Unified dark fluid with constant adiabatic sound speed and cosmic constraints

Abstract

As is known above 90% of the energy content in 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 2 s = α, which is in the range [0, 1], is studied. At first, via the energy conservation equation, its energy density, ρ d /ρ d0 = (1 − Bs) + Bsa −3(1+α) where Bs is related to integration constant from energy conservation equation as another model parameter, is presented. Then by using 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. *

Unified dark fluid with constant adiabatic sound speed: Including entropic perturbations

“…The above two equations reduce to the corresponding continuity and Euler equations as shown in our previous work [3] when the entropy perturbation vanishes. For the dark fluid in this paper, we assume the shear perturbation σ d = 0.…”

“…In the last few years, the unified dark fluid models [1][2][3][4][5][6][7][8][9][10][11][12] were investigated as a possible explanation to an accelerated expansion phase of our Universe [13,14]. These models are inspired by the facts that above 96% of the energy content in the Universe is made of unknown dark component.…”

“…Also, it can be treated as k-essence scalar field [3]. Interestingly, it can avoid the so-called averaging problem [16] when the perturbations become nonlinear [3]. So, in this paper, we are going to investigate this unified dark fluid model.…”

“…As pointed out by the authors [15], when α is negative, this model can be seen as the attractor solution of quintessence scalar field dynamics. Also, it can be treated as k-essence scalar field [3]. Interestingly, it can avoid the so-called averaging problem [16] when the perturbations become nonlinear [3].…”

“…On the contrary, when the adiabatic sound speed is preposed, the EoS can also be determined up to an integration constant in principle. Based on this point, the so-called ΛαCDM or CASS model was proposed [2,3,10] where a constant adiabatic sound speed c 2 s,ad = α was assumed. The case of α = 0 was discussed in Ref.…”