The properties of ordinary matter are strongly constrained by the anthropic principle. In soome cases you can even calculate non trivial things. E.g. the anthropic reasoning was used by Hoyle to prove the existence of an energy level of the carbon-12 nucleus.
Dark matter seems to be much less constrained. We know that there must exists a lot of dark matter, and this fact could be an observer selection effect. However, since dark matter is believed to have only very weak interactions with ordinary matter, the exact properties of dark matter particles seem to be irrelevant for observers.
Some time ago, I noted that annihilations of strongly interacting dark matter can generate large amounts of internal heat inside planets. This is interesting, because it is thought that 50% of Earth's internal heat is supplied by radioactive decay and 50% comes from cooling of the Earth's core. However, if you calculate back in time, the cooling rate implies that the Earth's core must have been completely molten about a billion years ago.
Now, today the inner core is solid (due to the high pressure), and this is essential for generating a magnetic field. Old magnetic rocks show that the magnetic field did exists more than 3 billion years ago. So, the geophysicists have a problem here, and I thought that dark matter could be the answer. If dark matter supplies some of the internal heat, the earth's core would be cooling less fast and that would solve the problem.
Anthropically, you could say that without a magnetic field life would have had a difficult time on Earth (because of cosmic and solar radiation) and we may not have been here. You would still need to show that we couldn't have evolved on a larger planet where the heat supplied from dark matter annihilations is not necessary.
Unfortunately, this idea does not work. Uranus has a very low inernal heat production, and this strongly constrains the properties of strongly interacting dark matter, making the above scenario impossible. See here 40 minutes from now:
http://arxiv.org/abs/astro-ph/0408341
Received on Thu Aug 19 2004 - 19:26:26 PDT