A theory of extra radiation in the Universe

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55 Citations (Scopus)


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, δNν>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, our scenario predicts a new gauge symmetry at TeV scale, and therefore may be confirmed at the LHC. As a concrete example, we show that such a light fermion naturally appears in the E6-inspired GUT.

Original languageEnglish
Pages (from-to)275-279
Number of pages5
JournalPhysics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics
Issue number4
Publication statusPublished - 2011 Mar 14


  • Extra radiation
  • Grand unified theory


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