Newsgroups:
sci.physics.plasma
From: ethan@astro.as.utexas.edu (Ethan Vishniac)
Organization:
McDonald Observatory, University of Texas @ Austin
Subject: Re: Big
Bang
My apologies for my slow response. I've got a lot going on right
now and between the semester
starting and the High Holidays September
is always my busiest month.
I'll
try to make this brief. This really
calls for just a little
clarification.
Dr. Lerner's last message contained several
misunderstandings.
1. All distances quoted in my last article were
written with a factor
of $h^{-1}$, where $h$ is Hubble's constant in units
of 100 km/sec/Mpc.
(Well, all but one.
I did leave it out once.) This is standard
practice to eliminate
ambiguity and doesn't constitute an endorsement of
any particular value
of Hubble's constants. I said this in
the original
article, but since Dr. Lerner seems to have missed this
point it's
possible other readers did too.
2. When I mentioned mean distances that
galaxies had to be displaced
to account for observed structures I was
using dynamical models
when possible (i.e. in the invocation of a
spherical shell model
for large structures and in the use of linear
perturbation theory).
Dr. Lerner implied that such estimates assumed that
galaxies not
actually in the structure didn't move at all. I have no idea where
that came
from.
3. The reports of
structure at scales of more than 100 $h^{-1}$
Mpc do not imply that all
galaxies are in such structures.
(That
would be absurd anyhow, from local observations.) The
Broadhurst et al. data shows the largest signal (in their
published
work) and suggests that roughly half of all galaxies are in
such
structures. Further
observations by those authors, and work
by competing groups (e.g. the
collaboration by Schecter, Shectman
and several others), suggest a
smaller number, whose exact value
is hotly disputed. A value of 0.3 was meant to be
generous.
4. It's worth mentioning that further work seems to show
that the
periodicity seen by Broadhurst et al. is real, in that
particular
direction, but does not show up in other surveys. The possibility
that this results from
a random realization of statistically
isotropic power on large scales is
present at about the 2 percent
level if we restrict ourselves to that data
set alone, and is
somewhat larger if we take into account other
surveys. This
is intriguing, but
while it may suggest problems with current ideas
on galaxy formation, it
can't be said to be for or against the
Big Bang unless one presents a well
defined alternative which explains
this naturally.
5. I assumed
a critical density when estimating peculiar velocities.
One can reconcile
observed structure with smaller velocities if
we live in a less dense
universe, which is actually favored by
some observations.
6. The IRAS data does not support a discrepancy
between the
gravitational instability hypothesis in the standard
model
and observed clustering and velocities. Interested readers
may wish to consult `IRAS Galaxies Versus POTENT Mass - Density
Fields,
Biasing and Omega' by Dekel, Bertschinger, Yahil, Strauss,
Davis,
and Huchra (Astrophysical Journal 412, 1, 1993). This is actually
just an example of a
rather larger literature, but does contain
many of the critical
references, as well as a discussion of the
IRAS catalog. The basic method employed by the POTENT
program is to
use the observed radial component of the peculiar
velocities, together
with the assumption that these velocities arise from
an irrotational
velocity field (as one would expect from gravitational
instability) to
reconstruct the underlying gravitational potential. One recovers the
large scale
distribution of mass, which turns out to be in good agreement
with the
observed large scale distribution of galaxies.
The
authors of this paper solve for the density parameter and the
biasing parameter (ratio of luminosity variations to density
density
variations) and obtain a constraint of Omega^{0.6}/b_I
= 1.28 (+0.75,
-0.59), where b_I is the biasing parameter appropriate
for the IRAS catalog
(which might well be one, on the basis of this
work). Note that this is a detailed *dynamical*
reconstruction, of the
kind that Dr. Lerner has not done, which finds that
the observed
structure is consistent with the observed motions in the
context of
gravitational instability in the standard BB model. This falls well
short of proving that
the observed structure *must* result from
this kind of process, but is
sufficient to show that Dr. Lerner's
claim that the observed structure and
peculiar velocities can
be used to rule out the standard model is utterly
wrong, to the
point of being dishonest.
I will quote Dr. Lerner
here. He says:
>But that
[Bertschinger's work] says nothing about how long it took to
>generate
these concentrations, which is the point under discussion.
For the
reasons cited above, this is exactly wrong and constitutes
a
misrepresentation of this work.
On light elements:
>The
lower limit from D of 23.8% (for He4) is well known
Not quite. Direct measures of D are not very useful
since it has been
destroyed in the course of the galactic evolution. Current abundances
are lower than the
primordial abundance and give an *upper* limit
for He4. If we just take the current measurements (D
about 1.6x10^-5)
we get He4 less than .251, which is not
restrictive.) We can do a
bit
better by trying to model the chemical history of the galaxy.
It is
generally thought to be more reliable to take the sum of
He3 and D which
yields a lower bound for He4 of approximately
0.238. That must be what you're talking
about.
>Dr. Vishniac ignores the Balser
>results, but he
can't or shouldn't just throw out data he
>dislikes. The He3 results require an He4 abundance of
at least
>24.4-24.9% for consitency with Big Bang predictions.
Unfortunately,
I didn't save your original reference to Balser. The
most recent work I could find is Balser et al. (ApJ 430,
667). In
this abstract they
say
`We report our progress in measuring the cosmic abundance of
3He.
...... If averaged over the
initial mass function stars are not
a net sink for 3He, then our lowest
abundance ratio places
a lower limit on the cosmological parameter eta,
the baryon to
photon ratio. The
3He lower limit is about the same as the upper
limit from D, 4He, and 7Li,
eta about 4x10^{-10}.'
They do report a lot of variation in their
results, which they
interpret as being due to local enrichment. It does not
correlate with galactic
chemical evolution in any obvious
way.
I'm not sure whether Dr. Lerner disagrees with their
interpretation
of their work, or whether he's referring to some
other work.
I've
deleted a bunch of other stuff which, IMHO, amounts to trying
to squeeze
preferred answers out of ratty data while making light
of the idea that
one should stick to defining the limits consistent
with the data. Needless to say, I think is poor
science.
--
"Quis tamen tale studium, quo ad primam omnium
rerum causam evehimur,
tamquam inutile aut contemnendum detractare ac
deprimere ausit?"-Bridel
Ethan T. Vishniac, Dept. of Astronomy, The
University of Texas at Austin
Austin, Texas, 78712
ethan@astro.as.utexas.edu