Galaxies without dark matter halos?

From: Ulf Torkelsson
You are actually forgetting one of the strongest arguments in favour
of the big bang. The abundances of the light elements. It is
tremendously difficult to make a universe consisting of 75% hydrogen
and 25% helium without a hot big bang, and it is virtually
impossible to make any deuterium in any other scenario. The big bang
at the same time explains these abundances and the microwave
background.

Please correct me if I'm wrong: Wouldn't you get the same universe
consisting of almost 75% H and 25% He, including some deuterium, so long as
the model includes any hot large explosion that produces all the matter we
observe? Is there something about the physics of that reaction that requires
that it not be a part of a larger universe?

-- Richard S. Sternberg

[Mod. note: top-posting fixed. Please don't top-post -- mjh.]

Richard S. Sternberg wrote:

Please correct me if I'm wrong: Wouldn't you get the same universe
consisting of almost 75% H and 25% He, including some deuterium, so long as
the model includes any hot large explosion that produces all the matter we
observe? Is there something about the physics of that reaction that requires
that it not be a part of a larger universe?

In order to be consistent with the high degree of isotropy of the
microwave background and the fairly homogeneous distribution of mass
on the largest scales, the nucleosynthesis must have taken place
uniformly throughout the universe that we can observer today.
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

Richard S. Sternberg wrote:

Please correct me if I'm wrong: Wouldn't you get the same universe
consisting of almost 75% H and 25% He, including some deuterium, so long as
the model includes any hot large explosion that produces all the matter we
observe? Is there something about the physics of that reaction that requires
that it not be a part of a larger universe?

In order to be consistent with the high degree of isotropy of the
microwave background and the fairly homogeneous distribution of mass
on the largest scales, the nucleosynthesis must have taken place
uniformly throughout the universe that we can observer today.
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

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. opinionBut 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

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. opinionBut 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