🚀 go-pugleaf

As someone who has spent some years playing with cosmology, I'd thought
I'd kick in a few comments on the Big Bang.

There are at least two senses in which people are using that
phrase.  In one sense it refers to the idea that are universe is
currently expanding, has been doing so for some time, and at some
finite time in the past (somewhere between 10 and 20 billion years ago)
was extraordinarily hot and dense.  In another sense it refers to 
the idea that the universe began with an initial singularity, and
"before" this point neither time nor space existed.  The first version
of this idea can be analyzed using  our current understanding of
physics (like GR, particle physics etc.) and by choosing to approximate
our universe as homogeneous and isotropic.  This usually goes by
the name of the "standard model". 

There is an impressive amount of evidence in favor of the first
idea and no compelling evidence against it.  Besides being consistent
with physics as we know it, it  can be tested by comparing the
current blackbody background temperature (probably 2.75pm.05)
with the estimates of the primordial abundance of certain light
elements (H, He3, He4, D, Li7).  Consistency between these
figures can be used to constrain the average baryonic density
of the universe (This does not represent a trivial prediction.
It is difficult to get consistency and easy to imagine answers
for the elemental abundances which would be inconsistent with this
picture.)  In addition, it has been used to predict the number
of light, or massless weakly interacting particles (i.e. neutrinos).
This prediction has just been verified.  It is probably worth
noting that the predicted abundance of baryonic matter is not 
inconsistent with dynamical estimates of the mass density of the
universe.  It has already been mentioned here that this model
has the only consistent explanation for the blackbody background.

The weakest aspect of this model is that it does not incorporate
galaxy formation.  There are a number of  galaxy formation models
that are consistent with the standard model, some of which cannot
be ruled out at the present time.  None of them is particularly
compelling, although models using cold dark matter are probably
ahead of the rest.  These models may fail if current observations
of large scale structure and motion hold up.  One of the tightest
constraints on all such models is the incredible isotropy of
the blackbody background.  The growth of gravitational structure
in an expanding universe is slow enough that the perturbations
that would have to grow into galaxies should have left some
remnant disturbance in the blackbody background.

Inflation is a way of explaining the features of the standard model
by using particle physics, and appealing to the high temperatures
present at *very* early times.  It is a nice idea, but has problems
and may not end up as part of the standard model.  As someone
else has noted it does seem to predict that the universe is
very close to critical density.  It does *not* predict that
the universe is closed or open (although specific models of inflation
do make such predictions).  The bulk of the observational evidence
seems to indicate that the universe has a much lower mean density.
Inflation (at least in some versions) also predicts a power spectrum
for temperature anisotropies in the blackbody background.

The idea of the Big Bang as a  real singularity in the fabric of
space-time is another issue.  This view results from an unreasonable
extrapolation of classical GR into temperature regimes where quantum
effects must be important, and there is no consensus view on how to
make  such corrections.   There have been many speculative papers on
how to do away with the initial singularity without changing the
standard model appreciably.  None of them command general assent, nor
are they likely to in the near future.

The bottom line is that the initial singularity and inflation could
be disproved (in some sense) tomorrow and our view of the universe
would not have changed a great deal.  I certainly wouldn't regard
it as the death of the "Big Bang Theory".
-- 
 I'm not afraid of dying     Ethan Vishniac, Dept of Astronomy, Univ. of Texas
 I just don't want to be     {charm,ut-sally,emx,noao}!utastro!ethan
 there when it happens.      (arpanet) ethan@astro.AS.UTEXAS.EDU
    - Woody Allen            (bitnet) ethan%astro.as.utexas.edu@CUNYVM.CUNY.EDU

These must be my opinions.  Who else would bother?