If You Could See Neutrinos...

...how far back into the universe could you see?

For the case of photons we look at the cosmic microwave background (CMB) and use it to construct an image of the so called 'surface of last scattering', which represents the moment when photons decoupled from and were able to escape the cosmic plasma. This corresponds to a t=380,000 years (with t=0 being the very beginning) in the current theory.

For the case of neutrinos we would look to the cosmic neutrino background (CNB) and try to use that to construct a surface of last scattering for the neutrinos. At what time did neutrinos decouple from the cosmic plasma? Current theories predict that this decoupling time occurred when the universe was less then a second old.

It would stand to reason then that one should be able to use neutrinos to 'look further back' into the universe. In other words, one would expect the last scattering surface for neutrinos to be further back than the last scattering surface for photons.

However, at least one paper finds the opposite to be true:

http://arxiv.org/abs/0907.2887

Somewhat counter-intuitively, the CNB last scattering surface turns out to be much closer and much thicker than the CMB surface--the paper suggests it has to do with the fact that neutrinos are able to have different velocities from each other.

So perhaps the neutrino's LSS is inside the CMB's, but where might the dark matter LSS be?