"GD" == George Dishman writes:

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

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

I'm looking at the discussion in MTW's _Gravitation_. They estimate
that neutrinos decoupled from matter at a time about 100 seconds or
when the scale factor was about 1E-9 of its current value. That would
suggest a redshift z ~ 1E9.

There's also some discussion at
URL:http://www.astro.soton.ac.uk/%7Etrm/PH421/notes/notes/node152.html
describing why the temperature of the cosmic neutrino background
should be T ~ 2 K, lower than that of the cosmic microwave
background. The neutrino temperature is lower because after they
decouple, their temperature simply decreases with the Universal
expansion. In contrast, the photons are heated by electron-positron
annihilation that occurs later.

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