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Re: Old Stars/Olber's Paradox




I wonder if it is worth mentioning that there is a difference in saying that
the *Universe* is expanding (which presumably Doppler shifts the wavelength)
and saying that *space* is expanding (so the wavelength lengthens as well.).
Yes, most might say they amount to the same result, but our students have a
hard time seeing the difference.

There is a great deal of difference between these two statements, at
least on conceptual grounds, since an attribution to Doppler effects
strongly suggests a local motion for the distant galaxy. On average,
there are no significant local motions -- the red shift happens on
the way through the expanding geometry.

This makes a difference because there is an overwhelming tendency to
imagine a preexisting geometry into which matter is flying as a
result of a tremendous explosion. There might even be an edge to the
distribution of matter, beyond which is empty space. This notion is
perpetuated by every astronomy textbook in the world, which all talk
about how the explosion idea is false and then present a picture of
galaxy red shifts with associated recession velocities. No wonder the
students are confused. It is confusing enough when you stick to the
truth without introducing extraneous and inconsistent falsehoods.

The universe has always been full of mass/energy. It does not fly out
into space. It in fact has no local motion (in the jargon, it all
qualifies as "comoving"). The geometry of spacetime is what is
changing.


Indeed, I find myself wondering what measurement can distinguish between a
distant galaxy's fleeing ours (*into* space) with some Doppler velocity and
a "stationary" distant galaxy with the space expanding between us and it.
Is there any more than the Hubble Red Shift?

Not really. Well, that and the Copernican notion that the Earth is
not the center of the universe (and so, by extension, neither is the
Milky Way). And I suppose we have to add the requirement of
isotropy. IF the universe is isotropic and IF we are not at the
center, THEN the Hubble observations imply an (isotropically)
expanding universe.

And why can't a distant galaxy
have a velocity within the Universe *and* the space of the Universe expand?
How can I tell the difference? I wonder if this is a meaningful question. (:-)

It can. For the most part, these local motions are negligible and
they seem to average to zero (i.e. there is no collective motion of
the whole contents of the universe other than the Hubble expansion).

The local group, for example, are all moving in more or less the same
direction toward some concentration of mass known as the Great
Attractor. This motion is negligible, however, from the cosmological
point of view.



And there was a question of why the background radiation was visible today:

Mechanism 3) is responsible for red-shifting the wavelengths of the cosmic
background 1000-fold since those photons decoupled from the matter which
produced them so that they have a meager intensity in the microwave band
rather than a bright intensity in the visible one. Without this red-shift
the sky would be very bright everywhere from the cosmic backgound in the
visible wavelengths whether or not there are enough point sources (stars,
galaxies, quasars, etc.) to also make the sky bright.

Well yes, David, that is why the background is at 3K, but why can we *see*
it?. I think the answer is that, if we assume that c has been constant since
the BB (we don't have much of a choice but to assume this), that space has
been expanding with the Universe (an unnerving thought), and that the
expansion of space has been <c (but what if it hasn't?), then the photons
have had more than enough time to fill the Universe no matter what the
mechanism of the BB.

Again, it is not a matter of something expanding outward from a
single point to fill space so we have to wait for it to get
everywhere. It already WAS everywhere since the entire space has
always been full of stuff and so every bit of it was producing
radiation which, after suitable cooling due to expansion, became the
cosmic background. You don't have to wait for it to get everywhere
since it was produced locally by the matter present in our vicinity.
The fact that we don't see pronounced variations in the effective
temperature is taken as evidence that the expansion was uniform and
isotropic so all the bits redshifted equally.

The expansion rate has, in fact, probably NOT always been <c. The
significant point of inflationary models is that there was a brief
time when the expansion rate was a great deal *larger* than c. This
solves a lot of the fine tuning problems associated with the
classical big bang model. For example, we receive cosmic background
radiation from regions of the universe that are so distant they could
never have been in causal contact in the classical BB but
nevertheless they seem on the basis of the background radiation to
have been always at the same temperature. Pre-inflation, this was
simply built in as an assumption albeit a very unlikely one. The
inflationary explanation is that these regions *were* in causal
contact before the inflation and had sufficient time to come to
thermal equilibrium. Then the inflationary period pushed them out of
causal contact but since the expansion is isotropic, their
temperatures both decreased by the same factor. There are a number of
other technical problems with classical BB cosmology that are solved
in a similar way by inflation but then inflation introduces some
idiosyncratic technical problems to replace them.


BUT this doesn't answer for me why I can see quasars (or Q's flashlight) --
which is where we were in the Old Star thread. (Maybe I have one of those
senility caused blocks like the one I still have re retrograde motion (:-))


You can see quasars because they are within your horizon distance (and
because they are very bright).

Paul J. Camp "The Beauty of the Universe
Assistant Professor of Physics consists not only of unity
Coastal Carolina University in variety but also of
Conway, SC 29526 variety in unity.
pjcamp@csd1.coastal.edu --Umberto Eco
pjcamp@worldnet.att.org The Name of the Rose
(803)349-2227
fax: (803)349-2926