A theory of extra radiation in the Universe
Kazunori Nakayamaa, Fuminobu Takahashib and Tsutomu T. Yanagidab
Abstract
Recent cosmological observations, such as the measurement of the primordial
4He abundance, CMB, and large scale structure, give preference to the existence of extra radiation component. The extra radiation may be accounted for
by particles which were in thermal equilibrium and decoupled before the big bangnucleosynthesis. Broadly speaking, there are two possibilities: 1) there are about 10 particles which have very weak couplings to the standard model particles and decoupled much before the QCD phase transition; 2) there is one or a few light particles with a reasonably strong coupling to the plasma and it decouples after the QCD phase transition.
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