Re: Galaxies without dark matter halos?

On Wed, 27 Aug 2003, Ulf Torkelsson wrote:

> Furthermore in order to get the observed element abundances the
> material that underwent the nucleosynthesis must have consisted of
> free protons and neutrons in the ratio expected a few minutes after
> the big bang.
>
> Ulf Torkelsson
>
A bit stronger than that: If the expansion & cool down is too slow
("nuclear reactions stay in equalibrium") then there is time for larger
atoms to built up and those with the most binding energy are favored
--Lots of iron and little hydrogen results. But if it cools too fast there
isn't a chance for helium to form and any neutrons tend to decay to more
hydrogen rather than be captured --lots of hydrogen and little of anything
else.

Since the heavier elements tend to be built up in steps, how much of one
element is made at one time is sensitive to how much of some particular
lighter elements were made ealier, so the final abundances are sensitive
to some details of the cool down.

Folding in the binding energies vs. tempurature, densities
etc. and running the cool down of the universe as predicted by the
standard (Friedman-Robertsosn-Walker metric) you get good agreement
with observed abunancies or the first few elements which are, admittedly,
hard to measure. <opinion>But it is impressive to get any agreement...
</opinion>

Some of this is covered at a popular level in George Gamow's _The creation
of the universe_ and in many (most?) cosmology texts, i.e. _The Early
Universe_ by Kolb & Turner.


 Note that this happens well after the time (below the energy
scale/temperature) of inflation but before recombination (CMB)

  3ch