The Cosmic Neutrino Background
In the very early universe matter was dense and hot. With the expansion of space, matter cooled down which eventually allowed for the formation of nuclei and later atoms, molecules and increasingly large structures. Atoms could be formed when the average energy of electrons decreased to a value so small that ionization became improbable - an event called recombination. Photons, which prior to recombination were scattered on the free electrons, could then travel almost undisturbed. This happened at a temperature of about 1 eV, or some thousand Kelvin. Due to the continuing expansion of the universe, the photons from that time became redshifted, but are still present today. Their temperature is now at 2.7 K, and they have become famous under the name Cosmic Microwave Background (CMB). The temperature fluctuations in the CMB carry information about the structure of matter at the time of the photons' decoupling from matter. WMAP has measured these temperature fluctuations with great accuracy. (We discussed the CMB and some of what we have learned from it here, here, here and most recently here.)
The photons that we are so used to rely on for "looking" do not allow us to learn anything about the early universe prior to recombination. But we can try to see by other means. Neutrinos are well known for being weakly interacting, which is why they are so difficult to detect. But that they interact only weakly also means neutrinos ceased to scatter on the hot matter in the early universe earlier than photons. This happens at the typical energy scale for the weak interaction, at about 1 MeV or 1010K, after which the scattering of neutrinos and anti-neutrinos to produce an electron-positron pair became very improbable and, briefly after this, nucleosynthesis took place. Today, the temperature of the cosmic neutrino background, C?B, is about 10-4 eV or 2 Kelvin* and it's all around us.
While we have not yet measured the absolute neutrino masses, but only have upper bounds, neutrino oscillations test for the differences of squares of masses. This allows us to conclude that at least some of the neutrino species must have cooled so much that their kinetic energy is smaller than their restmass, which means they are non-relativistic. This is interesting because these neutrinos will then clump in gravitational fields like that of our Milky way. As a consequence, the density of neutrinos on the path of planet Earth is roughly one to two orders of magnitude larger than the average density.
Still, these C?B neutrinos are very difficult to detect. But difficult is not impossible. Neutrino capture on tritium would, with some effort but presently available technology, yield a detection rate of maybe 10 C?B neutrinos per year [reference]. That would be enough to confirm the presence of the C?B, but to measure temperature fluctuations, with that procedure we'd probably have spend some million years doing nothing but gathering statistics, not to mention that tritium doesn't grow on trees. Alternative to tritium, it has recently been proposed to instead capture anti-neutrinos on Holmium, which, with some effort and some luck, might yield comparable detection rates. Direct detection of the C?B is the first step. Since the detection rate depends on the neutrino-density, it would not only confirm our theories about the creation of the neutrino-background, but give us information about the distribution of neutrinos in the gravitational field of our galaxy.
* It is (4/11)1/3 times the temperature of the CMB. The conversion factor is partly due to neutrinos being fermions while photons are bosons, and partly due to the photons gaining in density, and thus temperature, when electron-positron pairs annihilate to photons while the opposite reaction becomes increasingly improbable. When this happened, neutrinos had already decoupled.
