Barotropic fluid compatible parametrizations of dark energy
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Regular Article - Theoretical Physics
Barotropic fluid compatible parametrizations of dark energy Dalibor Perkovi´c1,a , Hrvoje Štefanˇci´c2,b 1 2
University of Applied Health Sciences, Mlinarska street 38, 10000 Zagreb, Croatia Catholic University of Croatia, Ilica 242, 10000 Zagreb, Croatia
Received: 9 May 2020 / Accepted: 3 July 2020 © The Author(s) 2020
Abstract Parametrizations of equation of state parameter as a function of the scale factor or redshift are frequently used in dark energy modeling. The question investigated in this paper is if parametrizations proposed in the literature are compatible with the dark energy being a barotropic fluid. The test of this compatibility is based on the functional form of the speed of sound squared, which for barotropic fluid dark energy follows directly from the function for the Equation of state parameter. The requirement that the speed of sound squared should be between 0 and speed of light squared provides constraints on model parameters using analytical and numerical methods. It is found that this fundamental requirement eliminates a large number of parametrizations as barotropic fluid dark energy models and puts strong constraints on parameters of other dark energy parametrizations.
1 Introduction Ever more precise observations of various cosmic phenomena [1–6] reveal a present state of universe which cannot be understood only in terms of General Relativity and forms of matter known from local physics, such as radiation or baryonic matter. Available observations point to the presently accelerated cosmic expansion, whereas the dynamics at the level of galaxies and clusters of galaxies, among other places, reveals additional gravitational interaction which could be explained by the presence of large quantities of, yet not directly observed, dark matter. The mechanism behind the accelerated cosmic expansion is usually attributed to a cosmic component with the negative pressure, called dark energy (DE). It has been shown that the concept of dark energy can be realized in many different ways such as cosmological constant, dynamical cosmological term, quintessence, phantom energy, k-essence or interacting dark energy [7–12]. Numera e-mail:
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ous alternatives have been proposed to both dark matter and dark energy, frequently as a modification of gravitational interaction at scales from galactic to cosmic [13–15]. Yet, even if the effects such as accelerated cosmic expansion or galactic rotation curve dynamics do not originate from cosmic components, concepts of dark matter and dark energy (including their unifications) remain very useful effective concepts. Present observational data reveal a large tension in the value of H0 inferred from low and high redshift measurements assuming the benchmark CDM model (for a recent review see [16]). Some of proposed solutions to this puzzle are nontrivial dynamics of dark energy [6] and dark matterdark energy interaction
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