Fred Lunnon <fred.lunnon@gmail.com> wrote:
"behind (or before) the CMB" puts the problem in a nutshell.
Note that those times are not in principle forever inacessible. Before photons uncoupled from matter, neutrinos did so. And even earlier gravitational waves did so. So there are at least three distinct backgrounds which probe very different eras of the early universe, and so far we've only looked at the last of them. It would be interesting to know whether they're all at the same temperature. The earlier universe was hotter, but perhaps the additional red shift from the earlier uncouplings perfectly compensates for that. Neutrinos may be problematic, since they have mass and don't travel at c. Hence the now-very-cold background neutrinos have had plenty of time to clump with each other and with matter (both dark and light), and lose all record of their earlier patterns. But that's not an issue for gravitational waves. Of course detecting cold thermal gravitational waves is *way* beyond today's technology, but perhaps not tomorrow's.