is due to the CMB, the second comes from the CB. The Standard Model with its three neutrino species predicts a value of , including a small correction caused by a non-thermal distortion of the spectra during e×e annihilation. The radiation density had a major impact on various physical processes in the early universe, leaving potentially detectable imprints on measurable quantities, thus allowing us to infer the value of from observations. Big Bang nucleosynthesis Due to its effect on the expansion rate of the universe during Big Bang nucleosynthesis (BBN), the theoretical expectations for the primordial abundances of light elements depend on Astrophysical measurements of the primordial and abundances lead to a value of = at 68% c.l., in very good agreement with the Standard Model expectation. CMB anisotropies and structure formation The presence of the CB affects the evolution of CMB anisotropies as well as the growth of matter perturbations in two ways: Due to its contribution to the radiation density of the universe (which determines for instance the time of matter–radiation equality), and due to the neutrinos' anisotropic stress which dampens the acoustic oscillations of the spectra. Additionally, free-streaming massive neutrinos suppress the growth of structure on small scales. The WMAP spacecraft's five-year data combined with type Ia supernova data and information about the baryon acoustic oscillation scale yielded = at 68% c.l., providing an independent confirmation of the BBN constraints. The Planck spacecraft collaboration has published the tightest bound to date on the effective number of neutrino species, at = . Indirect evidence from phase changes to the Cosmic Microwave Background (CMB) Big Bang cosmology makes many predictions about the CB, and there is very strong indirect evidence that the cosmic neutrino background exists, both from Big Bang nucleosynthesis predictions of the helium abundance, and from anisotropies in the cosmic microwave background. One of these predictions is that neutrinos will have left a subtle imprint on the cosmic microwave background (CMB). It is well known that the CMB has irregularities. Some of the CMB fluctuations were roughly regularly spaced, because of the effect of baryon acoustic oscillation. In theory, the decoupled neutrinos should have had a very slight effect on the phase of the various CMB fluctuations. In 2015, it was reported that such shifts had been detected in the CMB. Moreover, the fluctuations corresponded to neutrinos of almost exactly the temperature predicted by Big Bang theory