A conventional approach to the dark-energy concept

Abstract

Motivated by results implying that the constituents of dark matter (DM) might be collisional, we consider a cosmological (toy-) model, in which the DM itself possesses some sort of thermodynamic properties. In this case, not only can the matter content of the Universe (the baryonic component, which is tightly gravitationally-bounded to the dark one, also being included) be treated as a classical gravitating fluid of positive pressure, but, together with all its other physical characteristics, the energy of this fluid's internal motions should be taken into account as a source of the universal gravitational field. In principle, this form of energy can compensate for the extra (dark) energy, needed to compromise spatial flatness, while the post-recombination Universe remains ever-decelerating. What is more interesting, is that, at the same time (i.e., in the context of the collisional-DM approach), the theoretical curve representing the distance modulus as a function of the cosmological redshift, μ(z), fits the Hubble diagram of a multi-used sample of supernova Ia events quite accurately. A cosmological model filled with collisional DM could accommodate the majority of the currently-available observational data (including, also, those from baryon acoustic oscillations), without the need for either any dark energy (DE) or the cosmological constant. However, as we demonstrate, this is not the case for someone who, although living in a Universe filled with self-interacting DM, insists on adopting the traditional, collisionless-DM approach. From the point of view of this observer, the cosmologically-distant light-emitting sources seem to lie farther (i.e., they appear to be dimmer) than expected, while the Universe appears to be either accelerating or decelerating, depending on the value of the cosmological redshift. This picture, which, nowadays, represents the common perception in observational cosmology, acquires a more conventional interpretation within the context of the collisional-DM approach.

Dark energy and equation of state oscillations with collisional matter fluid in exponential modified gravity

“…In the expression above, ρ m stands for the rest mass density which remains unaffected by the internal motions of the gravitational cosmic fluid and the part containing the potential energy Π, is what renders the matter fluid collisional [42,43]. The collisional matter is considered to be a prefect fluid with equation of state,…”

“…The detailed description of a perfect matter fluid with interactions was given in [42,43] and reference therein (see also [41]). It worths recalling in brief, the basic features which we shall use in this article.…”

“…In a recent study [41], we investigated the effect of collisional matter [42] on the late time evolution of the universe, in the context of F (R) modified theories of gravities. The resulting picture of our investigation was that the behavior of the collisional matter filled universe, varied in a model dependent way, giving in some cases better fit to the ΛCDM model with respect to the non-collisional matter filled universe, always in the context of F (R) gravities.…”

Late-time cosmological evolution in f ( R ) theories with ordinary and collisional matter

“…Note that we are interested in the post-recombination era, at which the universe is filled with matter and dark energy (late time evolution of the universe). The models of matter we shall use, appear already in the literature [59][60][61][62] and shall briefly describe them in the following sections. The transition from deceleration to acceleration is mostly affected by the f (R) theory and hence the inclusion of this selfinteracting matter can affect, as we will see, the rate of the process and the exact value of the transition redshift, but not the qualitative behavior of the evolutionary process.…”

Late-time cosmological approach in mimetic f(R, T) gravity

“…As a novelty, we assume in this paper the matter content of the Universe to be collisional matter and relativistic matter (radiation). The model of collisional matter has been studied in some work before, leading to interesting results [53,[62][63][64]. This approach of considering forms of matter other than cold dark matter can teach us about the choice of the models of modified gravity.…”