What’s up with all the “missing matter” in the universe?

SDStaff Karen replies:

Astrophysicists may not be rocket scientists (*snork*) but at least they know the difference between “undetected” and “completely vanished.” When a black hole “gobbles” matter (we scientists prefer to say “accretes matter”) the matter doesn’t just vanish from the face of the universe. It sticks to the black hole. (Surprise! Dirt doesn’t vanish when you hoover it, either.) And the mass of a black hole isn’t “immeasurable”; it can be measured the same way we measure the mass of Jupiter or Alpha Centauri or any other celestial object: by its gravitational effects on nearby objects. I have right here a couple of papers, one (by R.P. van der Marel and F.C. van den Bosch) claiming evidence for a black hole with a mass of 3×10⁸ solar masses and another (by John Kormendy, et al.) claiming a 2×10⁸ solar mass black hole.

You are right about one thing: our universe is expanding, what with the explosion of the Big Bang flinging everything apart. There are three choices for what happens next: (1) there is enough mass in the universe such that the gravitational forces are strong enough to stop the universe from expanding and cause it to start to shrink back into tight little ball; (2) there is not enough mass in the universe to halt the expansion and the universe expands forever; or (3) there is exactly enough mass in the universe to exactly halt the expansion, but not cause any shrinking. Sure, in scenarios #2 and #3 the universe gets larger and more lonely, but do you really prefer a shrinking universe? Parking spots are already too hard to find.

The mass of the universe is usually expressed as Ω, the fraction of critical mass. Critical mass is the mass the universe needs in order to get Scenario #3, i.e., Ω = 1. Ω < 1 gives a forever-expanding universe and Ω > 1 gives eventual shrinkage.

There are a couple of ways to measure the mass of the universe. The first is to assume that most of the mass in the universe is in stars and shines (99.9% of the mass of our solar system is in the sun) and measure how much light you see. Measuring the mass this way you find Ω_vis is about 0.005, with an uncertainty of a factor of 2 (a precise science astrophysics ain’t). The second method is to measure the velocities of stars in spiral galaxies and calculate how much mass is required to create the gravity that causes those motions. This measurement shows that there is a lot of non-shining matter, called dark matter. In fact, Ω_dm = 0.05, ten times more than the visible matter! If you add in observations of other kinds of galaxies, clusters of galaxies, etc., you can get Ω_dm = 0.3, although these methods require some assumptions about how galaxies form.

I can only speculate what you mean by “missing matter,” as that is not a term used by astrophysicists. Some inflationary theories require Ω = 1, so if you are looking at Ω_{vis + dm} = 0.3 and your theory requires Ω = 1, then maybe that 0.7 difference is “missing” in the sense that I didn’t win $15 million in the Lotto, but I wish I did, I sure miss that $15 million. Some theorists are really attached to Ω = 1, and actually think some mass is missing, and many more lump it in with dark matter, but there is no evidence that that matter exists.

Dark matter is a hot topic of research. There could be conventional non-luminous celestial bodies, such as brown dwarfs, jupiters, MACHOs (MAssive Compact Halo Objects) and astrophysicists are actively searching for these types of objects. There could also be a lot of tiny little exotic particles that we haven’t seen before like WIMPs (Weakly Interacting Massive Particles). This is where the particle physicists and crackpots hop on board. All kinds of stuff has been postulated as dark matter; they’re categorized as CDM (cold dark matter), HDM (hot dark matter), WDM (warm dark matter), and Rocky Kolb adds ADMYW (any damn matter you want). Neutrinos are a favorite candidate. We know that lots of neutrinos exist, but so far we have considered them to be massless.

Even if they have a teeny weeny mass, there are enough of them to contribute to the dark matter. (Neutrinos with mass also cause lots of fun particle physics effects.) Other exotic particles, like axions and neutralinos, which were proposed for other reasons, may contribute to the dark matter. But in general, all you need is a particle that hasn’t been seen before, but that solves all your dark matter problems. Lots of wacky candidates have been proposed and at this point it becomes difficult to distinguish the serious, studied proposals from astroparticle geniuses and the incoherent rantings of crackpots. So by all means, throw your hat into that ring, your Nobel prize may come yet.

SDStaff Karen, Straight Dope Science Advisory Board

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