From: Eugene Salamin According to people in the UCSC Physics Department, it is consistent with current observations that dark matter interacts only gravitationally.? I'll ask again when I next go to the Physics Colloquium, in case that claim is now out of date.

--the paper that began this thread claims dark matter must interact in some manner that is not just gravitational, to satisfy the 0.1 cm^2/gram cross section lower bound, to meet observations.

The density of dark matter can be calculated from galactic velocity curves.? For the Milky Way, the velocity of orbiting visible matter is pretty much constant at 225 km/s, except near the center, where it decreases.? From m v^2 / r = G M(r) m / r^2, we get for the enclosed mass within radius r,

M(r) = (v^2 / G) r. The mass density is ?(r) = (dM(r)/dr) / (4?r^2) = (v^2 / 4?G) r^-2 ???? = (6.0e19 kg/m) r^-2

--note the large number "?" symbols. This is due to you using non-ASCII characters which my emailer barfs on. A common problem by some mathfun posters.

At our location 28000 ly = 2.8e20 m from the galactic center, ? = 7.7e-22 kg/m^3 = 0.43 (GeV/c^2) / cm^3. The gravitational acceleration varies as 1/r, so the potential ? varies as log r, and the Laplacian of ? varies as 1/r^2, just like ?, so that ? and ? satisfy the Poisson equation.? Also, for a suitable choice of m/T, ? = exp(-m?/kT), so that ? and ? satisfy the Boltzmann equation.? Thus the 1/r^2 density implies that the dark matter has equilibrated.

--not so: I also can, and have, derived such densities (in my paper-draft you haven't seen...) without any need for any equilibration ever. And anyhow Boltzmann law might be expected on maxent grounds without needing equilibration.

Now here's a question for which I don't know the answer.? If dark matter equilibrates with itself, it should presumably also equilibrate with ordinary matter.? But ordinary matter can dissipate energy and coalesce.? So why can't dark matter do so too using ordinary matter as an intermediate?

--dark and ordinary matter have very small cross section for interaction, which is why dark matter has never been directly detected yet. Any such "equilibration" would take longer than age of universe. The dark-dark interaction is now postulated to be considerably larger than the dark-normal interaction (although still small). Now the whole thought, which Eugene & I explored in posts so far, that I used to generate the conclusion that the dark-dark interaction had to be via a NEW force, not a member of the usual known forces {Higgs,weak,strong,EM,gravity}... I thought that was a new insight... but looking, it seems it, or anyhow something somewhat like it, was already thought of by the Dark Side. Various members of the Dark Side had already came to the conclusion there had to be a new "Dark Force" between dark matter particles to allow this. So we see the Dark Side is now not only postulating most of the universe is made of invisible particles we've never detected, but there also are new forces we've never seen. In fact, why stop there? You can add an entire new Dark Sector to the standard model containing however many new particles and forces it takes -- that gives you more power to fit observational data. Conceivably no matter what you added it still would be unable to match observation, in which case the DM hypothesis would be falsified... but as far as I can tell nobody knows how to prove that, which would mean it is effectively an unfalsifiable hypothesis, i.e "not science." They are constructing castles in the air. If an observation-based criticism comes, they overcome it by fantasizing further extensions to their castle. So, Dark Matter is not in a very healthy state. But, hell, maybe the whole Dark Sector thing is really true anyhow. In some sense it is completely free of new ideas: the Standard Model gives well known recipes for adding new forces and particles (so long as you stay within the allowed patterns) so there is nothing stopping you from adding them and in some sufficiently abstract mathematical sense this is "adding nothing new". -- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)