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It’s been over 20 years since astronomers first discovered that the expansion of the Universe is accelerating thanks to dark energy. And in these decades, astronomers still don’t have much evidence for what could be causing the increased expansion rate. Maybe there’s something else going on to explain it.
Show Notes
Escape Velocity Space News (CosmoQuest)
PODCAST: Ep. 4: The Search for Dark Matter (Astronomy Cast)
PODCAST: Ep. 11: A Universe of Dark Energy (Astronomy Cast)
The Nancy Grace Roman Space Telescope (NASA JPL)
South Pole Telescope (University of Chicago)
Physical Review Letters (APS)
Hot new early dark energy: Towards a unified dark sector of neutrinos, dark energy and dark matter (Physics Letters B)
Casimir Self-Interaction Energy Density of Quantum Electrodynamic Fields (Physical Review Letters)
Dark matter (CERN)
Dark Energy (Hubblesite)
Doppler Shift (Swinburne University)
Equation of state (cosmology) (Wikipedia)
Hubble’s Exciting Universe: Measuring the Universe’s Expansion Rate (Hubblesite)
Planck (ESA)
WMAP- Content of the Universe (NASA)
Supernovae Were Discovered in all These Galaxies (Universe Today)
What are Cepheid Variables? (Universe Today)
Carnegie Supernova Project II: The Slowest Rising Type Ia Supernova LSQ14fmg and Clues to the Origin of Super-Chandrasekhar/03fg-like Events (The Astrophysical Journal)
Research team discovers unique supernova explosion (Phys.org)
Planck and the cosmic microwave background (ESA)
Hydrogen Epoch of Reionization Array (HERA)
FOLLOW-UP: What is the ‘zero-point energy’ (or ‘vacuum energy’) in quantum physics? Is it really possible that we could harness this energy? (Scientific American)
First principle (Wikipedia)
WMAP Inflation Theory (NASA)
Could a Dark Energy Phase Change Relieve the Hubble Tension? (Universe Today)
Michael S. Turner (University of Chicago)
What is the Casimir effect? (Scientific American)
The Higgs boson (CERN)
Particle Fever (2013) (IMdB)
Transcript
Transcriptions provided by GMR Transcription Services
Fraser Cain: Astronomy Cast Episode 669, Challenges to Dark Energy. Welcome to Astronomy Cast, 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, 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 Cosmo Quest. Hey, Pamela, how you doing?
Dr. Pamela Gay: I am doing well. The third episode of our new TV show aired Saturday and we got it to the station 90 minutes before it was due, due to computer crashes, because computers know.
Fraser Cain: Yeah.
Dr. Pamela Gay: But if you haven’t checked it out yet, it’s Escape Velocity Space News. It airs on Now Media and I am gonna be putting together a podcast version and loading that up later today.
Fraser Cain: Congratulations, that’s amazing. It’s been over 20 years since astronomers first discovered that the expansion of the universe is accelerating thanks to dark energy. And in these decades, astronomers still don’t have much evidence for what could be causing the increased expansion rate. Maybe there’s something else going on to explain it.
If you go back into the archive of Astronomy Cast shows, one of the first episodes that we did was Dark Matter and then after that was Dark Energy, like within the teens anyway and we’ve brought it up a couple of times since them, but I had been expecting in the 15 years that we’ve been doing this show that we would have something more to say on the matter. But to be honest, not much has been figured out, apart from some really interesting new surveys, like the Dark Energy Survey Telescope, the development and eventual launch of the Nancy Grace Roman Telescope. We still have no idea what this is.
Dr. Pamela Gay: One of the things that actually caught me by surprise is I was expecting to spend today talking about the new results from the Dark Energy Survey and the South Pole Telescope, and there are actually two really cool papers that have both come out in Phys Rev Letters in the past couple of months that are both clearly the, hi, we’re theorists, we have predictions, there are only two of us on this paper, please give us the Nobel Prize when our predictions prove true.
Fraser Cain: Right.
Dr. Pamela Gay: And so, we may be getting to the point that theorists are starting to figure out how to get a handle on things, and they’re finding answers that may also help confine dark matter, which is kinda cool.
Fraser Cain: Well, that’s great. So, then, I guess, what is the evidence for dark energy?
Dr. Pamela Gay: Basically it comes down to when you measure the distance to a supernova and you measure the rate at which that galaxy is moving away from us using doppler shifts. We find that the universe is actually accelerating over time, which is not something that was in anyone’s predictions but it was in the math in the form of a constant to the equations of state for the universe. When Einstein originally came up with these equations there was an integration factor, when you integrate you have to add a constant, and he assumed that the constant would have a value that caused the universe to be static.
A few years later, Hubble came along, found the universe is expanding. Now we have a constant that makes sense for that. But it turns out that if you have a universe that is accelerating apart, that is a value for the equations of state, and so now what we’re finding is, in order to explain the geometry of our universe, which is flat, flat, flat, very flat, and an accelerating universe, you have to have 70% of the universe made of something that isn’t observable matter, that’s about 4-1/2 percent of the universe, that isn’t dark matter that gets observed through gravitational lensing, gets observed through rotation curves of galaxies, gets observed through the motions of galaxies and clusters, that’s 27-ish percent and instead you have dark energy.
Fraser Cain: So, this measurement really relies on how good the measurements to those type 1A supernovae are.
Dr. Pamela Gay: Yes. And it also comes down to, once we realized, okay, so we have from the Planck Observatory the flat geometry and then you combine it with everything else and you look at the mass density of the universe, the only way to explain the mass density of the universe is to have this extra stuff as well. So, you can get to dark energy from a couple of different ways but to get at the value that we’re seeing, yes, that came very clearly from the 1998 observations that the universe is accelerating with time and its expansion.
Fraser Cain: And we did some coverage on a new database that came out a couple of months ago, where astronomers had gone through and like recalibrated, normalized all the data for about 1,000 total type 1A supernova measurements and if anything, have gotten even more accurate.
Dr. Pamela Gay: Yeah.
Fraser Cain: You overlap the Cepheid variables with type 1A supernovae, the distance ladder is beautiful, the error bars are ever shrinking and the amount of dark energy in the universe is zeroing in on this. They’re really nailing this number.
Dr. Pamela Gay: And one of the wild things about this is we keep trying to find an excuse that maybe further back in the universe these kinds of supernovas, due to the change in the chemistry, would have different properties, and we do keep observationally finding random exceptions. There was a super cool type 1A supernova that went off while inside of another star, which is one way to blow up a white dwarf.
Fraser Cain: You can imagine that would pollute the results a little bit.
Dr. Pamela Gay: Right. But these one-offs that we’re finding are super cool but they are one-offs. The vast majority of the type 1A supernovae are just boring white dwarfs that ate more than they could hold without changing states.
Fraser Cain: Right. And so I guess one possibility is that the type 1A supernovae aren’t the standard candles that astronomers had always believed but the evidence is continuing to build that yes, indeed, they are.
Dr. Pamela Gay: Yeah.
Fraser Cain: Except for these ones where one star blows up from inside another star.
Dr. Pamela Gay: Yeah.
Fraser Cain: Right.
Dr. Pamela Gay: And so, I mean it’s just like saying most human adults are between five foot and six foot, yes, there are people who are only three foot and there are people who are only seven foot, but the vast majority of us are between five and six foot. So, yeah, it’s averages.
Fraser Cain: So, then, let’s talk about some of the largescale surveys that have been developed to try to get to the heart of dark energy. Not necessarily explain it, I guess, but at least to confirm it, map it, try and nail down its parameters.
Dr. Pamela Gay: So, the big one with the obvious name is the Dark Energy Survey, which was done from down in Chile where they observed with extreme sensitivity vast swaths of the sky with the goal of looking to see how the structure of the universe evolved with time. So, the idea here is, we know that the universe started out as pretty much smooth distribution of particles. It wasn’t even anything more fancy than particles initially, with slight over and under densities that were created by soundwaves moving through the early universe.
That mostly smooth distribution that we can measure in the cosmic microwave background, then had to collapse down into galaxies, stars at the smallest scales, but then clusters of galaxies, walls of galaxies, super clusters at the largest scale and it does that over time. And we have models that basically say okay, here is the CMB, here is the modern universe, let’s fill in in between and the Dark Energy Survey was designed to get at the more recent few billion years.
The next survey that is super exciting to look at is HERA, which is being done in the radio, looking at the redshifted 20-centimeter line of cold hydrogen, that the detectors for this, they’re looking at wavelengths of light that instead of being just 20 centimeters are instead many, many feet, so, longer than us. And in these longer wavelengths we are able to start seeing how cold hydrogen was distributed in the early universe, start piecing together how cold gas clumped and then got re-ionized in the era of re-ionization.
And between these different surveys, we’re working our way through measuring what was the structure over time so that we can better confine our models and say, okay, was the amount of dark energy constant over time. Was there some sort of a phase transition? Was there a kick somewhere? And these are the kinds of questions that folks are trying to answer, is what observables can dark energy give us that will help us confine our theories?
Fraser Cain: And so, in addition to the type 1A supernovae measurements, they’re able to now look at these galaxy clusters and then on top of that they’re looking at the concentrations of these regions of hydrogen gas. And so, you’ve got like three independent lines of observation that theoretically should allow you to map out the amount of dark energy.
Dr. Pamela Gay: Yes.
Fraser Cain: And is the assumption that they’re gonna find it? Like I know the Dark Energy Survey is still partway through its survey. You didn’t even mention the Nancy Grace Roman, it’s gonna be launching in 2025, and –.
Dr. Pamela Gay: Well, you don’t count your satellites until they’re orbiting.
Fraser Cain: Fine, fine, okay. Pamela, put your hands over your ears. Audience, the Nancy Grace Roman, one of its main jobs is going to be to characterize dark energy, doing kinda the same thing that the Dark Energy Survey is but from space. So, this is classic, right. Like if you don’t have an easy answer then you build more instruments, more observatories. You keep trying to characterize the nature of the problem, building hypotheses, testing them against your observations, removing bad ideas one after the other and hopefully trying to pin down the final thing. So, we may never know what’s causing dark energy but we’ll have it measured to Six Sigma accuracy.
Dr. Pamela Gay: Well, and the other side of it is the particle physics side. So, we know that dark energy, whether it’s a force, an energy, a field theory, whatever it is, puts into every cubic meter of space basically a proton-ish worth of energy. And so, how do you explain that energy existing? And by better understanding the quantum mechanic side of the universe, the particle physics, the vacuum energy, are sterile neutrinos actually a thing or not, this is another way of coming at the fullness of the universe by looking at the smallest factors inside of it.
Fraser Cain: All right. So, I guess, you found a few papers that have been proposing some alternative explanations that would explain the observations but not necessarily be new energy that’s being injected into every cubic meter of the universe, which I guess sounds satisfying. Like the fact that energy is appearing out of nowhere, that’s unnerving, so what are they proposing?
Dr. Pamela Gay: So, the first paper in here, I have to look at my notes, the first paper came out from Martin Sloth and Florian Niedermann, where they’re looking at new early dark energy. And the idea here is our universe has undergone phased transitions in terms of the energy of the entire universe. So, the first massive phased change occurred in the first fractions of a second, where we essentially went from every basically molecule sized bit of the universe expanding out via inflation, which we also don’t know what is, to be about the size of the observable universe according to some ways of looking at it.
And this massive, fairly instantaneous epic of inflation may have only been one of two phased transitions or there could have been a later phased transition that, if there testable ideas are right, leads through first principles to having a cosmological constant of 72, which is within error bars of what we see for the modern universe, and fixes the discrepancy we see with the old universe.
Fraser Cain: And so, sorry, so like, the idea of inflation –.
Dr. Pamela Gay: Yes.
Fraser Cain: Happening in just the first fraction of the Big Bang was developed I think back in the ’70s –.
Dr. Pamela Gay: Yeah.
Fraser Cain: To help explain a lot of the problems with the Big Bang. Like the Big Bang beautifully explains the universe as we see it today, but there are these flaws in the theory. How can vastly separated parts of the universe be similar temperature, there’s a bunch of these ideas. And so, one of the theories is that, in fact, there was this period of rapid inflation that carried everything away from each other really quickly and then it settled down.
So, I think baked into modern cosmology is already this idea, as you say a phased changed, a dramatic change in the expansion rate of the universe. So, it doesn’t seem that surprising that there could then have been others later on.
Dr. Pamela Gay: Right.
Fraser Cain: So, when would have this other, they’re calling it, what, early, what are they calling it, early –?
Dr. Pamela Gay: They’re calling it new, early dark energy.
Fraser Cain: Dark energy. Right, okay.
Dr. Pamela Gay: So, NEDE is the abbreviation.
Fraser Cain: New early dark energy, right. And so when would this have occurred in the timeline of the universe?
Dr. Pamela Gay: This still would have occurred during what they refer to as the dark times prior to the release of the cosmic microwave background. And it fits in with the way – Michael Turner, who is a prominent cosmologist, now professor emeritus from the University of Chicago, he says there’s basically these three unknown pillars of cosmology, inflation, dark energy which he actually named, dark matter. And so, this looks at that early period, makes some solid predictions for temperature details that we should be able to see with enhanced continuing to look at the cosmic microwave background, which tells us everything apparently.
But it also makes some finite predictions for what kinds of neutrinos should be out there, and what kinds of specific particles we can expect to find, thus, also perhaps explaining dark matter. So, we have one coherent theory spelled out in a letter-sized research article making concrete predictions. It seems good and just needs tested now but it’s not the only one out there.
Fraser Cain: All right. Well, let’s talk about the other one, then.
Dr. Pamela Gay: So, the other one is by Alexander, and I’m going to mispronounce this and I am sorry, this appears to be a Ukrainian last name, Tkatchenko, and the other one is Dmitry Federov. And they look at the vacuum energy of the universe, and vacuum energy is something we know is real because of the Casimir effect, which is just one of the best named effects in particle physics. The dude’s name was Casimir but it just sounds cool to say.
And what the Casimir effect says is if you take two plates that are capable of conducting electrons and you put them extremely close together but you’re not actually running charge through them and they’re not being exposed to any fields, nothing should happen I a vacuum. But the reality is, if you put these two plates just a couple nanometers apart within a vacuum, they will either attract or repel due to the constant creation and destruction of virtual particles that are in their creation and destruction, creating a field. So, if you have a proton spring into existence as a virtual particle between these two plates, that proton has a field and it creates the effect.
Fraser Cain: Right.
Dr. Pamela Gay: And we see this.
Fraser Cain: Right. And if I understand, like the Casimir effect, because the gap is so small the virtual particles of only certain sizes can pop into existence outside or as a field.
Dr. Pamela Gay: Right.
Fraser Cain: Outside the plates and then they are pushing inward, there is essentially a field on the outside and not so much field on the inside and it’s pushing on these plates. And once you move the plates farther and farther apart, now there’s room for the fields to appear both in between the plates and outside the plates.
Dr. Pamela Gay: Yeah.
Fraser Cain: And then that force goes away. And it sort of shows you the presence of this vacuum energy that is everywhere. And it’s been beautifully measured, so no one argues with the existence of vacuum energy. The assumption, though, is that this energy cancels out, goes away very quickly and doesn’t provide any ongoing force to the universe.
Dr. Pamela Gay: And that’s one school of thought. Another school of thought is, no, it’s totally providing dark energy-like forces, but when they do all the maths, they have consistently come up with an amount that is too large by a factor of 10 to the 120 for the theories that I like the best and that are among the prominent theories. And the best case for getting it down has, until this paper as far as I know, been 10 to the 30th. And when you’re off by somewhere between 10 to the 30 and 10 to the 120, it really doesn’t matter.
Fraser Cain: That’s a lot. Yeah.
Dr. Pamela Gay: It’s a lot. And this is where these two researchers, Tkatchenko and Fedorov, they said, well, okay, what if the vacuum energy has a polarizability, that the particles can actually have alignments that affect what can and can’t come into existence and annihilate. And they make again solid predictions that should be testable if we go looking with new technology that we don’t currently have laying around.
And it’s just one of those nice, simple, elegant ideas of we know stuff can be polarized. We know particles can be aligned. What if just the universe as a whole has this polarizability characteristic to it that we just hadn’t been including? And again, with this research, it makes set predictions on what to go look for. It looks at the particles that are out there and says, okay, here is what to see in particular physics and I don’t know which team I’m rooting for more. I like the polarizability one.
Fraser Cain: Why choose?
Dr. Pamela Gay: Well, I mean that’s – it’s two theory papers making predictions. Both of them have only two authors. Both papers are clearly gearing up for a Nobel Prize. It’s always good to cheer for people but I guess I’ll cheer for both.
Fraser Cain: Yeah. When you think about the winning Nobel Prizes, many of them start this way.
Dr. Pamela Gay: Yes.
Fraser Cain: That a theorist puts together a paper and says there should be a particle called the Higgs boson, right, and then 30 years later experimenters are finally able to find it, and this is the same thing. So, I mean I guess if dark energy makes you uncomfortable already –.
Dr. Pamela Gay: This doesn’t help.
Fraser Cain: Buckle up. Yeah.
Dr. Pamela Gay: Yeah.
Fraser Cain: You still got another 30 to 50 years of searching and scanning and trying theories to try and narrow in on an answer to it.
Dr. Pamela Gay: And if you want to get a feel for what it’s like within the field of particle physics to watch people so clearly chasing Nobel Prizes, watch the moving Particle Fever, it is very delightful. It shows the good, the bad and the ugly of scientists being scientists, and just the joy that comes from people getting to see their dreams come true through instrumentation.
Fraser Cain: I will check that out.
Dr. Pamela Gay: It’s really cool.
Fraser Cain: Yeah, that’s awesome. All right. Well, thanks, Pamela. I hope within the lifetime of Astronomy Cast we will do the show we finally know what dark energy is. After we do the show where we say we finally know what dark matter is, we will do both those shows. This is my promise to all of you.
Dr. Pamela Gay: And this is where it’s just sort of like we can hope, and when that finally happens is that when we retire?
Fraser Cain: No, no.
Dr. Pamela Gay: Okay.
Fraser Cain: Because there will be there will be a hundred new mysteries that are even –.
Dr. Pamela Gay: We keep going until we die.
Fraser Cain: Yeah, there will be a hundred new mysteries that are even more complicated and more troubling, so, no, that’s how this whole process works. All right, Pamela, thanks a lot.
Dr. Pamela Gay: Thank you. And thank you to all the patrons out there. I don’t actually have a thing of names, I just discovered, to pull out because it’s the very beginning of the month and the list hasn’t been generated yet but I do want to say thank you to all of you. We have been reading the names of just a subset of you on air, it’s one of the perks associated with the different levels but we love all of you, whether you’re a $1.00 a month contributor or a $100.00 a month contributor. You allow us, through our ongoing efforts, year after year to allow us to pay our humans and keep the show going, and occasionally replace cables from my camera when they die. So, thank you.
Fraser Cain: I will take this opportunity then to sort of mention a trend, a disturbing trend that you who are a fan of educational content should be aware of and that is artificial intelligence generated content.
Dr. Pamela Gay: Yeah.
Fraser Cain: That more and more websites are moving to this model of getting ChatGPT and other artificial intelligence to generate en masse the material and this trend is going to accelerate.
Dr. Pamela Gay: Yeah.
Fraser Cain: And it’s just another career that is gonna be –.
Dr. Pamela Gay: Go away.
Fraser Cain: Go away, sunsetted. The career of the science communicator and that’s because it’s way cheaper to let a large language model generate explainer content. And hopefully, Pamela and I have value and place in this society as more and more of this content shifts into being generated by enormous databases. I know it sounds shocking and surprising but this is where it’s all gonna go and it’s gonna happen startlingly fast. So.
Dr. Pamela Gay: We have had people write in and say, hey, why don’t you switch over to using ChatGPT to generate your scripts.
Fraser Cain: Right.
Dr. Pamela Gay: It’s already people are suggesting it.
Fraser Cain: Yeah, yeah.
Dr. Pamela Gay: And we won’t do that.
Fraser Cain: Yeah. I will get ChatGPT to recommend ideas but I won’t –. See, here’s the key, there are no scripts, that’s the trick.
Dr. Pamela Gay: Right, right.
Fraser Cain: So, how could we have a script if there are no scripts? Smart. Anyway. So, if the work that we’re doing, like if the loss of science communicators at the mainstream media was already troubling to you, now they’re coming for all science communicators. So, if supporting the work that we do is important to you to make sure that we can keep doing the show, paying the people on the team, keeping the servers running, all of that, join our Patreon.
Dr. Pamela Gay: Thank you.
Fraser Cain: Patreon.com/astronomycast. Thanks, everyone, and we’ll see you next week.
Dr. Pamela Gay: Bye-bye.