Big Bang nucleosynthesis with a stiff fluid

Sourish Dutta; Robert J. Scherrer

doi:10.1103/PhysRevD.82.083501

paper

Big Bang nucleosynthesis with a stiff fluid

arXiv:1006.4166 · doi:10.1103/PhysRevD.82.083501

Abstract

Models that lead to a cosmological stiff fluid component, with a density $ρ_{S}$ that scales as $a^{- 6}$ , where $a$ is the scale factor, have been proposed recently in a variety of contexts. We calculate numerically the effect of such a stiff fluid on the primordial element abundances. Because the stiff fluid energy density decreases with the scale factor more rapidly than radiation, it produces a relatively larger change in the primordial helium-4 abundance than in the other element abundances, relative to the changes produced by an additional radiation component. We show that the helium-4 abundance varies linearly with the density of the stiff fluid at a fixed fiducial temperature. Taking $ρ_{S 10}$ and $ρ_{R 10}$ to be the stiff fluid energy density and the standard density in relativistic particles, respectively, at $T = 10$ MeV, we find that the change in the primordial helium abundance is well-fit by $Δ Y_{p} = 0.00024 (ρ_{S 10} / ρ_{R 10})$ . The changes in the helium-4 abundance produced by additional radiation or by a stiff fluid are identical when these two components have equal density at a "pivot temperature", $T_{*}$ , where we find $T_{*} = 0.55$ MeV. Current estimates of the primordial $^{4}$ He abundance give the constraint on a stiff fluid energy density of $ρ_{S 10} / ρ_{R 10} < 30$ .

6 pages, 2 figures. Clarification added: element abundances derived using a full numerical calculation. Version accepted at PRD

Big Bang nucleosynthesis with a stiff fluid

Abstract

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