The role of expansion for the entropy of the Universe

Under assumption of the closed FRW-universe, the idea is presented that the cosmological expansion/contraction on its own, has an entropy balancing effectively the changing entropy of the cosmic fluid in such a way that at every epoch the total entropy of the Universe remains constant.     

Dynamics of some cosmological solutions in modified f(R) gravity

This paper is devoted to investigate the modified f(R) theory of gravity, where R represents the Ricci scalar respectively. For our current work, we consider the Friedmann-Robertson-Walker (FRW) space-time for finding solutions of field equations. Furthermore, some numerical solutions are examined by taking the Klein-Gordon Equation and using distinct values of the equation of state (EoS) parameter. In this way, we have discussed the solutions for acceleration expansion of the Universe, sub-relativistic Universe, radiation Universe, ultra-relativistic Universe, dust Universe, and stiff fluid Universe respectively. Moreover, their behaviours are examined by using power-law and exponential law techniques. The bouncing scenario is also discussed by choosing some particular values of the model parameters and observed the energy conditions, which are satisfied for a successful bouncing model. It is also concluded that some solution in f(R) theory of gravity supports the concept of exotic matter and accelerated expansion of the Universe due to a large amount of negative pressure.     

A primordial cosmological scenario with two scalar fields and matter

The presence of matter in its different forms in a hexadimensional space-time, where gravitation is coupled to a conformal field, generates a class of cosmological solutions with very peculiar features: the initial proper distance between any two points of the space-time is infinite, following successive contraction and expansion phases in the evolution of the Universe. This behaviour defines a Anti-Big Bang Singular Cosmology. In this article we analyze this primordial cosmological scenario considering different matter forms and the effects of a Conformal Transformation on the metric.     

The coincidence of friedmann integrals

The expansion of a two-component Universe with an arbitrary spatial curvature is considered. It is shown that the Friedmann integrals of an almost flat Universe do not coincide.     

On the nature of dark energy: the lattice Universe

There is something unknown in the cosmos. Something big. Which causes the acceleration of the Universe expansion, that is perhaps the most surprising and unexpected discovery of the last decades, and thus represents one of the most pressing mysteries of the Universe. The current standard ΛCDM model uses two unknown entities to make everything fit: dark energy and dark matter, which together would constitute more than 95 % of the energy density of the Universe. A bit like saying that we have understood almost nothing, but without openly admitting it. Here we start from the recent theoretical results that come from the extension of general relativity to antimatter, through CPT symmetry. This theory predicts a mutual gravitational repulsion between matter and antimatter. Our basic assumption is that the Universe contains equal amounts of matter and antimatter, with antimatter possibly located in cosmic voids, as discussed in previous works. From this scenario we develop a simple cosmological model, from whose equations we derive the first results. While the existence of the elusive dark energy is completely replaced by gravitational repulsion, the presence of dark matter is not excluded, but not strictly required, as most of the related phenomena can also be ascribed to repulsive-gravity effects. With a matter energy density ranging from ～5 % (baryonic matter alone, and as much antimatter) to ～25 % of the so-called critical density, the present age of the Universe varies between about 13 and 15 Gyr. The SN Ia test is successfully passed, with residuals comparable with those of the ΛCDM model in the observed redshift range, but with a clear prediction for fainter SNe at higher z. Moreover, this model has neither horizon nor coincidence problems, and no initial singularity is requested. In conclusion, we have replaced all the tough problems of the current standard cosmology (including the matter-antimatter asymmetry) with only one question: is the gravitational interaction between matter and antimatter really repulsive as predicted by the theory and as the observation of the Universe seems to suggest? We are awaiting experimental responses.     

Delay of emission from extragalactic gamma-ray burst sources as a test for selecting a model of the universe

We obtained an order-of-magnitude estimate for the dispersion of light caused by the effect of quantum fluctuations on the propagation of electromagnetic waves in four-dimensional spacetime. We calculated the delay of the photons from cosmological gamma-ray bursts (GRBs) for the flat, open, and closed cosmological models. This delay is attributable to the effect of expansion of the Universe on the propagation of a dispersive light wave in space. Analysis shows that the delay of GRB photons contains a regular component related to the expansion of the Universe. We conclude that cosmological models of the Universe can be selected by the delay of emission of various energies from GRBs; the accuracy of measuring the parameter Δt/ΔE _γ must be no lower than 10^?6 s MeV^?1.     

Evolution of the Universe through the Planck epoch

The problem of the Universe emerging out of the Planck epoch is discussed. It is pointed out that an earlier exponential expansion phase well before the onset of the GUTs phase transition is essential. Such an expansion can occur owing to the breaking of scale invariance at Planck energies in a unified theory of gravity with other interactions.     

On the equation of state for the Λ field

The recently detected accelerated expansion of the Universe is related to the existence of a new type of matter called the Λ field or quintessence. Constraints were obtained on its equation of state from the absence of clustering of this matter on scales much smaller than the cosmological horizon. The question of how these constraints affect the possibility of fitting the accelerated expansion by such cosmological models as the Chaplygin gas model is discussed.     

Cosmology with modified Newtonian dynamics (MOND)

It is well known that the application of Newtonian dynamics to an expanding spherical region leads to the correct relativistic expression (the Friedmann equation) for the evolution of the cosmic scalefactor. Here, the cosmological implications of Milgrom's modified Newtonian dynamics (MOND) are considered by means of a similar procedure. Earlier work by Felten demonstrated that in a region dominated by modified dynamics the expansion cannot be uniform (separations cannot be expressed in terms of a scalefactor) and that any such region will eventually recollapse regardless of the initial expansion velocity and mean density. Here I show that, because of the acceleration threshold for the MOND phenomenology, a region dominated by MOND will have a finite size which, in the earlier Universe ( z >3), is smaller than the horizon scale. Therefore, uniform expansion and homogeneity on the horizon scale are consistent with MOND-dominated non-uniform expansion and the development of inhomogeneities on smaller scales. In the radiation-dominated era, the amplitude of MOND-induced inhomogeneities is much smaller than that implied by observations of the cosmic background radiation, and the thermal and dynamical history of the Universe is identical to that of the standard big bang model. In particular, the standard results for primordial nucleosynthesis are retained. When matter first dominates the energy density of the Universe, the cosmology diverges from that of the standard model. Objects of galaxy mass are the first virialized objects to form (by z =10), and larger structure develops rapidly. At present, the Universe would be inhomogeneous out to a substantial fraction of the Hubble radius.     

Cosmology and a flat space

A theory of gravitation in a flat space is briefly described, which gives a completely new view on cosmological problems. Instead of expansion of the Universe, a contraction of bodies and clusters is obtained. This theory excludes the closed solutions as non-physical ones and gives a simple condition for the stability of gravitationally-bounded systems with respect to cosmological gravitational field.     

WD 1953−011: a magnetic white dwarf with peculiar field structure

We present a new interpretation of recent observations suggesting that the expansion of the Universe has recently started to accelerate. A cosmological model with a quintessence field driven by a potential motivated by M-theory is used to study the energy density and equation of state for the Universe. We find that late-time acceleration does not have to lead to the usual predictions of perpetual acceleration. The model allows another broad class of scenarios in which today's acceleration is a transient phenomenon, which is succeeded by a return to matter domination and decelerating expansion.     

General relativistic singularity-free cosmological model

We “explain”, using a Classical approach, how the Universe was created out of “nothing”, i.e., with no input of initial energy nor mass. The inflationary phase, with exponential expansion, is accounted for, automatically, by our equation of state for the very early Universe. This is a Universe with no-initial infinite singularity of energy density.     

Decelerating universes older than their Hubble times

Recent observations suggest that Hubble's constant is large, and hence that the Universe appears to be younger than some of its constituents. The traditional escape route, which assumes that the expansion is accelerating, appears to be blocked by observations of Type Ia supernovae, which suggest that the Universe is decelerating. These observations are reconciled in a model in which the Universe has experienced an inflationary phase in the recent past, driven by an ultralight inflaton, the Compton wavelength of which is of the same order as the Hubble radius.     

Interacting dark sector and the coincidence problem within the scope of LRS Bianchi type I model

It is shown that a suitable interaction between dark energy and dark matter in locally rotationally symmetric (LRS) Bianchi-I space-time can solve the coincidence problem and not contradict the accelerated expansion of present Universe. The interaction parameters are estimated from observational data.     

Primordial black holes

Primordial black holes (PBHs) are a profound signature of primordial cos-mological structures and provide a theoretical tool to study nontrivial physics of the early Universe. The mechanisms of PBH formation are discussed and observational constraints on the PBH spectrum, or effects of PBH evaporation, are shown to re-strict a wide range of particle physics models, predicting an enhancement of the ul-traviolet part of the spectrum of density perturbations, early dust-like stages, first or-der phase transitions and stages of superheavy metastable particle dominance in the early Universe. The mechanism of closed wall contraction can lead, in the inflation-ary Universe, to a new approach to galaxy formation, involving primordial clouds of massive BHs created around the intermediate mass or supermassive BH and playing the role of galactic seeds.     

Primordial black holes

Primordial black holes(PBHs) are a profound signature of primordial cosmological structures and provide a theoretical tool to study nontrivial physics of the early Universe.The mechanisms of PBH formation are discussed and observational constraints on the PBH spectrum,or effects of PBH evaporation,are shown to restrict a wide range of particle physics models,predicting an enhancement of the ultraviolet part of the spectrum of density perturbations,early dust-like stages,first order phase transitions and stages of superheavy metastable particle dominance in the early Universe.The mechanism of closed wall contraction can lead,in the inflationary Universe,to a new approach to galaxy formation,involving primordial clouds of massive BHs created around the intermediate mass or supermassive BH and playing the role of galactic seeds.     

Bianchi type–I dark energy cosmological model coupled with a massive scalar field

In the present investigation we are mainly concerned with a massive scalar field in an axially symmetric Bianchi type – I space-time. Einstein field equations are solved to obtain an exact cosmological model. We have used certain physically meaningful conditions for this purpose. Kinematical cosmological parameters are determined, and their dynamical aspects are discussed. It is observed that our model represents accelerated expansion of the Universe. It is observed that our model agrees with the scenario of accelerated expansion of the Universe confirmed by supernova 1a experimental data.     

The time evolution of cosmological redshift as a test of dark energy

The variation of the expansion rate of the Universe with time produces an evolution in the cosmological redshift of distant sources (e.g. quasar Lyman α absorption lines) that might be directly observed by future ultrastable, high-resolution spectrographs (such as the COsmic Dynamics Experiment) coupled to extremely large telescopes (such as the European Southern Observatory's Extremely Large Telescope). This would open a new window to explore the physical mechanism responsible for the current acceleration of the Universe. We investigate the evolution of cosmological redshift from a variety of dark energy models, and compare it with simulated data. We perform a Fisher matrix analysis and discuss the prospects for constraining the parameters of these models and for discriminating among competing candidates. We find that, because of parameter degeneracies, and the inherent technical difficulties involved in this kind of observations, the uncertainties on parameter reconstruction can be rather large unless strong external priors are assumed. However, the method could be a valuable complementary cosmological tool, and give important insights on the dynamics of dark energy, not obtainable using other probes.     

Dynamics of expansion of the Universe in the models with nonminimally coupled dark energy

The dark energy model with barotropic equation of state, which interacts with dark matter by gravitation and by other force, which causes the energy-momentum exchange between them, is considered. Both components are described in approximation of ideal fluid, which are parameterized by density, equation of state and effective sound speed parameters. The three types of interactions between dark components are considered: interaction independent from their densities, interaction proportional to energy density of dark energy, and interaction proportional to energy density of dark matter. The equations that describe the expansion dynamics of homogeneous and isotropic Universe and evolution of densities of both components for different values of interaction parameter are obtained on the bases of the general covariant conservation equations and Einstein’s ones. For three kinds of interactions, the existing of the range of values of parameters of dark energy for which the densities of dark components are negative was shown. The conditions of positivity of energy density of dark energy and dark matter were written for which the constraints on the value of parameter of interaction were derived. The dynamics of expansion of the Universe with these interactions of dark energy and dark matter is analyzed.     

LRS Bianchi-I anisotropic cosmological model with dominance of dark energy

The present study deals with spatially homogeneous and anisotropic locally rotationally symmetric (LRS) Bianchi type I cosmological model with dominance of dark energy. To get the deterministic model of Universe, we assume that the shear scalar (σ) in the model is proportional to expansion scalar (θ). This condition leads to A=B ⁿ , where A, B are metric potential and n is positive constant. It has been found that the anisotropic distribution of dark energy leads to the present accelerated expansion of Universe. The physical behavior of the Universe has been discussed in detail.