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Astronomer Stacy McGaugh on a Universe Without Dark Matter

*"This is Galileo-type stuff"* In 1934, Swiss astronomer Fritz Zwicky discovered something very amiss in the structure of galaxies. Something was missing. Mass, and lots of it. With the amount of mass that we can observe in a given galaxy, it...

“This is Galileo-type stuff”

In 1934, Swiss astronomer Fritz Zwicky discovered something very amiss in the structure of galaxies. Something was missing. Mass, and lots of it. With the amount of mass that we can observe in a given galaxy, it appears that that galaxy should not exist. This led to the postulation of dark matter, e.g. that missing mass, as stuff that doesn't interact with our world like anything we know. Just a blank "something."

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Dark matter's existence is taken as a given now by the science community. It hasn't been directly detected, but it must exist. Not only that, 80-percent of our universe should be made of dark matter. While physicists hunt for it deep in mines, old sewers, and within the Antarctic ice (all to get away from the interference of "normal" cosmic stuff), a small group of astronomers are questioning the very basic equations that dictate how stars should move in a galaxy. Maybe those are wrong, not that we're missing stuff. The hypothesis is called Modified Newtonian Dynamics (MOND).

One of those scientists is Professor Stacy McGaugh at the University of Maryland, who just published a paper in Physical Review Letters with new evidence that things might not be as they seem. I talked to him last week about how the search for dark matter might never end, super-symmetry, and the troubles of helming some very unpopular research.

What are the chances of actually detecting dark mater versus those of detecting the Higgs boson?
That's a really good question. For dark matter as we conceive of it now to be right, the existence of the Higgs is a necessary but not sufficient [for proof] condition. We infer that there's this unseen mass out there and we've developed a specific paradigm that has that dark matter, these non-baryonic cold dark matter particles, called WiMPs (weakly interacting massive particles).

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So these WiMPs live in what particle physicists call the super-symmetric sector. Every ordinary particle that we're familiar with in this hypothesis is presumed to have a super-partner. So neutrinos have the neutralino as its partner particle. It's sort of the best-bet in the model that the neutrilino is the most common WiMP. So that's the dark matter.

So these super-symmetric theories are just hypothesis. They are not part of the Standard Model. They're something that's part of the next possible extension [of the Standard Model]. So for there to be any credence at all to them, then you need the Higgs particle to exist. [The Higgs should complete the Standard Model.] If the Higgs particle is found at the LHC as people hope it is, then that allows this paradigm to survive and go on. [But] it doesn't guarantee that these things are there. On the flip-side, if you don't have the Higgs, then you don't have these [dark matter] particles.

Dark matter is said to account for 80-percent of all matter; how does this alternative theory explain all that way?
It's how we infer dark matter to be there. We infer it from its gravitational effects. What you get when you look at the universe and the stars, what you see when you try to use the law of gravity that we know about in the solar system and apply that to galaxies, [the galactic structure] doesn't work. You look at what you don't know about the galaxy and really the only option is to add some kind of mass that you can't see.

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And so the flip-side to that is that we're just saying that the equation that we're using in the solar system, we're insisting we use for the galaxies. What I'm saying is that well jeeze, what if the equation doesn't work?

How does MOND explain the gravity in galaxies then?
That's a trickier thing to describe perhaps. There is the equation that describes through gravity how mass relates to motion. So in the solar system the sun is the dominant mass and its gravity causes the planets to orbit around it. So in galaxies, same thing, either the dark matter is the dominant mass, and stars orbit wherever the dark matter tells them to or you change the equation and the stars orbit as they are based on what you see.

You want to write down a more general equation—this is what Einstein did. His General Relativity includes Newton's gravity in the appropriate limit. He just made a more general theory and showed that there are places where you could get more phenomena then Newton told us about. So that's what you have to do here; you need to modify the existing theories to [to include that in the] special regime of galaxies, something different happens.

What's different in galaxies?
The weird thing that's different about galaxies? You'd think it's that they're bigger, right? The thing that seems to matter is that the acceleration scale, the surface density scale. And that's very, very low in galaxies. So, the acceleration that a star orbiting in a galaxy feels, the centripetal acceleration to keep it in its orbit, is about one part in 10^11 [10 + 11 zeros] what we feel sittin' here on the Earth. It's a tiny, tiny scale. And the particular modification that Milgrom hypothesized and that I'm talking about and found worked surprisingly well, just posits that at some critical acceleration scale that's very small, the force law changes in a specific way.

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Yeah.

Can MOND be proven?
I don't think one can prove it. There's a real dichotomy as to what the evidence shows. So, on what cosmologists call small scales, that is individual galaxies, this theory MOND seems to work better than dark matter. But on big scales, the whole universe, MOND doesn't really make clear predictions. And the dark matter models do and they work fine there. It's comparing apples and oranges as to which one you think is better. What I'd like to be able to do is test whether or not MOND works at these bigger scales. And I've made some progress in that, but it's. . . it's hard work.

What if we don't detect dark matter?
If you run the direct detection experiments for five years and don't find anything, you can abandon all hope, or you can go back to your models and tweak things until the dark matter particle is harder to find. What I'm saying is that you can only hope to falsify specific dark matter candidates. You might be able to prove it's not neutralinos, for example. But that doesn't preclude you from coming up with something harder to detect.

And the thing that worries me is that the more general process does not have a falsifiable finish to it. It is hard for me to know when to say when we should we quit. I'm one of only a few scientists even thinking along these lines because there is this, what seems like, a crazy theory. Its prediction did come true in my data and I have to respect that. Even though I don't much like the theory myself. It got this right. If we're to be objective, we have to pay it some respect.

What if we did detect dark matter?
I would be relieved, honestly. I find it very awkward. I think it's great that the public is interested and it's wonderful that there is fundamental science to be done. This is Galileo-type stuff. Is the Earth at the center or not? Is the universe full of dark matter or not? But it costs me a lot of grief, personally and professionally. A lot of my colleagues think this is crazy to even talk about and, therefor, I must be crazy. I find myself having the same debates over and over with them. To every single one that I meet. It's unpleasant. If there were a clear dark matter detection then I could breath a sigh of relief.

Related:
A Possible Outing Of Dark Matter
Hunting the Universe’s Biggest “Something” in its Deepest Nothing
Our Neighborhood In the Cosmos Just Got A Lot Darker