A theory of extra radiation in the Universe
arXiv:1010.5693 · doi:10.1016/j.physletb.2011.02.013
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, Delta N_nu > 0. The extra radiation may be accounted for by particles which were in thermal equilibrium and decoupled before the big bang nucleosynthesis. 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. Focusing on the latter case, we find that a light chiral fermion is a suitable candidate, which evades astrophysical constraints. Interestingly, such a scenario may be confirmed at the LHC. As a concrete example, we show that such a light fermion naturally appears in the E_6-inspired GUT.
17 pages; minor corrections
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- Observable dark radiation from cosmologically safe QCD axion
- A Minimal Model of Majoronic Dark Radiation and Dark Matter
- Exploring the Role of Axions and Other WISPs in the Dark Universe
- Extra light fermions in -inspired models and the 3.5 keV X-ray line signal
- A natural and simple UV completion of the QCD axion model
- Anthropic Bound on Dark Radiation and its Implications for Reheating
- Reconciling BICEP2 and Planck results with right-handed Dirac neutrinos in the fundamental representation of grand unified E_6
- U(1) Symmetry Restoration and Effective Neutrino Species