MOND and more. Does the universe need a rethink?

MOND and more. Does the universe need a re-think?

Our current cosmology says cold dark matter helped small galaxies form in the early universe. But recent data from the James Webb Space Telescope are challenging that idea. Astronomers, like Stacy McGaugh of Case Western Reserve University, think that since our theories don’t match the observations, it might be time to rethink our story of how the universe formed in its extreme youth. According to McGaugh and his team, new research supports the MOND theory. EarthSky’s Dave Adalian down with Stacy McGaugh to talk about his team’s discoveries, what they means for cosmology and where we go from here on Monday, November 18. A transcript of that interview is below. It’s fascinating stuff!

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Dave Adalian
Hey, welcome back to EarthSky. This time, we are going to talk with astronomer Stacy McGaugh at Case Western Reserve University in Ohio. He studies dark galaxies, dark matter and theories of modified gravity, which are going to be key to what we’re talking about today. He is an expert on low-surface-brightness galaxies, which are objects where the stars are spread thinner than they would be here at home in the Milky Way. And he also demonstrated these dim galaxies appear to be dark-matter dominated and that they provide a unique set of tests for theories of galaxy formation and for modified gravity.

Stacey, the first thing that we want to ask you is, is the Big Bang safe? You’re not getting rid of the Big Bang on us, are you?

Stacy McGaugh
So no, the Big Bang is fine so far as I’m aware. We will entertain some pretty deep and challenging ideas in this talk. But I think it’s very well established empirically that the universe is expanding. It went through an early hot phase that gave us the nucleosynthesis that gave us the hydrogen, helium, lithium we have in the universe and also left behind a relic radiation field that we know is the cosmic microwave background.

So that framework I think is true, has to be true in any theory. It’s getting to the right theory that’s going to be the issue.

Dave Adalian
Awesome. OK, great. Well, that’s a load off my mind. I was worried that the Big Bang might be in trouble. But no. Not from you. Well, there is some evidence mounting, a little bit here and there, that the Big Bang might not be what we think it is, or maybe it is. We don’t know. Is that right?

Stacy McGaugh
Not from me.

Well, it’s always been a target, right? There are people who just don’t like the idea. And so there is a lot of crazy talk from that side. You have to be careful of that. On the other hand, one has to be careful not to dismiss legitimate things by association. And so it’s certainly possible that there might be some kind of evidence we should reconsider.

You know, I’ve spent a lot of time in my career trying to weigh how that is, what that is. So in my opinion, the Big Bang, as we know, it is not in serious jeopardy. I would say that the specific theory most of us believe in nowadays, the so-called Lambda Cold Dark Matter theory, that’s more open to question.

Dave Adalian
So that brings us back to the paper that you and your team put together. You’re the lead author here. And I understand you looked at some data from the James Webb Space Telescope all the way back, as far as you can see, to the earliest structures in the universe. And we’re talking about an era when the whole sort of general mishmash. Sorry about that. In other words, everything was brand spanking new. The cosmos was fresh-faced and innocent, and it had no idea what was was coming for it.

So I mean but we’re going to be talking about the most basic theory of how galaxies and early structures in the universe began right? How far back are we talking about?

Stacy McGaugh
I Would like to say all the way, but not quite all the way. So in the first billion years or so and especially the first half a billion years after the Big Bang. A billion years, give or take. That’s a long time to you and me. For the universe, that’s the first month on the calendar, right? The universe is 13, maybe 14, billion years old.

So we built James Webb Space Telescope to look and see how galaxies formed at the very earliest times. And it has succeeded marvelously in doing that. And it is revealing to us things that happened at such ludicrously high redshifts [Ed. note: Redshifts enable astronomers to measure distances and look-back times].

The very first billion years – you know, less than a month on that cosmic calendar – it’s all happening in January. And a lot of it’s happening in, you know, the second week or so.

Dave Adalian
So when we’re talking about redshift, and that’s in terms of z [Ed. note: astronomers often denote the value of a redshift by the letter z]. Z right now is 0. Is that correct? Z=0. And so the z that we’re going to be talking about is about 2 or 3. Is that right? Where are we looking? What are we looking at?

Stacy McGaugh
Yes, that’s correct.

Well, that is a great question. So redshift quantifies how much the light has stretched since it was emitted and the universe is expanding. So the further back in time, the closer to the start of the Big Bang, the more stretching. And so the higher the redshift. And so the relation between the look-back time, you know, a million years ago, a billion years ago, how long have those photons been traveling?

Most of cosmic time is between now and redshift one. And that’s a lot, right? That’s a lot of time, 7 billion years, give or take. And it corresponds to taking a spectrum and basically doubling all the wavelengths, right? So if you see some spectral line, it shifts by one. That means a whole 100%, basically. And that redshift can go arbitrarily high, and you just get more and more shifted. And so already redshifts 2 and 3 are pretty early in the universe. James Webb is showing us things at redshift 10 and above even. And so yes, wow.

You know, I remember some of the early claims in the 1990s when we decided we have to build this thing [the James Webb Space Telescope]. And there were claims of things at Redshift 7 and we’re like, nah, no way, that can’t be true.

And I remember talking to theorist friends who were categorical: There is nothing above Redshift 7.

Dave Adalian
You fast forward to now. And that was a reasonable attitude. It was back then. It’s not now, though.

So, ok, your paper is called Accelerated Structure Formation, the Early Emergence of Massive Galaxies and Clusters of Galaxies. And what did you see back there? You expected to see proto-galactics [galaxies in the process for being formed].

Do we need to talk about Lambda CMD here [aka the “standard model,” the most popular model of how our universe works, on the largest scales]? What did you see? Why don’t you just tell us what you saw, and what are the “protogalactic fragments?” And did you find them? Let’s start right there. How does that sound?

Stacy McGaugh
Absolutely.

Okay, so what are we talking about here? So, okay, so we live in galaxies today that are big at redshift 0, things like the Milky Way. And we look out in the universe around us and we see other island universes like that, basically big collections of stars, like our own Milky Way, with lots and lots of empty space between them.

And then that stretches out into larger scale structures, clusters, voids, where there basically are no galaxies. There are walls and filaments between them, this whole tapestry of the large-scale structure of the low-redshift universe.

And so we wanted to know how that came to be. And we have a lot of ideas for how that is. So that’s where this Lambda Cold Dark Matter theory comes from.

The Lambda part is what you also hear people talk about dark energy. That’s what makes the expansion rate of the universe accelerate. We don’t care too much about that for our purposes today. It just sort of sets the framework, in which all this happened. The background expanding universe is the hot Big Bang. The cold dark matter matters a lot. So, in the early universe, we’ve got observational constraints that predate even what the Webb telescope shows us.

You need something extra. Let me put it that way.

And our first best guess for what that something extra was, was this cold dark matter.

And one reason for that is that the cosmic microwave background, that cosmic relic radiation field from the hot Big Bang. That comes to us from a redshift of 1,000, even a little more than that.

The universe was only a few hundred thousand years old. So that’s sort of a baby picture of the universe and it had no structure to speak of. So we see this rich amount of structure today. The early universe is observed to be extremely uniform.

And so the question arises, how do you get here from there?

Dave Adalian
Where did the structure come from? Was there Cold Dark Matter that caused things to start clumping up? And then we built on a hierarchy of small to large. That’s the Lambda CMD theory. But then there’s another approach, isn’t there? Please continue.

Stacy McGaugh
Exactly. Exactly.

Well, so I think we should talk about the Cold Dark Matter a little more, because what you said is exactly right. But it’s worth elaborating, I think, what we expect from that theory.

And in fact, most of the paper is about that. And it’s important. A theory is only as good as its prior. What do you predict? Because you can always fudge it afterwards. So it’s really important to establish what you expected going in.

Dave Adalian
Okay. Okay, good.

Stacy McGaugh
And the idea of cold dark matter was to take that very smooth early condition of the universe and give something extra to make the structure grow. So gravity is an attractive force. We do see tiny density fluctuations in that early radiation field, but they’re tiny.

And so gravity will take those things and make the rich get richer. So something that starts out with a little extra density will become more and more dense. And the idea is that eventually becomes a galaxy.

The problem is that gravity as taught to us by Newton and Einstein is not strong enough to get here from there. You can’t get here from there. And so you need something to goose the process. You need some extra something.

Dave Adalian
Some kind of energy of activation.

Stacy McGaugh
And so the idea was that we would add in this cold dark matter, stuff that didn’t interact with the photons by choice and construction. And then it could grow without leaving too much of a scar on the microwave background. And so we need that [for the theory to work]. We need something extra, anyway, to make the structure grow to get here from there.

And so that was one of the reasons that we invented this stuff [dark matter] in the first place.

Now that brings us to … We need to get here from it, right? There are other mysteries like baryogenesis. Why is there more normal matter than antimatter? We still don’t really understand that.

So there are fundamental puzzles still, which is kind of cool. But – once we have the cold dark matter – we can calculate the basic solution that Einstein gives us for how the universe expands, and just tweak it a little and show how that structure should grow in the presence of the cold dark matter.

And that’s been a very successful theory. It shows how you can get here with lots of structure from there way back when, with very little structure.

Dave Adalian
Right.

So its predictions have held up for the most part, haven’t they? Lambda-CMD is a strong theory in that regard.

Stacy McGaugh
Yes, yes and no. I mean, Lambda-CDM is an excellent fit to all the data at low redshift for large-scale structure. We didn’t just get there. Our initial best guess was what we used to call the Standard Cold Dark Matter theory, without the Lambda. And that failed badly. We knew we needed the cold dark matter.

But that theory didn’t actually work to explain large-scale structure by itself. We had to do something different. Long story short, that became the Lambda. So we migrated from the ’80s, believing in the Standard Cold Dark Matter theory, having all sorts of issues in the ’90s. And, by the end of that, we decided, no, it’s Lambda Cold Dark Matter.

And I mention that in part because it’s easy to look at the successes, and there are plenty. You can look at Lambda-CDM now and think, well, it’s predictive. But it’s not that predictive. It was fitted. Because we started at something different. Now, having said that, it fits lots of different things that it wouldn’t necessarily do. There are independent constraints. So that I find impressive. But …

We did not expect the amount of large-scale structure that we see. We were surprised by that. We were gobsmacked as we made surveys of nearby galaxies.

Dave Adalian
So we’re talking now about the data that you examined from Webb that led to the paper. And what you found were not fragments or building blocks, protogalactic fragments. You didn’t find those in the numbers that you expected to. Is that correct?

Stacy McGaugh
That’s correct. So basically what we had before the Webb telescope observations was a very smooth starting point, and a very lumpy finishing point. We wanted to know what goes on in between.

So that’s what the Webb is telling us now. And what Lambda-CDM predicted was that you had to build up very slowly and gradually out of this smooth initial condition. And that’s what makes these protogalactic fragments.

Dave Adalian
Right.

Stacy McGaugh
It is the hierarchical merging of these things, little things merging to form bigger things, and those merging to form still bigger things … that process is supposed to form the galaxies we see today. That makes perfect sense mathematically, but it takes a long time to do.

And so that’s what we’re not seeing. At these very high redshifts that the Webb is revealing, we expected that there would be lots and lots of small protogalaxies. And, instead, what we’re seeing is a surprising number of what looked like, here’s a galaxy. And here’s a galaxy.

There are big island universes already in place at very early times, bigger than they should have been in this standard theory.

Dave Adalian
And, in some of them, the star formation had essentially ceased. Is that correct? I mean, that’s indication of age, isn’t it?

Stacy McGaugh
So yes, so again, stepping back in time, we’ve been studying galaxies for a long time. And without knowledge of that Lambda-CDM theory that we were just talking about, people just looked at the properties of galaxies and inferred already that some of them were old. They stopped forming stars a long time ago. And it looked like they formed almost all of their stars very early on.

The brightest elliptical galaxies fall into this kind of category.

Naively, from this picture, you would think that the biggest things are these elliptical galaxies They would have formed last and so they should have the youngest stars. But they have the oldest stars. And so this has a reasonable explanation in Lambda-CDM. It’s that there’s a difference between when the stars form and when they assemble into a galaxy

Dave Adalian
Wait, what?

Okay.

So older stars form into new galaxies?

Stacy McGaugh
Exactly. So these little protogalactic fragments could be where all that star formation happened. And then they merged to assemble into the bigger giant galaxy. That was the picture going in. That’s the idea.

Dave Adalian
Okay.

So that was the theory, right? Ok. But that’s not what you saw. That’s not what Webb discovered, is it?

Stacy McGaugh
That’s not what Webb is seeing. And let me give credit to the entire astronomical community here. There are many, many astronomers working with Webb on these data. We’ve played a fairly small part in analyzing it. And it really is a community effort first to build this really elaborate facility, launch it into space, and then contend with the data that come back, a proverbial fire hose.

So, yes, but what people recognized very early from the first images basically was that there were brighter galaxies at higher redshift than were expected in the scenario we just talked about.

Dave Adalian
Okay. And so in the paper, you and the team make an argument that what you’re seeing is supported by an alternate theory of cosmological formation, which is modified Newtonian dynamics, right?

Stacy McGaugh
Correct. That’s right.

Dave Adalian
What the heck is that?

Stacy McGaugh
What the heck indeed? I’ve been asking myself that question for going on 30 years here.

Dave Adalian
I hope you came up with some good answers you can share.

Stacy McGaugh
Well, sometimes yes, sometimes not. It’s a really a challenging thing. Having it put it that way, this is where we’re challenging deeper theory, if not the Big Bang itself.

And looking back at the history of science, it takes a long time to wrap your head around these ideas. Newton introduced the idea of the universal law of gravity. And we just take that for granted and expect our first year students to pick it up right away. You know, he wasn’t sure where it came from himself. And a lot people gave him a hard time, because it seemed to them magical that there was action at a distance. How could that possibly happen? The sun is reaching out and grabbing the Earth or something like that.

And there are lots of aspects that we just have built into what we now call Newtonian dynamics that we take for granted. But it took a long time to develop.

For example, the Poisson equation is an essential tool that we use in the sort of analyses that we do of the dynamics. That came decades later. It takes a long time to wrap your head around these things. And so I think we’re still in that phase with these modified dynamics. It’s telling us something. But we don’t really know what that is yet.

Dave Adalian
Ok. Ok. So we don’t know. The observations don’t match the Lambda-CMD theory as well as they should. In fact, they don’t match it very well at all. But what you did find supports MOND.

Stacy McGaugh
Hahaha.

Yes.

Dave Adalian
But how does that allow us to keep the Big Bang then? Let’s go back there. I’m still worried about the Big Bang.

Stacy McGaugh
Well, so … That’s healthy. We should worry about everything.

It’s just that some things are so well established, we kind of forget that we should worry about them. And dark matter is such an example. I mean, when I first encountered this issue, I kept banging my head. How can this stupid theory, MOND, have any of its predictions come true when there’s so much evidence for dark matter?

And I really struggled with that question for a long time until I realized that there’s evidence that something is wrong. We don’t actually know which it is. And so you need additional observations like this to try to distinguish where the theories make different predictions. And the prediction in the case of MOND was that galaxies would form quickly.

Dave Adalian
In the past.

Stacy McGaugh
And well, in the past now. You have to ask, which is stronger? We’ve invented the cold dark matter to speed up the formation process. And we invented MOND for the same kinds of reasons, to give more force. But it’s a different invention.

So you have to see how things behave. And if you simply ask, okay, here’s some region of the early universe that’s expanding, but it’s obeying MOND, how long does it take to collapse and form a galaxy sized object? And the answer is about half a billion years. You can do that calculation. It’s pretty straightforward. And what should take a couple billion years conventionally happens in that much faster timeframe.

And that’s what Webb is seeing, that there are things that already seem to have formed that big, that fast.

Dave Adalian
So this doesn’t confirm MOND, but it adds support to that particular model.

Stacy McGaugh
Yeah, it’s a priori, right? I mean, as I said, you want to make your predictions before the observation. And in Lambda-CDM, we had a clear prediction, and it was that we should have all these little protogalactic fragments. In MOND, we had a prediction is that the galaxies would get big fast. That’s what we’re seeing.

Dave Adalian
So are there other theories of cosmological formation besides those two that might fit the data? Or are these basically it for now?

Stacy McGaugh
Well, yes and yes. So yes, there are lots and lots of ideas out there, and I don’t want to dismiss them all. And it’s tempting to dismiss MOND. I certainly tried hard to do that, early on.

But these are the two main games in town at this juncture.

You put it this way, there’s a contrast in scale on the very large scales in the local universe. Things have to look like Lambda-CDM. It fits enough of the data that whatever your final answer is has to show that same thing for MOND, and there are lots of observations like this one, like the dynamics of individual galaxies have to look like MOND.

And it’s those two contrasts that really confuse me because it depends on how you weigh the different lines of evidence. Here are these two incommensurate things that we’ve observed. And if you’ve put all your weight on, and only care about, large-scale structures, Ok, well, Lambda-CDM is the right answer. But if you care about the details of dynamics and how galaxies actually work and how they could possibly form this early, well then, MOND is a better theory.

And we haven’t figured out how to make those things mesh.

Dave Adalian
Okay, so we have two really good theories, but they don’t mesh at the right spot, sort of like the gravitational theory and the quantum theory.

Stacy McGaugh
Yeah, they hate each other, yeah.

Dave Adalian
Excuse me. Okay, folks, if you are just joining us, we are talking with Professor Stacey McGaugh, who is an astronomer, and we’re talking about a paper that he’s written about the formation of structure in the early universe. And what they found is that galaxies are bigger than expected, if we’re basing our understanding on the Lambda Cold Dark Matter theory of cosmic evolution. Dr. McGaugh’s work, instead, it seems to support modified Newtonian dynamics, aka MOND.

So what’s wrong with MOND? It doesn’t describe everything. It has some weaknesses. Is that right?

Stacy McGaugh
For sure. There are two big things I would highlight. One is theoretical, and one is observational. And again, there’s a dichotomy on which you care about. As an observer, I’m more concerned about the observational one.

Dave Adalian
Okay.

Stacy McGaugh
But of course, there are lots of people who want to know the right theory. So they’re concerned by the fact that MOND as a theory is an extension of Newtonian dynamics. So basically, Newton is a subset of MOND. But it is not compatible with Einstein’s general relativity. And that’s a big deal, right? General relativity is our foundational theory of gravity now. It has been tested in lots of precise ways.

And so, again, you have to recover all those successes if you’re going to build some grander theory still that incorporates both general relativity and MOND. As you might imagine, that’s not a trivial task. And so there have been attempts. I’m not convinced that any of them are correct. Close maybe.

Dave Adalian
Okay, so, yeah, we wouldn’t say that Newton was wrong. We’d say that Einstein was more correct and developed upon that theory. And so that is that what we’re seeing here?

Stacy McGaugh
Yeah, yeah, exactly. Exactly. And so what we would like to see develop theoretically is a still more correct theory that can encompass both of these things.

Dave Adalian
Something more robust that more exactly describes what we find when we look back into the earliest moments of our galaxy.

Stacy McGaugh
Well, yes, exactly.

Dave Adalian
Stacey, where do we go from here with your exploration of the universe?

Stacy McGaugh
Well … I don’t know. So let me answer your other question, finish answering it, because the other problem, big problem that MOND has to my mind is in the data for clusters of galaxies.

MOND is extraordinarily good at explaining the dynamics of individual galaxies. It does so over a remarkably large dynamic range, going from little bitty dwarf galaxies with a million stars up to things bigger than the Milky Way with hundreds and hundreds of billions of stars.

And that is striking because, you know, as you mentioned, one of my early discoveries was that these low-surface-brightness galaxies were dark matter dominated. And what attracted my attention to MOND was the fact that none of us, including myself, predicted that. But MOND did predict it. Basically how dark matter dominated you appear to be as a galaxy depends on how spread out your stars are. The more spread out, the more dark matter domination. And so that was the only theory that got it right.

It’s still a theory. It’s the only theory I know where you can use it to say, okay, there’s that galaxy in the sky, and I can measure the distribution of its stars and gas. I can solve the Poisson equation for what Newton says it should be, and that doesn’t work [in MOND]. That’s why we need something extra like dark matter.

But the extra thing that MOND tells you is it’s basically an extension of Newton. So it says, ok, you start with what Newton tells you, but here’s an amplifier. And it lets you quantitatively predict how much amplification you get based on what you see.

That makes no sense in terms of dark matter, because especially in these low-surface-brightness galaxies. There, all the mass is in the dark matter. The stars don’t matter, supposedly.

Dave Adalian
So if the dark matter wasn’t there, galaxies wouldn’t hold together? Is that right?

Stacy McGaugh
Exactly. They would just fly apart. And so you need something extra to hold them together. And that’s what we thought was the dark matter. But then the dark matter should be telling the stars what to do. And that’s not what we observe.

Instead, it’s the other way around. We can use this strange formula that’s in MOND. And that tells us what we are going to observe for the kinematics [the study of objects in motion without regard to external forces], and where the dark matter needs to be. It’s, you know, a tail wags the dog kind of situation.

Dave Adalian
Right.

Stacy McGaugh
So that’s one reason why MOND seems to work better than dark matter. You do the same experiment in clusters of galaxies, which were now systems of hundreds, even thousands of individual galaxies all moving around. And it should succeed in the same way. And it almost does, but doesn’t quite. It doesn’t go far enough to explain that.

Dave Adalian
So close, yet so far. I don’t know. Where do we go now?

Stacy McGaugh
Yeah, so it’s tempting. Where do you take it from here? And it’s, it’s tempting to dismiss the theory because of this one failing. But if we did that, we would have thrown dark matter away a long time ago, too.

Dave Adalian
Right.

Stacy McGaugh
And so it feels like, you know, the old parable of the blind man and the elephant. There’s a problem, and there are different aspects to the problem, but they all feel like different things depending on how we approach the problem.

And we’ve yet to figure out what the whole is.

Dave Adalian
So Lambda-CMD isn’t entirely wrong, and MOND isn’t entirely correct. And somewhere out there is a theory that we can use to unite these disparate observations into a singular explanation of how the whole general mishmash got this way.

Stacy, such a great explanation. I would like to say I understand it better …

Stacy McGaugh
I would, too.

Dave Adalian
And probably where we’re headed next is more research, more observation, more description, more theorizing. What are you going to do?

Stacy McGaugh
Yes, you always want to look more, right? And the Webb observations did come as a surprise to us. And that’s uncomfortable. But it’s also where we learn new things. And that’s why we do science. So we want to keep exploring. We want to look deeper into the universe, both in time, the early redshift, but also broader, in space, to lower-mass galaxies, you know, these low-surface-brightness galaxies that I’m interested in. I got interested because at the time they were barely known, and so it was new territory, and we learned a lot of new things from them, including things that falsified my own dark-matter-based theories. And so, you know, we have to always be exploring and we also have to be open to the fact that maybe we’re wrong.

We’ve done that a lot, so it shouldn’t come as too much of a surprise when it happens again. But it’s hard to accept, especially with really well-determined theories, you know, or seemingly so.

Dave Adalian
I guess you have to remember that the folks who are out on the adventure, they don’t have any idea how the adventure is going to end. And so you’re out there adventuring. You’re out there discovering new things and surprising yourself, it sounds like, too.

This was really good, Stacey. Thank you.

Stacy McGaugh
It’s a lot of fun to talk about these things, whatever you want to believe in.

Dave Adalian
Thanks again so much for joining us. Remember, folks, one Earth, one sky, EarthSky. We’ll see you next time.

Bottom line: There’s increasing evidence that astronomical theories don’t match observations. Is it time to rethink our story of how our universe came to be?

Source: Accelerated Structure Formation: The Early Emergence of Massive Galaxies and Clusters of Galaxies

Via Case Western Reserve University