A theory of extra radiation in the Universe

TL;DR

This paper explores theoretical models explaining extra radiation in the universe, focusing on light fermions that could be detected at the LHC, and discusses their compatibility with cosmological and astrophysical constraints.

Contribution

It proposes a new scenario involving light chiral fermions from E_6-inspired GUTs as candidates for extra radiation, linking cosmology with particle physics experiments.

Findings

Light fermions can account for extra radiation without conflicting with astrophysical constraints.

Such fermions may be detectable at the LHC, providing a test for the theory.

The scenario aligns with cosmological observations of primordial helium, CMB, and large-scale structure.

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.