Re: Old Stars/Olber's Paradox

[Jim Green <Jim.Green@Snow.edu>]

David Bowman has this wonderful bent of giving vivid and compact overviews
of the issue -- but this time has not quite answered the question --
probably because the question was not well formed:

I meant to ask something like "How does the answer to Obler's Paradox
involve the rather gymnastic posts re the Old Star thread?"

Let me comment a bit on David's post and add a question or two:

I mumbled in part the following question: 

> > So somebody please tell me why the sky is dark -- in view 
> > of this discussion about "Old Stars, quasars, and Q's flashlight.  

David ably answered as follows:

> There are 4 possible suggested mechanisms (that I can think of) 
> as to why the sky would be dark in most directions. 

> 1) The universe is finite in spatial extent giving only a finite 
> number of point light sources.

> 2) The universe is finite in time.  Since it has only been around 
> for a finite amount of time there is a
> light-travel-time-since-the-beginning horizon beyond which no 
> light can be seen.


> 3) The universe is expanding--causing the light from the 
> most distant sources to be red-shifted away to ever lower 
> (& dimmer) photon energies.

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.
  
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?  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. (:-)


> 4) The luminous matter of the universe is hierarchically 
> arranged in a fractal arrangement whose Hausdorff dimension 
> is less than 3 so that the sources of light are a set of 
> measure zero in space (yielding an average density of zero)
> allowing there to be not enough light sources to light up 
> the sky in most directions.

Now this is new thinking to me.  Is this commonly discussed?  A bit of a
tutorial would help here -- at least for me -- others??.

> A few years ago a consensus developed as to which one was 
> more important, but I can't remember which way it came out. 
> (Maybe a legitimate list-resident general relativist can 
> help us out here.)

Yes, yes, yes -- a tutorial would help here as well.

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.

Also if this were not the case then it would not be that the sky would be
"very bright everywhere" in the visible, the photons would be well past the
gamma and we would be long ago fried.

This story seems to me to be as important as the resolution of Obler's
Paradox to defend the idea of a BB.

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 (:-))


Still babbling (but not drooling yet) over here in my little corner


..




Jim.Green@Snow.edu                             


Hey, let's have some new cliches.