Re: Postulates of Relativity and The Cosmic Background - Question

Tom Roberts <tjroberts@xxxxxxxxxx> wrote:
> Paul Valois wrote:

[...]
>> I have read about the Cosmic Background Radiation left over from shortly
>> after the Big Bang. This radiation consists of an almost perfectly
>> uniform distribution of photons, neutrinos and gravitons.

> Just electromagnetic radiation. The mechanism for generating the CMBR
> does not apply to either neutrinos or gravitons.

Oops. The rest of Tom's post is fine, but this is wrong. At high
enough temperatures, in the very early Universe, neutrinos are also
in thermal equilibrium with electromagnetic radiation and electron-
positron pairs. They "freeze out" at a slightly lower temperature
(basically from a count of degrees of freedom of electron-positron
pairs vs. neutrinos). The prediction is that there should be a 1.95K
cosmic neutrino background. The problem is that these are very, very
low energy neutrinos, and the neutrino cross-section depends strongly
on energy; for now, cosmic background neutrinos are not detectable.

Gravitons are a bit different -- the Universe is essentially transparent
to gravitational waves up to near the Planck temperature, so they won't
have been in equilibrium. But inflationary theories typically predict
cosmic background gravitational radiation, produced by the same type of
process that leads to the initial density fluctuations. The amount of
such radiation is model-dependent, though; for some models it may be
detectable by a next generation of space-based laser interferometer
detectors (e.g., the proposed Big Bang Observatory), but for others it
won't be.

Steve Carlip
.

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