Sound speed and viscosity of semi-relativistic relic neutrinos

Lawrence Krauss, Andrew J. Long

Research output: Contribution to journalArticlepeer-review

1 Scopus citations


Generalized fluid equations, using sound speed ceff2 and viscosity cvis2 as effective parameters, provide a convenient phenomenological formalism for testing the relic neutrino "null hypothesis," i.e. that that neutrinos are relativistic and free-streaming prior to recombination. In this work, we relax the relativistic assumption and ask "to what extent can the generalized fluid equations accommodate finite neutrino mass?" We consider both the mass of active neutrinos, which are largely still relativistic at recombination m2 / T2 ∼ 0.2, and the effect of a semi-relativistic sterile component. While there is no one-to-one mapping between mass/mixing parameters and ceff2 and cvis2, we demonstrate that the existence of a neutrino mass could induce a bias to measurements of ceff2 and cvis2 at the level of 0.01 m2 / T2 ∼ 10-3.

Original languageEnglish (US)
Article number002
JournalJournal of Cosmology and Astroparticle Physics
Issue number7
StatePublished - Jul 4 2016


  • cosmological neutrinos
  • cosmological parameters from CMBR
  • neutrino properties

ASJC Scopus subject areas

  • Astronomy and Astrophysics


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