( compared to a prediction of 1.95 K), and exactly three types of neutrino, the same number of neutrino flavours currently predicted by the Standard Model. Prospects for the direct detection of the CB Confirmation of the existence of these relic neutrinos may only be possible by directly detecting them using experiments on Earth. This will be difficult as the neutrinos which make up the CB are non-relativistic, in addition to interacting only weakly with normal matter, and so any effect they have in a detector will be hard to identify. One proposed method of direct detection of the CB is to use the capture of cosmic relic neutrinos on tritium i.e. 3H, leading to an induced form of beta decay. The neutrinos of the CB would lead to the production of electrons via the reaction while the main background comes from electrons produced via natural beta decay These electrons would be detected by the experimental apparatus in order to measure the size of the CB. The latter source of electrons is far more numerous, however their maximum energy is smaller than the average energy of the CB-electrons by twice the average neutrino mass. Since this mass is tiny, of the order of a few eVs or less, such a detector must have an excellent energy resolution in order to separate the signal from the background. One such proposed experiment is called PTOLEMY, which will be made up of 100 g of tritium target. The detector should be ready by 2022. See also Cosmic background radiation Dark matter Diffuse supernova neutrino background Gravitational wave background Notes References Physical cosmology Cosmic background radiation Neutrino astronomy