Fluid interpretation of Cardassian expansion

Paolo Gondolo and Katherine Freese
Phys. Rev. D 68, 063509 – Published 24 September 2003
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Abstract

A fluid interpretation of Cardassian expansion is developed. Here, the Friedmann equation takes the form H2=g(ρM) where ρM contains only matter and radiation (no vacuum). The function g(ρM) returns to the usual 8πρM/(3mpl2) during the early history of the Universe, but takes a different form that drives an accelerated expansion after a redshift z1. One possible interpretation of this function (and of the right-hand side of Einstein’s equations) is that it describes a fluid with total energy density ρtot=(3mpl2/8π)g(ρM)=ρM+ρK containing not only matter density (mass times number density) but also interaction terms ρK. These interaction terms give rise to an effective negative pressure which drives cosmological acceleration. These interactions may be due to interacting dark matter, e.g. with a fifth force between particles Frα1. Such interactions may be intrinsically four dimensional or may result from higher dimensional physics. A fully relativistic fluid model is developed here, with conservation of energy, momentum, and particle number. A modified Poisson’s equation is derived. A study of fluctuations in the early Universe is presented, although a fully relativistic treatment of the perturbations including gauge choice is as yet incomplete.

  • Received 28 October 2002

DOI:https://doi.org/10.1103/PhysRevD.68.063509

©2003 American Physical Society

Authors & Affiliations

Paolo Gondolo

  • Case Western Reserve University, Department of Physics, 10900 Euclid Avenue, Cleveland, Ohio 44106-7079, USA

Katherine Freese

  • Michigan Center for Theoretical Physics, University of Michigan, Ann Arbor, Michigan 48109, USA

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Vol. 68, Iss. 6 — 15 September 2003

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