"Steve Willner" wrote in message ...
In article ,
"George Dishman" writes:
We are now getting good data from the CMBR in particular
from WMAP but that only goes back to around 300k years
at z=1089+/-97.

The most obvious previous observable would be relic
neutrinos. I have not been able to find an estimate
of z for them but it would obviously be much higher
than that for the CMBR and since even solar neutrinos
are hard to detect, relic neutrinos will be even more
difficult.

As you say, they decouple earlier than the recombination epoch, and
then their temperature drops as R^-4 while the temperature of
everything else is dropping as R^-3. Thus they will be much colder
than the microwave background.

I remember seeing that a few times now you mention it. The
rate was higher in the 'radiation dominated' era.

Can anyone tell me a rough value for z for neutrinos
from when they decoupled and say whether there is a
credible prospect for producing a detector capable
of imaging them like WMAP?

It must be approximately at the epoch when temperature corresponded
to a few MeV, i.e., about when deuterium and helium are being
created, but I bet there's a more precise number around somewhere.

Ouch, that's low. Thanks Steve, that gives me another keyword
or two for searches, I appreciate your reply.

Detecting such low-energy neutrinos looks hopeless to me, but there
are plenty of people cleverer than I am.

Given what SuperKamiokande and the new one in the Antarctic
are achieving, I wouldn't rule out anything over the coming
decades.

best regards
George