K. Kleidis

Polytropic dark matter flows illuminate dark energy and accelerated expansion

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 ,w hich 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.

A conventional approach to the dark-energy concept

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: The Shadowy Reflection of Dark Matter?

In this article, we review a series of recent theoretical results regarding a conventional approach to the dark energy (DE) concept. This approach is distinguished among others for its simplicity and its physical relevance. By compromising General Relativity (GR) and Thermodynamics at cosmological scale, we end up with a model without DE. Instead, the Universe we are proposing is filled with a perfect fluid of self-interacting dark matter (DM), the volume elements of which perform hydrodynamic flows. To the best of our knowledge, it is the first time in a cosmological framework that the energy of the cosmic fluid internal motions is also taken into account as a source of the universal gravitational field. As we demonstrate, this form of energy may compensate for the DE needed to compromise spatial flatness, while, depending on the particular type of thermodynamic processes occurring in the interior of the DM fluid (isothermal or polytropic), the Universe depicts itself as either decelerating or accelerating (respectively). In both cases, there is no disagreement between observations and the theoretical prediction of the distant supernovae (SNe) Type Ia distribution. In fact, the cosmological model with matter content in the form of a thermodynamically-involved DM fluid not only interprets the observational data associated with the recent history of Universe expansion, but also confronts successfully with every major cosmological issue (such as the age and the coincidence problems). In this way, depending on the type of thermodynamic processes in it, such a model may serve either for a conventional DE cosmology or for a viable alternative one.

Effects of Finite-time Singularities on Gravitational Waves

We analyze the impact of finite-time singularities on gravitational waves, in the context of F(R)

Geodesic motions versus hydrodynamic flows in a gravitating perfect fluid: Dynamical equivalence and consequences

F(R)

Loop quantum cosmology-corrected Gauss–Bonnet singular cosmology

gravity. We investigate which singularities are allowed to occur during the inflationary era, when gravitational waves are considered, and we discuss the quantitative implications of each allowed singularity. As we show, only a pressure singularity, the so-called Type II and also a Type IV singularity are allowed to occur during the inflationary era. In the case of a Type II, the resulting amplitude of the gravitational wave is zero or almost zero, hence this pressure singularity has a significant impact on the primordial gravitational waves. The case of a Type IV singularity is more interesting since as we show, the singularity has no effect on the amplitude of the gravitational waves. Therefore, this result combined with the fact that the Type IV singularity affects only the dynamics of inflation, leads to the conclusion that the Universe passes smoothly through a Type IV singularity.

On the adiabatic expansion of the visible space in a higher dimensional cosmology

Stimulated by the methods applied for the observational determination of masses in the central regions of the active galactic nuclei (AGN), we examine, in the context of the theory of general relativity, the exact conditions, under which, in the interior of a gravitating perfect-fluid source, the geodesic motions and the adiabatic hydrodynamic flows are dynamically equivalent to each other. Dynamical equivalence rests on the functional similarity between the corresponding, covariantly expressed differential equations of motion and is obtained with the aid of a conformal transformation between the metric tensors of the original fluid, on the one hand, and the so-called virtual fluid on the other. In the latter, the hydrodynamic flow motions are formally the same as the geodesic motions. The conformal factor so obtained is written in terms of the specific enthalpy of the original fluid, and hence it is attributed a clear physical interpretation. The components of the virtual fluids energy-momentum tensor are determined, through the invariant field equations, in terms of the original fluids corresponding quantities, the conformal factor and its spacetime derivatives. In the Newtonian limit, the extra contribution to the original energy density results in an extra inertial-energy density and hence in an extra mass, both of which are always non-vanishing. The associated results indicate that, in the determination of the masses in the central regions of the AGNs, the observationally determined nuclear mass is being underestimated with respect to the real physical one. Accordingly, we evaluate the corresponding mass deficit, which, in typical cases of AGNs, is not always negligible compared with the mass of the central dark object, and it can be comparable to the total rest mass of the circumnuclear gas involved. Finally, the implications of the results are discussed, on the assumed form of the mass-density distribution law for the circumnuclear gas and the corresponding form of the extra inertial-energy density. We find that, under certain conditions, the density index is directly related to the polytropic index in the fluids adiabatic equation of state.

A conventional form of dark energy

In this work we investigate which Loop Quantum Cosmology (LQC)-corrected Gauss–Bonnet F(𝒢) gravity can realize two singular cosmological scenarios, the intermediate inflation and the singular bounce scenarios. The intermediate inflation scenario has a Type III sudden singularity at t = 0, while the singular bounce has a soft Type IV singularity. By using perturbative techniques, we find the holonomy-corrected F(𝒢) gravities that generate at leading order the aforementioned cosmologies and we also argue that the effect of the holonomy corrections is minor to the power spectrum of the primordial curvature perturbations of the classical theory.

Newton's Law Modifications due to Extra Dimensional Spaces with Dirichlet or Neumann boundaries

In the context of higher-dimensional cosmologies, with isotropic visible and internal space and multi-perfect fluid matter, we study the conditions under which adiabatic expansion of the visible external space is possible, when a time-dependent internal space is present. The analysis is based on a reinterpretation of the four-dimensional stress-energy tensor in the presence of the extra dimensions. This modifies the usual adiabatic energy conservation laws for the visible universe, leading to a new type of cosmological evolution which includes large-scale entropy production in four dimensions.

Magnetohydrodynamics and Plasma Cosmology

Motivated by recent results, indicating that the dark matter (DM) constituents can be collisional, we assume that the DM itself possesses also some sort of thermodynamical properties. In this case, the Universe matter-content can be treated as a gravitating fluid of positive pressure, and, therefore, together with all the other physical characteristics, the energy of this fluids internal motions should be taken into account as a source of the universal gravitational field. In principle, this form of energy can compensate, also, 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, fits the Hubble diagram of an extended sample of SN Ia events quite accurately. However, as we demonstrate, this is not the case for someone who, although living in a Universe filled with collisional DM, insists in adopting the traditional, collisionless-DM approach. From the point of view of such an observer, the 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.