Black hole dynamics in power-law based metric f(R) gravity

Chinese Physics C

The objective of this work is to generate a general formalism of gravity in the context of dark energy under the framework of K-essence emergent geometry with the Dirac-Born-Infeld (DBI) variety of action, where is the familiar Ricci scalar, is the DBI type non-canonical Lagrangian with , and ϕ is the K-essence scalar field. The emergent gravity metric ( ) and the well known gravitational metric ( ) are not conformally equivalent. We have constructed a modified field equation using the metric formalism in -gravity incorporating the corresponding Friedmann equations into the framework of the background gravitational metric, which is of Friedmann-Lemaître-Robertson-Walker (FLRW) type. The solution of the modified Friedmann equations have been deduced for the specific choice of , which is of Starobinsky-type, using the power law expansion method. The consistency of the model with the accelerating phase of the universe has been shown when we restrict ourselves to consider the value of t...

arXiv: General Relativity and Quantum Cosmology, 2015

Here, we investigate the growth of matter density perturbations as well as the generalized second law (GSL) of thermodynamics in the framework of $f(R)$-gravity. We consider a spatially flat FRW universe filled with the pressureless matter and radiation which is enclosed by the dynamical apparent horizon with the Hawking temperature. For some viable $f(R)$ models containing the Starobinsky, Hu-Sawicki, Exponential, Tsujikawa and AB models, we first explore numerically the evolution of some cosmological parameters like the Hubble parameter, the Ricci scalar, the deceleration parameter, the density parameters and the equation of state parameters. Then, we examine the validity of GSL and obtain the growth factor of structure formation. We find that for the aforementioned models, the GSL is satisfied from the early times to the present epoch. But in the farther future, the GSL for the all models is violated. Our numerical results also show that for the all models, the growth factor for ...

2016

A consistent theoretical model describing expansion of the universe from a central spherical uncharged Schwarzschild black hole is formulated with some modifications. The behaviour of the time dependent conformal factor, which is a temporal part of the geometric mass of the said black hole, is studied exhaustively in the different stages of the expansion. The concept of the dimensionless radius of the core black hole embedded by the space-time is emerged from this formulation. It is interpreted that the nature of such dimensionless radius plays a key role to govern the acceleration or deceleration of the universe. An inverse relation between the conformal factor and the scale factor evolves from the idea that the strength of the core black hole gets lowered to result an accelerated expansion of space-time. The nature of conformal factor and its effect towards the dynamics of the universe is established at different stages of expansion.

The fourth chapter of the collection of problems in cosmology, devoted to black holes. Consists of basic introduction, sections on Schwarzschild and Kerr black holes, a section on particles' motion and collisions in general black hole space-times, and the astrophysical part. This version contains only formulations of 137 problems. The full collection, with solutions included, is available in the form of a wiki-based resource at www.universeinproblems.com. The cosmological community is welcome to contribute to its development.

Monthly Notices of the Royal Astronomical Society, 2023

Observations support the idea that supermassive black holes (SMBHs) power the emission at the centre of active galaxies. Ho we ver, contrary to stellar-mass BHs, there is a poor understanding of their origin and physical formation channel. In this article, we propose a new process of SMBH formation in the early Universe that is not associated with baryonic matter (massive stars) or primordial cosmology. In this no v el approach, SMBH seeds originate from the gravitational collapse of fermionic dense dark matter (DM) cores that arise at the centre of DM haloes as they form. We show that such a DM formation channel can occur before star formation, leading to heavier BH seeds than standard baryonic channels. The SMBH seeds subsequently grow by accretion. We compute the evolution of the mass and angular momentum of the BH using a geodesic general relativistic disc accretion model. We show that these SMBH seeds grow to ∼10 9-10 10 M in the first Gyr of the lifetime of the Universe without invoking unrealistic (or fine-tuned) accretion rates.

International journal of scientific advances, 2022

In this paper we talk about the formation mechanism of the stellar gravitational singularities. To begin with we talk about the formation and evolution of stars and understand the Hertzsprung Russell diagram that teaches us how to classify stars. With the help of the diagram, we categorize the stars based of their physical parameters such as color, temperature, and mass. We then talk about the death cycle of different mass stars and what comes after when their fuels have been exhausted. Stars below the Chandrashekhar limit form a white dwarf at the end of their lives, while stars above the limit form a neutron star of a singularity. Further to find which of the heavy mass stars forms a singularity we look at the Tolman-Oppenheimer-Volkoff limit that states stars with mass above the limit will form singularities. The types of singularities formed depend on the solution of the general theory of relativity given by Schwarzschild, Kerr, Kerr-Newman, and Reissner-Nordström. Each of the theory aspect of the four solutions has been described to give a better understanding of the structure of the singularity formed. The paper also explains theories such as Wormholes and time travel in brief to try and explain what can replace the singularity.

Physical Review D, 2009

We show that within the class of f (R) gravity theories, FLRW power-law perfect fluid solutions only exist for R n gravity. This significantly restricts the set of exact cosmological solutions which have similar properties to what is found in standard General Relativity.

Physical Review D, 1998

We reconsider in this work the effects of an energy absorption term in the evolution of primordial black holes (hereafter PBHs) in the several epochs of the Universe. A critical mass is introduced as a boundary between the accreting and evaporating regimes of the PBHs. We show that the growth of PBHs is negligible in the Radiation-dominated Era due to scarcity of energy density supply from the expanding background, in agreement with a previous analysis by Carr and Hawking, but that nevertheless the absorption term is large enough for black holes above the critical mass to preclude their evaporation until the universe has cooled sufficiently. The effects of PBH motion are also discussed: the Doppler effect may give rise to energy accretion in black-holes with large peculiar motions relative to background. We discuss how cosmological constraints are modified by the introduction of the critical mass since that PBHs above it do not disturb the CMBR. We show that there is a large range of admissible masses for PBHs above the critical mass but well below the cosmological horizon. Finally we outline a minimal kinetic formalism, solved in some limiting cases, to deal with more complicated cases of PBH populations.

Journal of High Energy Physics, 2011

We consider the generalized laws of thermodynamics in massive gravity. Making use of explicit black hole solutions, we devise black hole merger processes in which i) total entropy of the system decreases ii) the zero-temperature extremal black hole is created. Thus, both second and third laws of thermodynamics are violated. In both cases, the violation can be traced back to the presence of negative-mass black holes, which, in turn, is related to the violation of the null energy condition. The violation of the third law of thermodynamics implies, in particular, that a naked singularity may be created as a result of the evolution of a singularity-free state. This may signal a problem in the model, unless the creation of the negative-mass black holes from positive-mass states can be forbidden dynamically or the naked singularity may somehow be resolved in a full quantum theory.

In the last ten years, increasing attention has been devoted to Extended Theories of Gravity with the aim to understand several cosmological and astrophysical issues such as the today observed accelerated expansion of the universe and the presence of Dark Matter in self-gravitating structures. Some of these models assume modifications of General Relativity by adding higher order terms of curvature invariants like the Ricci scalar R, the Ricci tensor Rμν and the Riemann tensors Rμνλσ, or the presence of suitable scalar fields like the former Brans-Dicke theory. It is therefore natural to ask for black hole solutions in this context since, on the one hand, black holes signatures may be the test-bed to compare new models to the Einstein gravity; on the other hand, they may lead to rule out models which disagree with observations. Although black holes are one of the most striking predictions of General Relativity, they remain one of its least tested concepts. Electromagnetic observations allow indirectly to infer their existence, but direct evidences remains elusive. In the next decade, data coming from very long-baseline interferometry and gravitational wave detectors should allow to image and study black holes in detail. Such observations will test General Relativity in the non-linear and strong-field regimes where data are currently lacking. Testing strong-field features of General Relativity is of utmost importance to physics and astrophysics as a whole. This is because the black holes solutions, such as the Schwarzschild and Kerr metrics, enter several calculations, including accretion disk structure, gravitational lensing, cosmology and gravitational waves theory. These black hole solutions could indicate strong-field departures from General Relativity with deep implications for the still unknown fundamental theory of gravity. Beside the physical interest, black hole solutions represent a very active area for mathematical physics investigations. Here we review the problem of black holes in a particular class of Extended Theories of Gravity, the so called f(R)-gravity, discussing some resolution techniques, obtaining exact solutions and comparing results with standard General Relativity. Furthermore, we discuss the problems of hydrostatic equilibrium and stellar structure in the context of f(R)-gravity showing that new features could emerge. The observation of such features could both explain the physics of exotic self-gravitating objects and constitute a signature for Extended Theories of Gravity.