Re: A Question on cosmic background radiation

["John S. Denker" <jsd@MONMOUTH.COM>]

I asked, rhetorically:
> > Just what is it that is accelerating?
> >   -- The background-radiation photons are on lightlike trajectories,
> >  and always have been, so you're going to have a hard time accelerating
> >  or decelerating them.
> >   -- Unruh's argument, as I understand it, involves acceleration
> >  of a black-body source.  Now the source of the cosmic background
> >  radiation ceased to exist many billions of years ago, so I fail
> >  to see how any expansion or exacerbation at the present time could
> >  have any relevance thereto.

Robert B Zannelli replied:
> ... just to answer your question, what is accelerating is the
> scale factor of those astronomical structures not sufficiently
> gravitationally bound. If this is the case, then event boundaries are being
> created as da/dt increases.

This literal answer bypasses the core of the question:
What is there _of any relevance_ that is expanding?
  *) Scale factors are not the source of the radiation.
  *) Unbound astronomical structures (clusters of galaxies etc.)
     are not the source of the radiation.
  *) Event boundaries (whatever those are) are not the source
     of the cosmic background radiation.

There is some very specific, very conventional physics to which
we ascribe the origin of the background radiation: hot plasma
cooling to form atoms, that's all.

> If Hawking and Unruh are correct then ....
> since the CMB is
> primarily the signature of the so called creation event.

I would be very surprised if Hawking or Unruh considered the
cosmic background radiation to be the signature of any so-called
"creation event".  It is the signature of something that happened
at roughly t=300,000 years (T=3000 K) in the standard model.
That's not t=0, not by a long shot.

Cosmic background radiation is not the same as Hawking radiation.
No connection at all, as far as I can see.

==============================

If you take a gas of photons (or anything else!) and put it in
a box of size a(then) and expand it to size a(now), the wavelengths
will expand by a factor of a(now)/a(then).  This is quite independent
of the trajectory of a() as a function of time, provided the
trajectory is not so weird as to make the expansion non-adiabiatic.
And I don't think anybody is suggesting that non-adiabaticity is
the issue here.  In particular, the ratio is independent of the
present-day expansion rate, and independent of the present-day rate
of change of the expansion rate.  This is an elementary, non-tricky
calculation, and I see no reason for distrusting it.