We continue our series of excerpts (and discussion) from the outstanding survey paper by George F. R. Ellis, Issues in the Philosophy of Cosmology.
Neutrinos and gravitational waves will in principle allow us to peer back to much earlier times (the time of neutrino decoupling and the quantum gravity era respectively), but are much harder to observe at all, let alone in useful directional detail. Nevertheless the latter has the potential to open up to us access to eras quite unobservable in any other way. Maybe they will give us unexpected information on processes in the very early universe which would count as new features of physical cosmology.
The cosmic microwave background (CMB, T = 2.73 K) points us to a time 380,000 years after the Big Bang when the average temperature of the universe was around 3000 K. But there must also exist abundant low-energy neutrinos (cosmic neutrino background, CNB, CνB, relic neutrinos) that provide a window to our universe just one second after the Big Bang during the radiation dominated era. That’s when neutrinos decoupled from normal baryonic matter.
As the universe expanded, these relic neutrinos cooled from a temperature of 1010 K down to about 1.95 K in our present era, but such low-energy neutrinos almost never interact with normal matter. Even though the density of these relic neutrinos should be at least 340 neutrinos per cm3 (including 56 electron neutrinos per cm3 which will presumably be easier to detect), detecting them at all will be exceedingly difficult.
Neutrinos interact with matter only through the weak nuclear force (which has a very short range), and low-energy neutrinos are much more difficult to detect than higher-energy neutrinos—if they can be detected at all. If neutrinos have mass, then they will also interact gravitationally with other particles having mass, but this interaction is no doubt unmeasurable due to the neutrino’s tiny mass and the weakness of the gravitational force between subatomic particles.
The cosmic gravitational background (CGB) points us to the time of the Big Bang itself. Faessler, et al. (2016) state
The inflationary expansion of the Universe by about a factor 1026 between roughly 10-35 to 10-33 seconds after the BB couples according to General Relativity to gravitational waves, which decouple after this time and their fluctuations are the seed for Galaxy Clusters and even Galaxies. These decoupled gravitational waves run since then with only very minor distortions through the Universe and contain a memory to the BB.
Ellis, G. F. R. 2006, Issues in the Philosophy of Cosmology, Philosophy of Physics (Handbook of the Philosophy of Science), Ed. J. Butterfield and J. Earman (Elsevier, 2006), 1183-1285.
Faessler, A., Hodák, R., Kovalenko, S., and Šimkovic, F. 2016