Polytropic dark matter flows illuminate dark energy and accelerated expansion

Abstract

Currently, a large amount of data implies that the matter constituents of the cosmological dark sector might be collisional. An attractive feature of such a possibility is that, it can reconcile dark matter (DM) and dark energy (DE) in terms of a single component, accommodated in the context of a polytropic-DM fluid. In fact, polytropic processes in a DM fluid have been most successfully used in modeling dark galactic haloes, thus significantly improving the velocity dispersion profiles of galaxies. Motivated by such results, we explore the time evolution and the dynamical characteristics of a spatially-flat cosmological model, in which, in principle, there is no DE at all. Instead, in this model, the DM itself possesses some sort of fluidlike properties, i.e., the fundamental units of the Universe matter-energy content are the volume elements of a DM fluid, performing polytropic flows. In this case, together with all the other physical characteristics, we also take the energy of this fluid's internal motions into account as a source of the universal gravitational field. This form of energy can compensate for the extra energy, needed to compromise spatial flatness, namely, to justify that, today, the total energy density parameter is exactly unity. The polytropic cosmological model, depends on only one free parameter, the corresponding (polytropic) exponent, Γ. We find this model particularly interesting, because for Γ ≤ 0.541, without the need for either any exotic DE or the cosmological constant, the conventional pressure becomes negative enough so that the Universe accelerates its expansion at cosmological redshifts below a transition value. In fact, several physical reasons, e.g., the cosmological requirement for cold DM (CDM) and a positive velocity-of-sound square, impose further constraints on the value of Γ, which is eventually settled down to the range −0.089 < Γ ≤ 0. This cosmological model does not suffer either from the age problem or from the coincidence problem. At the same time, this model reproduces to high accuracy the distance measurements performed with the aid of the supernovae (SNe) Type Ia standard candles, and most naturally interprets, not only when, but also why the Universe transits from deceleration to acceleration, thus arising as a mighty contestant for a DE model.

Accelerated expansion of the Universe in the presence of dark matter pressure

“…Applying the Archimedean-type coupling of the DM with DE to study the late-time accelerated expansion confirms that the Archimedean-type coupling provides the Universe evolution to be a quasiperiodic and/or multistage process [23]. Studying the dynamical characteristics of a spatially-flat cosmological model in which instead of DE the DM possesses some sort of fluid-like properties indicates that the pressure becomes negative enough, so that the Universe accelerates its expansion [24].…”

On the cosmological models with matter creation

“…Polytropes have very wide applications in astrophysics and the related fieldssee the extensive monograph [11]. Polytropic gas models of dark energy provide alternative explanation of the accelerated expansion of the Universe -see the review [10] and the references therein. These models follow the steps of the Cardassian expansion cosmological models [12] for which the right-hand side of the Friedmann equation H 2 = (1/3)ρ is modified to involve an additional, polytropic, term: H 2 = (1/3)ρ + Bρ n , where B is some constant and n < 2/3 in order to achieve accelerated expansion.…”

Viscous self interacting dark matter and cosmic acceleration

“…It has been argued that one should attribute thermodynamic properties like internal energy with DM owing to its self-interactions [2]. In addition, DM can also be provided with a polytropic EoS [3]. Both the above attributes are capable of explaining the Type Ia supernovae (SNe-Ia) data without any additional dark energy component in cosmic fluid.…”