Long you live and high you fly
Smiles you’ll give and tears you’ll cry
All you touch and all you see
Is all your life will ever be. -Pink Floyd
In part I of this series, we talked about a number of different ways — all using gravity — to measure the amount of matter in galaxies, clusters of galaxies, and the entire Universe. We got the same measurement no matter which method we used, finding out that 25-30% of the total energy of the Universe is in some type of matter. But, only about 0.5% of the total energy is in stars, which means that nearly all of this matter doesn’t give off light! So what is the rest of this matter?
Well, one possibility could be that it’s just normal matter like us: protons, neutrons, and electrons. After all, we are massive and we don’t give off light! Maybe our Solar System — where over 99% of the mass is in the Sun — is a rarity? Or, as one commenter put it:
Why would it be surprising that there is mass in the universe that does not emit appreciable light? If that’s all that’s meant by ‘dark matter’, it seems obvious that it exists (Phobos and Pluto don’t emit much light, for instance).
Well, as a cosmologist, there are a few tricks we can use to put this to the test. In fact, off the top of my head, I can think of four tests we can do to find out how much of this gravitational matter is made up of the conventional stuff: protons, neutrons, and electrons.
1.) Gas fraction of clusters. When we look at clusters of galaxies, many of them emit X-rays. The X-rays are created by hot, energetic gas at different temperatures throughout the clusters. From this information, we can deduce what percentage of the mass in a galaxy cluster comes from gas and dust, which is all normal matter. The answer — pretty much always — comes out to 13-15% for every cluster. This is much more than the 2% that’s in stars, but still much less than the 100% total.
2.) Cosmological clustering of galaxies. Galaxies and clusters group together due to gravity, but the way they group together is highly dependent on what they’re made out of. You make everything out of normal matter (what’s called baryons by physicists), and things group together differently than if they’re made out of dark matter. In this graph from one of my favorite cosmology textbooks, you can see the drastic difference between a Universe made up of normal matter (with the baryons label) and a Universe made up of either Hot Dark Matter (HDM), Cold Dark Matter (CDM), or a mixture of these (MDM):
What do we actually observe? Something that matches up very well with the CDM graph, but actually has some small, tiny wiggles in it. Altogether, we can draw the conclusion that the mass in the Universe is about one-sixth baryons and about five-sixths Cold Dark Matter.
3.) Nucleosynthesis. This is my favorite method of discriminating between baryons and “other” matter, perhaps because it’s been around the longest. It’s also one of the simplest. In the very early Universe, things are hot and dense enough that nuclei haven’t formed yet; everything is just a bath of free photons, protons, neutrons, and electrons. When the Universe cools enough, however, protons and neutrons come together to form Deuterium, Helium, and Lithium. How much of these elements will be formed? That is highly dependent on how much normal matter (i.e., baryons) is in the Universe. In fact, that’s pretty much all it depends on. We do our measurements, and then our calculations, and we find that no more than about 4-5% of the total energy in the Universe can be baryons, otherwise the abundances of these elements would be off by too much.
4.) The Cosmic Microwave Background. These tiny little temperature fluctuations — just a few millionths of a degree — can tell us an incredible amount about what’s in the Universe. You can generate for yourself (using CMBfast) what these temperature fluctuations would look like in a Universe filled with 25-30% baryons as the matter in the Universe, and what it would look like with 4-5% baryons and the rest of the matter being something else. The differences are here, and they leave no doubt: the matter we’re looking for cannot all be baryons.
So you can pick any one of these observations, or you can (like me) listen to all four. Any way you slice it, all of this matter can not be made up of the normal stuff we know and love. So, you would conclude, this means that there’s some new type of matter out there, exerting its gravity but not emitting any light.
But all of these observations are based on an assumption: that we’ve got the right theory of gravity. Do we really? Or is it possible to tweak gravity to explain these observations without introducing a new type of matter? You’ll have to come back for part 3 to find out.