Ask A Physicist: Why Believe In Dark Matter?

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On today's episode of our "Ask A Physicist" series, Dr. Goldberg tackles the darker side of the universe - Dark Matter, that is. What is this curious cosmic stuff, and why should we care?

Today is the final installment in our four-part "Ask the Physicist" series. Our question comes from NorcalRushfan, who should email me to get a free copy of "A User's Guide to the Universe." He asks:


Is dark matter/energy necessary to explain the universe as we see it, or is dark matter/energy an observational error? Saw something that brought into question the need for dark matter and am wondering if this is widely accepted.


I'm going on the defensive for this one. I've written articles before talking about Dark Matter, and I almost always get some sort of response accusing physicists of being lazy, stubborn, and intellectually dishonest. To them, Dark Matter just seems like some grotesque fudge to fix all of our ignorance of the universe.

Let's go back a bit. In the 1920's, the astronomer Fritz Zwicky found that galaxies were moving around in clusters so quickly that unless there was a lot of matter that wasn't in the form of stars or gas, the clusters should fly apart in no time. Vera Ruben showed the same thing for individual galaxies in the 1970's. A rough estimate is that about 85% of all of the matter in the universe is in some form of Dark Matter.


Breaking it down by the fraction of the cosmic inventory, ordinary matter makes up about 5% of the total, Dark Matter makes up about 25%, and Dark Energy makes up about 70%.

And don't even get me started about Dark Energy. It's the stuff that accelerates the universe, and if you think you've got a problem with Dark Matter, wait'll you see Dark Energy. It's no so much that we don't understand where Dark Energy could come from; it's just that the "natural" value (the one that comes out of reasonable assumptions based on vacuum energy) is about 10^100 times the density that we actually observe. For my money, this is the absolute biggest problem in physics.


All we know with high certainty about Dark Matter is spelled out in the name. It doesn't have any electrical charge (light interacts strongly with charged particles, so it'd be easy to spot), and it isn't made of atoms, because those are also relatively easy to see. The reason that some people are so dismissive of Dark Matter is that they're focusing on 90-year-old arguments. They are judgy because a) We've never detected a Dark Matter particle, and b) We're not even really sure what it is.

But there are LOTS of reasons to believe in Dark Matter, besides the obvious (and damn compelling, if you ask me) fact that galaxies would fly apart with out it. Here are three good ones:

1.) Gravitational Lensing
Light gets deflected as it travels past massive objects. We can use the distorted images of background galaxies to make mass maps. For those interested in Dave Goldberg background info, this is the area that I work in. Lensing reconstruction has been done (and found lots of extra mass) for tons of galaxy clusters, but one system has gotten a lot of attention in the press. In the image of the "Bullet Cluster" up top, two clusters of galaxies have recently collided with one another, stripping out the gas (shown in red). The blue indicates where gravitational lensing shows the mass to be. Whatever is making up the mass is neither gas nor stars. This image is the closest to "seeing" Dark Matter so far.


Lensing does double duty in the Dark Matter game. We can use a different effect, called "microlensing" to look for dark stars (or black holes, perhaps) in our own Galaxy. Whenever one of those passes in front of a more distant star, the star gets magnified. Cool idea, but we can now definitively say that our Galaxy (and presumably the others) isn't filled with enough black holes to solve the Dark Matter problem.

2.) The Big Bang
Even if you believe that there is missing mass, that doesn't mean that Dark Matter is anything particularly exotic. Maybe it's just ordinary stuff that we don't see, somehow. The problem is that we understand very well how the light elements were created during the Big Bang. The chemistry is so simple, in fact, that the only number that really goes into it is the density of atomic matter in the universe. For example, if all of the matter in the universe were ordinary stuff, we'd see a thousand times less deuterium than we actually do.


3.) Historical Precedent
If you're put off by the fact that we've never seen a Dark Matter particle, don't be. Rutherford predicted the neutron (also a neutral particle, and thus difficult to detect) in 1920, 12 years before it was discovered. Dirac predicted anti-matter 27 years before it was first detected. Pauli predicted the existence of neutrinos in 1930, 26 years before they were discovered. We have a very good track record for figuring out what's out there long before we see it. And we have some good ideas. I wouldn't be surprised if the LHC or some other experiment detects a "Lightest Supersymmetric Particle" – one of the prime theoretical candidates for Dark Matter. I also wouldn't be surprised if it turned out to be something else.

And that's not all. Our cosmological picture – including the exact contribution of Dark Matter – fits together very well, and explains everything from the distribution of hot and dark patches in the cosmic microwave background to the age of the universe to the evolution and structure of galaxies. And they don't work without Dark Matter.


Still, at least one of you is likely to write in with something about whether Modified Newtonian Dynamics (MOND, for short) could explain away the need for Dark Matter. Basically, you're asking, what if Einstein was wrong? He could have been, of course, in which case one or two of our arguments (but not all of them) are in trouble. But MOND has an awfully big hurdle to overcome; it has to explain everything that relativity gets right, and then some. I'll put my money on Dark Matter and general relativity any day.

Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty." (Wiley: 2010). He is an associate professor of Physics at Drexel University. He is in the process of answering questions from readers on io9.