Re: Is the Sun mostly dark matter ?

carlip-nospam@xxxxxxxxxxxxxxxxxxx wrote:
YKhan <yjkhan@xxxxxxxxx> wrote:
[...]
Presumably when DM was
made in the Big Bang, DM must have come out with all kinds of energy
levels, some travelling fast, and some slow. Some of the DM must be
moving slow enough that it can be captured in orbit around stars and
planets too, but apparently DM only comes out in high-speed versions
that can only be captured in orbit by the gravity of galaxies.

In the extremely early universe, dark matter was in thermal equilibrium
with the rest of the universe. At thermal equilibrium, there is certainly a distribution of energies, but the distribution is determined by the
temperature, and at the very high temperatures of the extremely early
universe, "slow" speeds are scarce.

As the universe expanded, energies and densities decreased, and at some
(still quite early) time, dark matter "decoupled" from ordinary matter.
The details of this process depend on the exact nature of dark matter,
which we don't know, but the basic ingredient is that the density became
too low of particles and antiparticles of dark matter to "find" each other
to annihilate, and the energy became too low for annihilation of ordinary
matter and antimatter to produce dark matter.

After decoupling, dark matter continued to cool, but (essentially) only because of the expansion of the universe. This means that the temperature,
and therefore the velocity distribution, evolved in a simple, predictable way. This can be calculated, and we can predict how much dark matter is captured by various gravitational sources.

Also why doesn't DM congregate around intergalactic gas?

It does. You just have to calculate how much.

There is
apparently enough of this gas around to create billions more galaxies,
yet the Bullet Cluster collision clearly shows that DM doesn't get
attracted to the gas, just to the galaxies. This despite the fact that
the gas outweighs all of the galaxies by several fold in that
collision.

No, that's not what the Bullet Cluster shows. The collision of gas in the Bullet Cluster drastically slowed the motion of the gas, while not affecting the dark matter, which kept moving. Eventually the gravitational interaction will pull some of the gas and dark matter back together, but this takes time -- after all, if you throw a ball into the air, the Earth's gravity doesn't *instantaneously* stop it and pull it to the ground. (I don't know off hand
whether the present relative velocity of the gas and dark matter is faster
than the escape velocity; if it is, most of the gas and dark matter will continue to separate forever.)

(The stars in the galaxies are pretty much bystanders. As you say, their mass
is small compared to that of the gas. The fact that they move with the dark
matter doesn't have so much to do with gravity -- they are just keeping their
original velocities, and so is the dark matter.)

Some of this can be a bit subtle, and it's fine to ask questions. But please
don't assume that no one has ever thought about these issues before. There
is very detailed modeling of the degree of clustering of dark matter, for
example -- I gave a qualitative answer, but if you look in the literature you
will find very careful, quantitative calculations and simulations.

Steve Carlip


Thanks!
.

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