Narrow Results

We analyze the universe as a thermodynamic system, homogeneously filled up by exotic matters popularly named as dark energies. Different dark energy models are chosen. We start with the equation of continuity and derive the time and scale factor relations for different EoSs of different dark energy models. To do the time-scale factor relation analysis, nature of dependences on different dark energy modeling parameters have been studied. For this, the help of different plots are used. In general, different dark energies show different properties while occurrences of future singularities are considered. Those properties can be supported by the graphical analysis of their cosmic time-scale factor studies.

We consider a homogeneous and isotropic Universe, described by the minisuperspace Lagrangian with the scale factor as a generalized coordinate. We show that the energy of a closed Universe is zero. We apply the uncertainty principle to this Lagrangian and propose that the quantum uncertainty of the scale factor causes the primordial fluctuations of the matter density. We use the dynamics of the early Universe in the Einstein–Cartan theory of gravity with spin and torsion, which eliminates the big-bang singularity and replaces it with a nonsingular bounce. Quantum particle production in highly curved spacetime generates a finite period of cosmic inflation that is consistent with the Planck satellite data. From the inflated primordial fluctuations we determine the magnitude of the temperature fluctuations in the cosmic microwave background, as a function of the numbers of the thermal degrees of freedom of elementary particles and the particle production coefficient which is the only unknown parameter.

To justify the 20-year old distant Ia Supernova observations which revealed to us that our universe is experiencing a late-time cosmic acceleration, propositions of existence of exotic fluids inside our universe are made. These fluids are assumed to occupy homogeneously the whole space of the universe and to exert negative pressure from inside such that the late-time accelerated expansion is caused. Among the different suggested models of such exotic matters/energy popularly coined as dark matter/dark energy (DE), a well-known and popular process is “introduction of redshift parametrization” of the equation of state (EoS) parameter of these fluids. We, very particularly, take the parametrization proposed by Barboza and Alcaniz (BA) along with the cosmological constant. We use 39 data points for Hubble’s parameter calculated for different redshifts and try to constrain the DE EoS parameters for BA modeling. We then constrain the DE parametrization parameters in the background of Einstein’s general relativity, loop quantum gravity and Horava–Lifshitz gravity one after another. We find the $1\sigma$, $2\sigma$ and $3\sigma$ confidence contours for all these cases and compare them with each other. We try to speculate which gravity is constraining the parameters most and which one is letting the parameters to stay within a larger domain. We tally our results of 557 points Union2 Sample and again compare them for different gravity theories.

The main motivation of our study is to explore Rényi holographic dark energy, Sharma–Mittal holographic dark energy, Rényi new agegraphic dark energy, and Sharma–Mittal new agegraphic dark energy in the context of generalized Rastall gravity with the two-fluid system, dark energy, and dark matter. In this regard, we have considered the scale factors into two distinct categories, one of which corresponds to the future singularity, whereas the other represents initial singularity. By employing future event horizon as infrared (IR)-cutoff, different cosmological quantities like deceleration parameter, equation of state (EoS) parameter are evaluated, and their ramifications have been described graphically. Analyzing the squared speed of sound, we have seen classically stable and unstable behavior for each of the considered models. Finally, we study ${\omega }_D$–${\omega }_D^{\prime }$ plane and find the thawing/freezing regions.