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Bibekananda Nayak

Publications and source records attributed to Bibekananda Nayak.

13 recordsLinked to original sources

Generalized Interacting Dark Energy Model and Loop Quantum Cosmology

The recently observed accelerated expansion of the universe has put a challenge for its theoretical understanding. As a possible explanation of this, it is considered that the most part of the present universe is filled with a form of energy that exerts a negative pressure called dark energy, which drives the acceleration. In the present work, we assume a dynamical dark energy model, where dark energy interacts with matter and grows at the expense of the latter. Using this model, we discuss the evolution of the universe within the context of loop quantum cosmology. Our work successfully explains the presently observed accelerated expansion of the universe, by predicting that the present universe is phantom dominated. We also found that in the past, the expansion of the universe was decelerated one and transition from deceleration to acceleration would occur at $t_{q=0}=0.688t_0$, where $t_0$ is the present age of the universe. Again, our analysis predicted that at the transition time, the universe would be dominated with quintessence type dark energy.

physics.gen-ph↗

Black Hole Dynamics in Power-law based Metric $f(R)$ Gravity

In this work, we use power-law cosmology to investigate the evolution of black holes within the context of metric $f(R)$ gravity satisfying the conditions provided by Starobinsky model. In our study, it is observed that presently accelerated expansion of the universe can be suitably explained by this integrated model without the need for dark energy. We also found that mass of a black hole decreases by absorbing surroundings energy-matter due to modification of gravity and more the accretion rate more is mass loss. Particularly the black holes, whose formation masses are nearly $10^{20}$ gm and above, are evaporated at a particular time irrespective of their formation mass. Again our analysis reveals that the maximum mass of a black hole supported by metric $f(R)$ gravity is $10^{12} M_{\odot}$, where $M_{\odot}$ represents the solar mass.

gr-qc↗

Interacting Holographic Dark Energy, the Present Accelerated Expansion and Black Holes

We study the evolution of the universe by assuming an integrated model, which involves interacting dark energy and holographic principle with Hubble scale as IR cutoff. First we determined the interaction rate at which matter is converting to dark energy. In the next step, we evaluated the equation of state parameter which describes the nature of dark energy. Our result predicts that the present state of the universe is dominated by quintessence type dark energy and it will become phantom dominated in near future. Again our analysis successfully addresses the problem of present accelerated expansion of the universe and softens the coincidence problem. We also found that the universe was previously undergoing a decelerated phase of expansion and transition from deceleration to acceleration would occur at a time $t_{q=0}=0.732 t_0$, where $t_0$ is the present age of the universe. Finally, we discuss the evolution of Black Holes in this environment.

gr-qc↗

Effect of Interacting Dark Energy on Expansion of the Universe

In our present work, we study the evolution of the universe by assuming an interacting dark energy model, where dark energy interacts with matter. Basing on this model, first we calculated the dark energy density parameter and using this we have picturised the expansion of the universe. From our analysis, we found that presently observed accelerated expansion of the universe can be explained by interacting model, if the dark energy is quintessence type. Though equation of sate parameter of dark energy $γ_ϕ$ for quintessence varies between $0$ and $-1$, our results predict that accelearted expansion is only possible for $γ_ϕ$ less (more negative) than $-0.166$. It is also found that in early time the universe was undergoing an decelerated phase of expansion and transition from deceleration to acceleration would occur in recent past. Further, our model predicts that in near future again expansion of the universe will undergo a second transition from accelerated phase to a decelerated one and finally decelaration parameter will take a constant positive value $1/2$ as early universe indicating a constant rate of deceleration in far future like distant past.

gr-qc↗

Accretion of radiation and rotating Primordial black holes

We consider rotating Primordial black holes (PBHs) and study the effect of accretion of radiation in the radiation dominated era. The central part of our analysis deals with the role of the angular momentum parameter on the evolution of PBHs. We find that both the accretion rate and evaporation rate decrease with increase in angular momentum parameter, but the rate of evaporation decreases more rapidly than the rate of accretion. This shows that the evaporation time of PBHs get prolonged with increase in angular momentum parameter. We also note that the lifetime of rotating PBHs increase with increase in accretion efficiency of the radiation as in the case of nonrotating PBHs.

gr-qc↗

Vacuum energy and Primordial Black Holes in Brans-Dicke Theory

In our work, we study the evolution of primordial black holes within the context of Brans-Dicke theory by assuming present universe as vacuum dominated. We also consider the accretion of radiation, matter and vacuum energy only during respective dominant periods. From our study, we found that the accretion rate is slower in both radiation and vacuum energy dominated eras in Brans-Dicke theory in comparision with General Theory of Relativity. Thus the PBHs evaporate at a faster rate in Brans-Dicke theory than Standard Cosmology, if we consider the presence of vacuum energy in both cases. We also find that vacuum energy accretion efficiency should be less than 0.61.

gr-qc↗

Evolution of Primordial Black Hole Mass Spectrum in Brans-Dicke Theory

We investigate the evolution of primordial black hole mass spectrum by including both accretion of radiation and Hawking evaporation within Brans-Dicke cosmology in radiation, matter and vacuum-dominated eras. We also consider the effect of evaporation of primordial black holes on the expansion dynamics of the universe. The analytic solutions describing the energy density of the black holes in equilibrium with radiation are presented. We demonstrate that these solutions act as attractors for the system ensuring stability for both linear and nonlinear situations. We show, however, that inclusion of accretion of radiation delays the onset of this equilibrium in all radiation, matter and vacuum-dominated eras.

gr-qc↗

Evolution of Primordial Black Holes in Loop Quantum Gravity

In this work, we study the evolution of Primordial Black Holes within the context of Loop Quantum Gravity. First we calculate the scale factor and energy density of the universe for different cosmic era and then taking these as inputs we study evolution of primordial black holes. From our estimation it is found that accretion of radiation does not affect evolution of primordial black holes in loop quantum gravity even though a larger number of primordial black holes may form in early universe in comparison with Einstein's or scalar-tensor theories.

gr-qc↗

Effect of Vacuum Energy on Evolution of Primordial Black Holes in Einstein Gravity

We study the evolution of primordail black holes by considering present universe is no more matter dominated rather vacuum energy dominated. We also consider the accretion of radiation, matter and vacuum energy during respective dominance period. In this scenario, we found that radiation accretion efficiency should be less than 0.366 and accretion rate is much larger than previous analysis by B. Nayak et al. \cite{ns}. Thus here primordial black holes live longer than previous works \cite{ns}. Again matter accretion slightly increases the mass and lifetime of primordial black holes. However, the vacuum energy accretion is slightly complicated one, where accretion is possible only upto a critical time. This critical time depends on the values of accretion efficiency and formation time. If a primordial black hole lives beyond critical time, then its lifespan increases due to vacuum energy accretion. But for presently evaporating primordial black holes, critical time comes much later than their evaporating time and thus vacuum energy could not affect those primordial black holes. We again found that the constraints on the initial mass fraction of PBH obtained from the $γ$-ray background limit becomes stronger in the presence of vacuum energy.

gr-qc↗

Brans-Dicke Theory and primordial black holes in Early Matter-Dominated Era

We show that primordial black holes can be formed in the matter-dominated era with gravity described by the Brans-Dicke theory. Considering an early matter-dominated era between inflation and reheating, we found that the primordial black holes formed during that era evaporate at a quicker than those of early radiation-dominated era. Thus, in comparison with latter case, less number of primordial black holes could exist today. Again the constraints on primordial black hole formation tend towards the larger value than their radiation-dominated era counterparts indicating a significant enhancement in the formation of primordial black holes during the matter-dominaed era.

gr-qc↗

Accretion, Primordial Black Holes and Standard Cosmology

Primordial Black Holes evaporate due to Hawking radiation. We find that the evaporation time of primordial black holes increase when accretion of radiation is included.Thus depending on accretion efficiency more and more number of primordial black holes are existing today, which strengthens the idea that the primordial black holes are the proper candidate for dark matter.

gr-qc↗

Note on nonstationarity and accretion of Primordial Black Holes in Brans-Dicke theory

We consider the evolution of primordial black holes by including non-stationarity in their formation process and accretion of radiation in Brans-Dicke theory. Specifically, we focus on how $η$, the fraction of the horizon mass the black hole comprises capturing nonstationarity, affects the lifetimes of these primordial black holes. Our calculation reveals that the primordial black hole dynamics is controlled by the product $fη$ where $f$ is the accretion efficiency. We also estimate the impact of $η$ through $fη$ on the primordial black holes' initial mass fraction constraint obtained from the $γ$-ray background limit.

gr-qc↗