# Can we look further than the CMB?

As I understood it, the map of the cosmic microwave background (CMB) corresponds to the time when photons decoupled from matter, making the universe transparent. In this sense, we might expect the CMB to be a limit to how far we can see.

Nevertheless, is it possible to say that we can look further than that? For example, if we look at the CMB fluctuations, or at the time-evolution of the CMB spectrum, can we infer/look beyond the time at which photons decoupled?

EDIT: as mentioned in a comment, what I was suggesting is that observing something other than electromagnetic radiation would allow to go further. The standard model does not give many options, and gravitational waves seems to be the best one.

• This applies only to electromagnetic radiation. In fact, observatories like LIGO and VIRGO strive to be able to observe gravitational waves from 'the big bang', which would go further than the CMB. Dec 11 '19 at 11:14
• "can we infer/look beyond the time at which photons decoupled" - now I'm confused: when you say "look", do you mean literally looking beyond (as in detecting electromagnetic radiation from before) the CMB, or figuratively looking beyond it by analyzing the CMB itself without observing any radiation from before? Dec 11 '19 at 11:24
• Ther should be a cosmic neutrino background too. But it's unlikely we will be able to have direct evidences of its existence! Dec 11 '19 at 17:05

1. The cosmic neutrino background is formed in a similar way to the cosmic microwave background. It occurs when the universe is $$\sim 1$$ second old, the temperature has fallen to $$\sim 3\times 10^{10}$$K and the matter becomes transparent to neutrinos. The decoupled neutrinos are initially ultra-relativistic, but their de Broglie wavelengths are stretched by the expansion of the universe, such that they are expected to have energies of order 0.1 meV now. The low energy neutrino background might be detectable by the capture of electron neutrinos by tritium nuclei, transforming it to helium via beta decay. A careful analysis of the beta electron energy spectrum (e.g. Faessler 2016). Such instruments are being built - e.g. the KATRIN neutrino experiment might be capable of detecting this signature, but really needs a bigger tritium source and better spectral resolution. A better bet will be the PTOLEMY experiment (Betti et al. 2019), which uses a similar principle and is currently in a research and development phase.