Ep. 668: The Crisis in Cosmology

Astronomers have made extremely accurate measurements of the expansion rate of the Universe and come up with different results. And the error bars for the observations don’t overlap, so there’s something strange going on. What’s the answer and how can the Crisis in Cosmology be resolved?

Download MP3 | Show Notes | Transcript

Show Notes

See Comet ZTF (C/2022 E3) Dash Between Big and Little Dippers (Sky & Telescope)

Comet Hale-Bopp (NASA JPL)

Comet Hyakutake (NASA JPL)

Comet NEOWISE, the best comet of 2020 (EarthSky)

Comet McNaught over the Pacific Ocean (ESO)

Hubble Tension Headache: Clashing Measurements Make the Universe’s Expansion a Lingering Mystery (Scientific American)

Hubble telescope refines universe expansion rate mystery (Space.com)

Megaparsec (Swinburne University)

Messier 87 (NASA)

Andromeda galaxy: All you need to know (EarthSky)

Ask Ethan: Is there a better way to measure cosmic time? (BigThink)

Type Ia Supernova (Swinburne University)

How do astronomers measure the brightness of something? (Astrobites)

Luminosity (Swinburne University)

Standard Candle (Swinburne University)

What are Cepheid Variables? (Universe Today)

Gravitational Lensing (Hubblesite)

Dr. Adam Riess (Space Telescope Science Institute)

Astronomical deep-sky photometry and spectroscopy (BBC Sky at Night)

Gaia (ESA)

Baryon Acoustic Oscillations (NASA)


Planck (ESA)

LAMBDA – ΛCDM Model of Cosmology (NASA)

Astronomers Grapple with JWST’s Discovery of Early Galaxies (Scientific American)

How Did Inflation Happen — and Why Do We Care? (Space.com)

The Big Bang (NASA)


Cosmic Inflation Theory Faces Challenges (Scientific American)

Sloan Digital Sky Survey

The Dark Energy Survey


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Transcriptions provided by GMR Transcription Services

Fraser Cain:                 AstronomyCast, Episode 668, “The Crisis in Cosmology.” Welcome to AstronomyCast, your 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 CosmoQuest. Hey, Pamela, how you doing?

Dr. Pamela Gay:         I am doing well. How are you doing?

Fraser Cain:                 Doing great. Yeah, nothing to report.

Dr. Pamela Gay:         That, I think, in 2023, is really the best any of us could ever ask for.

Fraser Cain:                 Yeah, the weather is fine, the snow is gone, I’m getting out and doing a bunch of hikes and stuff in the nature, even though it’s wintertime, but still, garden’s coming along.

Dr. Pamela Gay:         Dang.

Fraser Cain:                 I saw the comet last night. It sucks, but…

Dr. Pamela Gay:         Oh well.

Fraser Cain:                 Yeah, it’s gotten quite diffuse at this point, so although it’s bigger and brighter, it’s also more of just a cloud.

Dr. Pamela Gay:         It’s spread out, so the light from any given place – its surface brightness is really low.

Fraser Cain:                 That’s right, so you really don’t get that nice, little, tight nucleus with the tail, you just get this – what looks like a little cloud in the sky. But still, it’s easy to find. It’s so easy to find because it was right beside Ursa Minor, right beside the Little Dipper, and then, it’s moving towards Cassiopeia. So, if you have never seen a comet – you can’t see it with your eyes, but you can see it in a pair of binoculars or a small telescope. It’s easy to find, and that’s nice, as opposed to one where it’s in a fairly difficult constellation to discover. But unfortunately, completely inaccessible now to the folks in the Southern Hemisphere, so this one is just for the folks in the north. So, if you haven’t already –

Dr. Pamela Gay:         Who do not have cloud like the Midwestern folks in the north.

Fraser Cain:                 Right, yeah. It’s gonna peak in just a couple of days from now, so now is your chance, and then it’s just gonna get – but it mostly sucks. I always compare comets to Hale-Bopp and Hyakutake, and people are even like, “Oh, didn’t you like Comet NEOWISE?” I’m like, no.

Dr. Pamela Gay:         No.

Fraser Cain:                 No, it sucked. I could see it with my eyes. That does not a good comet… That is necessary – sufficient but not necessary? Anyway, just barely being able to see a comet with the unaided eye does not – you do not declare victory in the comet world. No, you want the one that is gigantic –

Dr. Pamela Gay:         The tail.

Fraser Cain:                 The tail spans multiple handspans across the sky, that you can see it even in light-polluted skies. That’s a comet, and everything else the universe is sending our way right now is mediocre, and I reject them. So, no, NEOWISE sucked, McNaught sucked, this one sucks. We demand better. I will wait, but I’ve been patient for too long. Come on, comet!

Dr. Pamela Gay:         I…I can’t argue with that. All of that is true. It’s all true.

Fraser Cain:                 Yeah. For people who are like, “Oh yeah, Comet NEOWISE was fine,” no, it wasn’t! It wasn’t, and you’re settling. You deserve better. I deserve better. We deserve better comets. The universe can provide it; it’s done it in the past. It’s time to put up or shut up. All right.

Dr. Pamela Gay:         Yeah…

Fraser Cain:                 Astronomers have made extremely accurate measurements of the expansion rate of the universe and come up with different results, and the error bars of the observations don’t overlap, so there’s something strange going on. What’s the answer, and how can the crisis in cosmology be resolved? So, what is the crisis in cosmology?

Dr. Pamela Gay:         So, some people call it the Crisis, some people call it the Hubble Tension. A lot of us just put “WTF?” and call it a day. So, what’s happening is before the supernova teams did such an amazing job of measuring the present expansion rate of the universe, we were like, “Nah, the universe is expanding somewhere between 50 kilometers per second per megaparsec to 100 kilometers per second per megaparsec,” and I had so many profs that were like, “Just use 100. It makes the numbers easier.” It was pleasing.

Fraser Cain:                 Right. Just so people understand this idea, that you take a megaparsec of space, which is about 33 million lightyears of space, and when you think about that, that is the distance between us and faraway galaxies, like Andromeda’s really close. We’re talking about galaxies that are 30 million lightyears away, like M87 is kind of in that – when you think about the supernova image. Every second that goes by, those objects are now 100 kilometers farther apart, or 50 kilometers farther apart.

Dr. Pamela Gay:         And it is a function of how far something is away from you, so the further something is away, the faster it appears to be moving away from you. A lot of people use a raisin bread analogy on this because the raisins stay the same size as the bread dough expands, so two raisins that start really close together will end up a little further apart, two raisins that are really far apart initially will end up seriously far apart by the time that bread is done rising.

Fraser Cain:                 And why is knowing the expansion rate of the universe important?

Dr. Pamela Gay:         It’s one of those things that allows us to put together all the rest of our cosmological ideas of how you go from our universe being a single point to expanding out, to forming hydrogen and helium, trace amounts of lithium and beryllium, to – the whole story is tied up, and it slowed down or it sped up, and understanding what rate we’re going now, since there’s no accelerator pedal that we know about, what we see has to be defined by the physics of our universe, and we can start to define all that physics of the universe if we know this one number that refuses to be measured.

Fraser Cain:                 I apologize, I was off by a factor of 10 there, so, sorry, and you should have caught me – it’s surprising you didn’t – but a megaparsec is 3.3 million lightyears, not 33, so Andromeda, roughly, is in that ballpark range. So, apologies.

Dr. Pamela Gay:         Yes. The way I think about it is someone in Andromeda looking back at us would be seeing Neanderthals.

Fraser Cain:                 Right, but astronomers don’t think in lightyears, they think in megaparsecs, so if I get off by a factor of 10, that’s fine by you because you don’t even think about it, so apologies in –

Dr. Pamela Gay:         It’s true.

Fraser Cain:                 Yeah, the general public thinks in lightyears while astronomers only think in parsecs and megaparsecs, but yeah, so, apologize, let’s continue. So, why – you were discussing why knowing the expansion rate of the universe is important.

Dr. Pamela Gay:         It basically just gives us this reference point that we can work all the other maths back from.

Fraser Cain:                 So, how long the universe has been around for?

Dr. Pamela Gay:         How long the universe has been around for, basically –

Fraser Cain:                 What will happen in the future…?

Dr. Pamela Gay:         The one that gets me is by understanding the current expansion rate, we can actually figure how fast the universe went from being a mostly smooth distribution of gases to forming galaxies, to forming galaxy clusters. The rate at which we formed large-scale structure, at a certain level, hinges on how fast our universe is expanding. It’s everything.

Fraser Cain:                 Right. And so, in the olden days, we used to get “How old is the universe?” and people would say, “Well, it’s kind of somewhere between 10 and 20 billion years old,” and that’s that range of measurement. If you get 50 kilometers per second per megaparsec, you get one age of the universe because you just measure how long the universe is expanding, but if you get 100, you get a different one, and they are very different, and knowing that is important. So, how do astronomers measure the expansion rate of the universe at the close and at the far?

Dr. Pamela Gay:         So, there are two totally different suites of mechanisms. The “local time” way of doing it is we look for supernovae, which give off a set amount of light if they’re Type 1A supernovae – explode a white dwarf star, and you get essentially the same explosion over and over and over again, with errors that we’ve discussed in other episodes.

Measure how bright that explosion appears, measure how fast the galaxy the supernova is in is moving, and this tells you the distance using measured brightness and known luminosity, and it tells you the expansion rate by looking at the Doppler shifting. So, we’re literally measuring how much the colors of the different bands of atomic lines have been shifted by the galaxy’s motion, and that gets us a velocity.

Fraser Cain:                 Right. And so, we have all of these standard candles, from the Cepheid variables, to the supernova, to – I saw a list. There must have been 30 potential standard candles overlapping, going from – some of which are very well known, others of which are poorly known, but you go from local measurements using parallax that overlaps with Cepheid variables that overlaps with Type 1A supernova, and you just get this really beautiful, smooth measurement, and what number did we get from the local methods of measuring the expansion rate of the universe?

Dr. Pamela Gay:         So, we’re getting around 70 kilometers per second per megaparsec, and this is using not just supernovae, but as you point out, there’s a bunch of other methods. So, folks are looking at red giants, they’re looking at planetary nebulae, they are looking even at the distant gravitationally lensed galaxies that we’re able to see multiple versions of using crazy geometry when we can see the galaxies’ lenses do the same thing at different times. All these methods are giving us definitely over 70, and narrowing in on 74, so it seems pretty constant.

Fraser Cain:                 Right, and the error bars are really tightening up. The quality of the observations is exquisite. I talk to a lot of astronomers, and they talk about how good of a job they’ve done with those observations, and they just gush.

Dr. Pamela Gay:         The SHOES survey by Adam Reiss – they’re quoting an error of 1.3%, and they are basically going from the nearby Cepheids that they have taken some of the most precise photometry of that anyone has ever taken, then using Gaia parallax data, and then working all the way out. How often does anything in astronomy get done with that level of accuracy? We know this. The local value is basically 74 kilometers per second per megaparsec.

Fraser Cain:                 Right. And so, let’s go the other end of the range because there is another group of measurements that are taken not locally.

Dr. Pamela Gay:         Right, and this is where things are squirrely. In the cosmic microwave background, we see these baryonic oscillations, these soundwaves that move through the early universe, causing slight over- and underdensities, and we can map so beautifully this distribution with our theoretical models, and by combining our understanding of, okay, the universe had this much regular matter, this much dark matter, this much – putting all of these base understandings that we come at from the theory, combining our average temperature information that we got from COBE and other missions, putting it all together, it gets us in the 60s, 68, generally.

Fraser Cain:                 But the most accurate version of this was the Planck satellite from the European Space Agency.

Dr. Pamela Gay:         Right, and this is where it’s important to note the Planck data was used to get at that distribution of baryonic oscillations, and that was used in combination with a mean temperature that they were, in a lot of the papers, referring back to COBE data. So, Planck got us, very specifically, deviations about the mean, and we just fed all the data together in the context of what’s called lambda cold dark matter.

This is a theoretical framework that says that our universe is not just expanding, but it’s accelerating as it expands, that the dark matter, the stuff that we’re not really sure what it is, that may be related to neutrinos in some way – whatever it is, it wasn’t moving extremely fast early in the universe, so that’s where the “cold” part comes in. So, we have lambda, the dark energy, and cold dark matter, two things we have very poor understanding of. When you combine those with the data, it gets you, again, roughly 60 kilometers per second per megaparsec with error bars that don’t overlap.

Fraser Cain:                 Right, and this is the key. So, you look at the local neighborhood and you get a measurement that’s in the low 70s with very tight error bars, you look at the early universe, you get 68 with very tight error bars. Both are exquisite observations, both are trying to tell you the same thing, and they disagree with one another, and this, at the heart, is the crisis in cosmology.

Dr. Pamela Gay:         Correct.

Fraser Cain:                 So, this is the crisis in cosmology, so what’s the answer?

Dr. Pamela Gay:         Well, this is where I personally am a bit excited, and I don’t know how many people are with me on this one because I wasn’t at the meeting, but at the American Astronomical Society meeting, there was a lot of discussion about how JWST images of gravitationally lensed early galaxies appear to be showing from two different studies that have both made it through peer review that there were already well-formed galaxies 350 million years after the Big Bang, and that’s early.

There is other work that is being done that is still going through peer review that is showing there may have already been galaxies – massive ones – at 200 million years after the Big Bang. So, with galaxies forming this early in the history of the universe, it tells us that that model we have, lambda cold dark matter, is off somewhere because while we thought there would be a couple, a few massive galaxies early on, those baryonic oscillations didn’t lead us to believe there would be as many as we are now finding.

And so, we have to figure out a new way to get from mostly smooth universe with the cosmic microwave background to galaxies forming in bigger and probably larger numbers than we anticipated to our present structure, and folks are putting out ideas like maybe there was a bit of leftover inflation, maybe the value of dark energy hasn’t been constant, and all of these different ideas – I don’t think we can really throw anything out yet, and I am the first person to want to throw out ideas.

Fraser Cain:                 Right. So, the challenge here is that you’ve got – the most obvious possibility is that one or both measurements is incorrect –

Dr. Pamela Gay:         Yes.

Fraser Cain:                 – which is what would be everybody’s first instinct, that someone’s wrong, that one of these measurements is incorrect. But, because of this dichotomy, both measurements have been scrutinized and scrutinized, and teams have gone back, and all they’re doing is narrowing the error bars. They’re not finding a large discrepancy. So, that’s the one that is most likely, and yet, that seems to be less and less of the case, and so, you’re left with the universe –

Dr. Pamela Gay:         New physics.

Fraser Cain:                 New physics, right, that our understanding of Einstein, our understanding of what those acoustic oscillations should be in the cosmic microwave background, is wrong.

Dr. Pamela Gay:         And this is where I’m not gonna lay blame on anybody. Relativity seems to work so far, it doesn’t seem to be the problem, but our understanding of the distribution of kinds of matter, the way different forces interplayed, whatever the heck inflation might be – we have no idea what inflation might be – it is somewhere in this physics of how we get from the Big Bang to now that we are missing something, and it’s kind of awesome.

We don’t get a whole lot of surprises anymore, it feels like, some days. We’ve found all the particles in a standard model. We didn’t find any of the particles from supersymmetry. This gives us something new to chase, and the fact that even the folks using gravitational waves to measure distances – they’re still getting the same local universe numbers.

Fraser Cain:                 And that’s harder to get wrong.

Dr. Pamela Gay:         Yeah!

Fraser Cain:                 I know I’ve talked to some people working in gravitational wave observatories, and I’ve had this conversation, and effectively, the more precise, the more powerful these gravitational wave observatories get, the farther they’re able to see out into the universe. You can rely on their measurements, so it’s another layer of observation, but one that’s very trustworthy, and it only goes so far. It doesn’t take you all the way up to the end of the universe, but maybe some future observation will.

So then, new physics – so, maybe we don’t understand how the early cosmic microwave background worked, maybe we don’t understand how Cepheid variables work, maybe we don’t understand all these different pieces, but there have been a few hints. Maybe Type 1A supernovae aren’t the standard candles that we thought they were.

Dr. Pamela Gay:         And this is where there are so many different things that are getting us that local 73-ish that, yeah, I’m happy saying there are discrepancies from one Type 1A supernova to another in weirdo special cases. We’ve got to talk about some of those. If the white dwarf ends up inside of another star, that explosion’s gonna be a bit different, and that happens, but it really seems like there is something about how you get from there – cosmic microwave background – to here – gravitational waves, Cepheids, planetary nebulae, supernovae, all these other mechanisms. There’s something in the science that we have yet to uncover.

Fraser Cain:                 And then, another possibility is if the rate of expansion of the universe changed. So, perhaps there was a – the model that I’ve heard is this idea of late inflation.

Dr. Pamela Gay:         Yeah, that’s the one I was looking at as well, where whatever it was that caused us to initially blow up, there’s a little bit of that left over that caused another kick, but we don’t understand what’s going on there.

Fraser Cain:                 Right, right. And so, if you had an expansion rate of 74 early on – or, sorry, 68 early on, and then it slowed down, you could get the one that’s today, and that’s even taking into account dark energy. I’m sure people are like, “What about dark energy?” That’s layered on top of this. That’s accounted for. And so, you would have this almost – instead of the universe smoothly applying the accelerator on the gas, it was like putting the accelerator a little harder, and then pulling the foot off the pedal a little bit, and then putting it on harder again, and who knows what kind of shenanigans it got up to in the intervening period? So, what is the way forward at this point? What is the way out of the crisis in cosmology?

Dr. Pamela Gay:         We need to basically do a survey of just what was the distribution of galaxies and galaxy clusters in the early universe. We have done a beautiful job, first with the Sloan Digital Sky Survey, doing a volume around our galaxy. Then, with the Dark Energy Survey, we have pushed out even farther in some areas of the sky. With JWST, we’re going to be able to continue pushing the survey of structure size further and further out, watching how the universe goes from being Swiss cheese with giant holes in it to being Swiss cheese with smaller and smaller holes in it –

Fraser Cain:                 Right.

Dr. Pamela Gay:         – and by measuring how that large-scale structure changes over time, that will start to put a different form of constraint on our models. We need to be out there, counting early galaxies.

Fraser Cain:                 Right, and so, you’ve got this structure of the cosmic microwave background radiation, and the hot spots and cold spots should map to equivalent clusters and distributions of galaxies, and so, you’ll know that this transition from the farthest that you can see to more recent is smooth –

Dr. Pamela Gay:         Yes.

Fraser Cain:                 – and then, that will tell you, and you can keep moving forward at that point, but that’s a much harder observation. Weird as it sounds, the galaxies are much dimmer and harder to spot and map out than the cosmic microwave background radiation, which is everywhere in all directions.

Dr. Pamela Gay:         Yeah. More telescopes.

Fraser Cain:                 Yeah. And then, the other side of that is going farther with gravitational waves, and hopefully, you’ll get to this point where the two overlap, where the gravitational waves reach the cosmic microwave background, or shortly after.

Dr. Pamela Gay:         That is technology that someone maybe someday will fund, and we’re at that frustrating point where the next big discovery beyond what we can do with the new, massive radio telescopes that have started to catch star formation at earlier periods, and what we can do with JWST – it’s gonna take multiple nations getting together to build these 30-, 80-, however-many-meter telescopes that are being discussed to be able to look back.

Fraser Cain:                 All right, place your bets. It’s the close observations are wrong, the CMB observations are wrong, or there’s new physics.

Dr. Pamela Gay:         New physics.

Fraser Cain:                 Really? That’s the most exciting outcome possibility, is new physics, so if that’s true, that would be wonderful.

Dr. Pamela Gay:         Or at least a new understanding of that cold dark matter temperature.

Fraser Cain:                 It would be huge thing, like there was a revision to relativity that nobody saw coming.

Dr. Pamela Gay:         So, again, I’m not sure saying a revision to relativity is the right way to say it because I think that what we’re looking at is something coming out of the realm of particle physics, and particle physics and relatively do not talk to one another, and I really think it’s going to be something about how particles interact in different regimes, and whatever the heck this dark energy is that’s gonna be what gets us to the solution to this discrepancy.

Fraser Cain:                 I think this “crisis” makes it sound like a bad thing, but you talk to astronomers, and they couldn’t be more excited. They’re so happy to not understand something, what was considered to be this bedrock idea, because the problem is bedrock is you get this ossification. Suddenly, you have this space that has opened up, where the solution is in there somewhere, and a lot of interesting ideas and theories, and a lot of brainstorming, and a lot of intellectual power gets to be put onto this problem, and they love it. They love it. So, I feel said that the term “crisis” – because you get a lot of pseudoscientists sort of rolling their eyes at scientists at this thing.

Dr. Pamela Gay:         I like “the Hubble Tension.”

Fraser Cain:                 “Hubble Tension” – yeah, but “Crisis in Cosmology” is a better name, so I’d rather reel them in with “the Crisis in Cosmology,” and then help people understand that, in fact, astronomers couldn’t be more excited and happy to have this opportunity. All right, Pamela, thank you so much.

Dr. Pamela Gay:         Thank you, Fraser, and thank you to everyone out there who makes this show possible through your patronage at Patreon.com/AstronomyCast. This week, I would like to thank Camy Raissian, Gabriel Gauffin, Benjamin Davies, Steven Coffey, john öiseth, Arcticfox, Dean, Corinne Dmitruk, Bart Flaherty, The Lonely Sand Person, John Drake, Nate Detwiler, Lew Zealand, Brian Kilby, Naila, The Air Major, Ron Thorrsen, Arthur Latz-Hall, Leigh Harborne, Jason Kardokus, Robert Hundl, Kim Barron, Paul Esposito, Ruben McCarthy, Bob Zatzke, Jordan Turner, Timelord Iroh, Daniel Donaldson, Frank Stuart, Ian Abdilla, and Geoff MacDonald. Thank you all so much for making everything we do possible.

Fraser Cain:                 Thanks, everyone, we’ll see you next week.

Dr. Pamela Gay:         Bye-bye.

Voiceover:                   AstronomyCast is a joint product of Universe Today and the Planetary Science Institute. AstronomyCast is released under a Creative Commons attribution license, so love it, share it, and remix it, but please credit it to our hosts, Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website, AstronomyCast.com.

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