the people who think that scientists are trying, are living in some kind of dogmatic hegemony where they just chant from the scriptures, the accepted truths that have been handed down for generations. Like, no, they love it. They love the change.
They love to see new evidence and they love to change their minds and, and consider the possibilities as new evidence comes in. But they also place a high bar for where things must stand to be convincing. And, and now.
Dr. Pamela Gay: We do find hills to die on. Sanditian Devocalors each had a hill that they did literally die upon.
Fraser Cain: Right, right. But most don’t. And I think most love the, the greatest delight is to be proven wrong.
Dr. Pamela Gay: Yeah.
Fraser Cain: Thank you, Pamela.
Dr. Pamela Gay: Thank you, Fraser. And thank you so much to all our patrons out there. This week, I would like to thank Keith Murray, Thomas Gazzetta, Steve Rootley, Maxim Lovett, Bebop Apocalypse, Dr. Woe, Danny McGlitchie, Jean-Baptiste Lemontier, Frodo Slavo, who’s given up on trying to teach me how to pronounce his name. I’m so sorry. Just Me and the Cat, Van Ruckman, TC Starboy, Michael Prichata, Burigowin Boreantrolevsval, Ed David, Buzz Parsek, Joe Holstein, Kenneth Ryan, WandererM101, Felix Goot, Dr. Jeff Collins, Greg Davis, MHW1961, Supersymmetrical, Bart Flaherty, Matthew Horstman, J. Alex Anderson, Kimberly Reich, James Roger, Scott Bieber, Daniel Loosley, Greg Vylde, Mark Steven Raznick, Janelle, Michelle Cullen, The Air Major, The Big Squish Squash, Justin Proctor, Don Mundus, Mark Phillips, Larry Dotz, Stephen Miller, Paul Esposito, Ron Thorson, and Daniel Donaldson. Thank you all so very much. Thanks, everyone.
And we will see you next week. Bye. AstronomyCast is a joint product of Universe Today and the Planetary Science Institute.
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