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Neutrino Cosmology by Sergio Pastor, Gianpiero Mangano, Gennaro Miele, Julien Lesgourgues

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3

Neutrinos in the early ages

A Tale of Three Numbers: 1, 3 and .1 This might be a good subtitle for this chapter, which is about the problem of baryogenesis in the early universe.

We have already mentioned in Chapter 1 that observations of the light nuclear yields produced during primordial nucleosynthesis and the features of the CMB anisotropies single out a definite value for 1 parameter, the baryon-to-photon density ratio ηB ~ 6 × 10−10 at low temperatures, much below the nucleon mass. If the universe’s expansion were starting with symmetric initial conditions, i.e., a zero initial baryon number per comoving volume, and no baryon-violating interactions were at work at all stages of this expansion, ηB would be expected to be much smaller, ...

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