In article <JOudnb4iKeopf03anZ2dnUVZ_u6rnZ2d@[EMAIL PROTECTED]
>,
max@[EMAIL PROTECTED]
(Erik Max Francis) says...
> Larry Caldwell wrote:
>
> > I have been reading the popular scientific articles about Dark Matter,
> > and am having trouble digesting the concept. My big problem is
figuring
> > out how it cooled off enough after the Big Bang to settle down and
form
> > galactic halos. When normal matter was formed in the Big Bang, it was
> > very hot. It cooled by emitting huge amounts of EM radiation. Dark
> > Matter doesn't emit EM radiation. Where did the energy go? The weak
> > force is far too short range to provide a pathway.
>
> It went away through cosmological red****ft, the same way all the other
> energy of the Big Bang went away. As the Universe expands, everything
> in it cools, including things that don't interact with anything else.
> Energy is not globally conserved in general relativity.
I'll have to think about that. It's not apparent how red ****ft cooling
could result in the observed distribution of Dark Matter. We can detect
the original hard radiation from the big bang as the CMB because, no
matter what wavelength, photons always travel at the speed of light. It
took those photons a long time to get here, and Dark Matter would have
to travel even farther. If Dark Matter doesn't interact with itself, it
left the Big Bang before the universe achieved EM transparency, probably
at the same time density dropped far enough to achieve neutrino
transparency. It's not intuitive to me how Dark Matter ended up closely
associated with normal matter, when it started out separately. OTOH, if
dark matter has its own gauge particle, the universe could have achieved
Dark Matter transparency about the same time it achieved EM
transparency, and both types of matter could have cooled at
approximately the same rate.
> > It's fairly easy to postulate some form of radiation that interacts
with
> > Dark Matter that does not interact with normal matter, but that just
> > leads me to another point of confusion. I don't see how this
> > hypothetical 'sneaky energy' could behave like the Dark Energy the
> > astronomers talk about.
>
> Dark energy has nothing to do with dark matter in terms of a direct
> relation****p. Dark matter is the name for matter we can't see; dark
> energy is the name for a repulsive energy whose cause isn't yet clear.
> But there's no reason to believe that they are directly related
> phenomena; they're just gaps in our knowledge. One name is a spinoff of
> the other, but that doesn't indicate any kind of actual relation****p
> between the two.
I'm not certain that the acceleration in expansion is not an artifact of
the topology of the universe. Topology has fallen out of favor in
cosmology for some reason. Maybe it's just hard to write about. I
don't know of any good description of predicted observations from the
inside of a singularity. Most people don't even seem to realize we are
still inside the big bang. Every point in the universe is moving at c
WRT the big bang, and always has been. The only way to reconcile that
with observation (and the passage of time) is through multi-dimensional
topology. The observed acceleration in expansion has interesting
implications for the shape of space-time.
> > It would have to be an analog of the EM
> > momentum exchange that allows normal matter particles to bump into
each
> > other and shed momentum. If Dark Matter can collapse into a galactic
> > halo, can it form dense aggregates.
>
> That doesn't follow, since present-day cool dark matter is cool enough
> to collect around galaxies, but not smaller objects.
That's a pretty narrow range of coolness. :) Particles in the galactic
halo would be orbiting pretty slowly. If Dark Matter does carry a weak
charge, it should be stopped by any collision with a neutron star, among
other means. Massive objects should continually gain mass until they
collapse into black holes. If Dark Matter is as common as it is thought
to be, the gain in mass, and transfer of momentum, should be measurable.
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