Re: Gravitational astronomy

[Leigh Palmer <palmer@SFU.CA>]

> > Observationally we know there is a limit to how far
> > we can see. The three degree background radiation sets that limit. It comes
> > from a time when the universe became transparent, further than that cannot
> > be seen since radiation swamps out any information. This physical
> > limitation makes the number of galaxies uncountable physically.
>
> I agree that the microwave background sets an observational limit for
> photon astronomy, but what about gravitational radiation?  Isn't it
> possible, if only in principle, to use the gravitational waves emitted
> before the recombination time, when the universe became transparent to
> photons, to study the structure of the early universe?

What this means is that there is an observational limit on how far into
the past we can see. It is, as yet, not certain that gravitational waves
can be detected (they never have been detected). Neutrino astronomy, on
the other hand, provides in principle an observational window on an
earlier time. Photons decoupled at about t = 300,000 y in the standard
model, in the time of atomic recombination (a misnomer). Neutrinos
decoupled at the time of primordial nucleosynthesis, at age t = 300 y
or less. This may seem like "only 299,700 years earlier", but it is
really better seen as another factor of a thousand earlier in time,
which is very significant, since the universe is thought to have been
very different at that time.

Leigh