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# Re: Olbers

• From: John Denker <jsd@AV8N.COM>
• Date: Fri, 13 Feb 2004 03:30:11 -0800

Quoting Mark Sylvester <msylvest@XNET.IT>:

Both the finite age of the universe and the fact that it's expanding
contribute to explaining Olbers' Paradox, and I'm told that the former is
the main contribution. I also sometimes read that both are *necessary*. Why
is this the case, if indeed it is?

As a preliminary, we have to decide whether we are trying to explain
just the Olbers observation (i.e. that the night sky is not
blazingly hot and bright) ... or trying to explain that and all
the other observational data.

Turning to the main question at hand: Finite age and expansion
are both required elements _of the usual explanation_ but that's
not the same as saying they are both necessary in the absolute
sense.

To see why, let's investigate BC's argument, and see why it is
not satisfactory. Yes, stars live a finite time. If you follow
a straight line, you will eventually smack into the surface of a
star, and if that happens to be a burned-out star, it contributes
less to the sky-brightness than an actively burning star ... but
how much less? Suppose you have just a bunch of cinders at
temperature T. They are in thermal equilibrium with each other.
Now add some stars to the mix. Just a few. Over time, they will
raise the temperature of the cinders. Given enough time, the
cinders will warm up until they are in thermal equilibrium with
the stars. Sooooo, either
a) you need some way of cooling down whatever stuff (matter and
radiation) was left over from earlier times, or
b) you need finite age, i.e. some time in the past when there
were a lot fewer stars than we have now, or
c) both.

That is: If the Olbers observation is the only fact at your disposal,
a) You cannot rule out a Hoyle-style steady-state expansion, i.e.
expansion with no age limit.
b) You cannot rule out a creationist deus-ex-machina scenario
where X billion years ago the stars were turned on. This of course
doesn't explain anything; it essentially says we see what we see
because some huge exception to the laws of physics made it so.

Given non-Olbers data, e.g. the microwave background and/or the
background galaxies, such theories become quite unattractive in
comparison to the standard model.
http://www.nobel.se/physics/laureates/1978/wilson-lecture.html
http://nedwww.ipac.caltech.edu/level5/Tyson2/Tyson4.html