A theory of extra radiation in the Universe
  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.
