  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.
