If you have any sort of passing interest in the universe around you, you’ve almost certainly heard of “Dark Matter” a mysterious substance which makes up about 85% of the matter in the universe. We’ve never held it, can’t see it, and it isn’t even predicted by our current theories of particles.
Nevertheless, it’s real. Really real. It’s not a mathematical fudge. It’s not a delusion.
Sure, I’ve seen posts around the interwebs (and even the rantings of one or two olde-tyme physicists around my own department) claiming otherwise, but I’ve got 5 very good reasons for believing in it, and 9 times out of 10, anti-darkmatterists only focus on the first of these. Here’s your arsenal of Dark Matter defense:
- There’s no school like the old school – Galaxies are rotating way too fast.For nearly a century now, we’ve been aware that if you measure the rotations of galaxies, or the motions of galaxies within clusters, and then very simply applied Newton’s laws of motion and gravitation, these giant structures should simply spin themselves apart. Based on everything we know about stars and gas, there shouldn’t be enough gravity to hold them together.
In the 1970’s Vera Ruben showed that galaxies couldn’t possibly be held together by the gravity of just the stars and gas within them:
And it’s not just true of galaxies. Entire clusters of galaxies have a great deal of missing mass, as much as 85% of it unaccounted for by “ordinary” (atomic) stuff. Rather than show you a cluster (which would do little to illustrate the point), here’s the ever curmudgeony Fritz Zwicky trying to illustrate that there isn’t enough matter to hold clusters together.
Because I’m generous, I’m going to throw a bunch of other arguments into reason #1. For instance, virtually every cosmological model/measurement is consistent with a universe containing way more matter than is accounted for with ordinary atoms and molecules. The cosmic microwave background:
which says that about 28% of the universe is made of gravitating matter, while the models of how atoms form in the big bang (and virtually every measurement) says that only 4.5% of the universe is made of “ordinary” stuff.
People try to get around all of this by supposing that Einstein just got gravity wrong. MOdified Newtonian Dynamics (MOND) has been suggested, but to my mind it creates an even more hodgepodge universe than one containing Dark Matter. For one thing, MOND generally says that there’s a particular length scale where the measurements and predictions won’t match up, but there seems to be “missing matter” on scales ranging all the way from individual galaxies out to the scale of the entire universe.
- Supersymmetry is all the rage, and it predicts lots of particles that we haven’t seen.One of the biggest complaints about Dark Matter is that we haven’t actually found a Dark Matter particle yet. This is supposed to be a big deal because everybody knows that we’ve already discovered (nearly) all of the particles in the “Standard Model”:
The only one we haven’t seen yet is the Higgs boson, and hopefully, the good folks at the LHC (or maybe Fermilab!) will be producing this in the next couple of years. There are no more missing particles for Dark Matter to be and hence (goes the argument) Dark Matter can’t exist.
But the good news is that the Standard Model can’t possibly be the end of the story, as we’ll see below. One of my favorite ideas is something called Supersymmetry. Essentially, every fermion (quark, neutrino, electron, and the like — the particles of matter) has a supersymmetric boson partner: a sneutrino, for example. Every boson (photon, gluon, W and Z particle, and [maybe] graviton — the particles of force) has a supersymmetric fermion partner: a photino, for example, is the partner of the photon.
Since heavy particles decay and neutral particles are very hard to detect, the “Lightest Supersymmetric Partner” (the neutralino, perhaps) might be all around us, invisible to detection.
Are we sure supersymmetry is right? Of course not. If it were, it would be part of the Standard Model. But we do know a couple of things:
- The Standard Model is incomplete. It can’t explain, for example, why there is an excess of matter over anti-matter in the universe.
- A lot of very popular models of how the universe works (like string theory) are premised on supersymmetry.
- We have a great track record for predicting particles long before they are detected.Don’t believe the theory? Well, the particle physics community has done a bang-up job in predicting particles long before they were actually detected.
A few examples will suffice:
- 1920 – Ernest Rutherford noticed a pattern in the periodic table. Helium has twice the charge of hydrogen, but 4 times the mass. He proposed the neutron, which was discovered by James Chadwick in 1932.
- 1928 – P.A.M. Dirac derived the famous “Dirac Equation” which, among other things, predicted the existence of anti-matter. The anti-proton was discovered experimentally in 1955.
- 1930 – Wolfgang Pauli predicted the existence of neutrinos based on the measured energies and momenta of protons and electrons in the decay of a neutron. He posited that there must be an unseen 3rd particle (an anti-neutrino, as it happens) that carried away the extra.
- 1964 – Murray Gell-Mann and George Zweig developed a counting device to explain the known hadrons (heavy particles like protons and neutrons). They introduced what was later known as “quarks,” and which were later developed directly.
Point is, just because we haven’t seen it yet, doesn’t mean it’s not there.
- Failure to see them is well within expectations.In particular, we might expect not to see Dark Matter particles because they are very, very difficult to see. In most models of Weakly Interacting Massive Particles (WIMPs), Dark Matter would be relatively rare and relatively massive. We might imagine making some (albeit relatively few) in the LHC, or detecting cosmic Dark Matter in the Xenon 100 experiment or others. Maybe we’ll even see WIMP/Anti-WIMP annihilation using the Fermi Gamma ray observatory. But why haven’t we done so yet?
Well, the short answer is that the weak force is very weak. Cosmic neutrinos, for example, which are comparable in number to photons in the universe are never detected directly, and they, too, only interact using the weak force. Even those created by the sun, nuclear reactors, or the atmosphere, are typically only collected in units of a few to a few dozen per day.
Absence of evidence is not evidence of absence.
- Actually, we have seen them! (Sort of).This is true in exactly the same sense in which you could “see” Sue Storm if she happened to be clothed. Dark Matter makes gravity, and gravity bends space-time. We can detect this directly using “gravitational lensing.” Essentially, distant luminous galaxies appear distorted to us by the gravitational field of massive galaxies and clusters between us and them. By figuring out how much distortion, we figure out how much mass, and where the mass is:
This is actually the field of astrophysics that I work in, and recently, my friend, Richard Massey, became somewhat famous by making a 3-dimensional plot of the Dark Matter in the universe.
If you don’t find that convincing, consider this plot of the “bullet cluster” from Douglas Clowe, Marusa Bradac, and their collaborators from 2004:
These are two colliding clusters of galaxies. You can see the stars (in white), and the gas, done in red. The mass, discovered through lensing analysis, is in blue. The basic story is that these two clusters passed through one another and the gas, which amounts for about 80% of the atoms mass, got completely separated from the stars and the dark matter. The gas, you see, unlike stars or dark matter, exerts pressure, hence the “bullet” shock feature. The upshot is that we see lots of mass where there shouldn’t (naively) be any. This image rightly got huge attention a few years ago because it’s the first direct sighting of Dark Matter.