scholarly journals A logarithmic correction in the entropy functional formalism

2016 ◽  
Vol 25 (07) ◽  
pp. 1650080 ◽  
Author(s):  
Fayçal Hammad ◽  
Mir Faizal
Keyword(s):  
Quantum Gravity ◽  
Modified Gravity ◽  
Correction Term ◽  
Modified Theories Of Gravity ◽  
Logarithmic Correction ◽  
Functional Formalism ◽  
Entropy Formula ◽  
Entropy Functional ◽  
Modified Gravity Theories ◽  
Theories Of Gravity

The entropy functional formalism allows one to recover general relativity, modified gravity theories, as well as the Bekenstein–Hawking entropy formula. In most approaches to quantum gravity, the Bekenstein–Hawking’s entropy formula acquires a logarithmic correction term. As such terms occur almost universally in most approaches to quantum gravity, we analyze the effect of such terms on the entropy functional formalism. We demonstrate that the leading correction to the micro-canonical entropy in the entropy functional formalism can be used to recover modified theories of gravity already obtained with an uncorrected micro-canonical entropy. Furthermore, since the entropy functional formalism reproduces modified gravity, the rise of gravity-dependent logarithmic corrections turns out to be one way to impose constraints on these theories of modified gravity. The constraints found here for the simple case of an [Formula: see text]-gravity are the same as those obtained in the literature from cosmological considerations.

scholarly journals Modified gravity from an entropy functional

2014 ◽  
Vol 23 (09) ◽  
pp. 1450073 ◽  
Author(s):  
Fayçal Hammad
Keyword(s):  
Modified Gravity ◽  
Higher Order ◽  
Functional Formalism ◽  
Entire Series ◽  
Entropy Functional ◽  
Modified Gravity Theory ◽  
Horizon Entropy ◽  
Modified Gravity Theories ◽  
Gravity Theories ◽  
Higher Order Corrections

We extend Padmanabhan's entropy functional formalism to show that, in addition to the Gauss–Bonnet (GB) or the entire series of Lanczos–Lovelock Lagrangians already obtained, more general higher-order corrections to General Relativity, i.e. the so-called modified gravity theories, also emerge naturally from this formalism. This extension shows that the formalism constitutes a valuable tool to investigate, at each order in the curvature, the possible structure the higher-order modified gravity theories might have. As an application, the extended formalism is used to evaluate the horizon entropy in a modified gravity theory of the second-order in the curvature. Our findings are in agreement with previous results from the literature.

On Modified Gravity

2021 ◽  
Vol 6 (1) ◽  
Author(s):  
Viktor Toth ◽  
Jean-Pierre Luminet
Keyword(s):  
Dark Matter ◽  
Modified Gravity ◽  
Modified Theories Of Gravity ◽  
Theories Of Gravity

Viktor Toth adds theoretical insights to the modified theories of gravity that aimed to solve the dark matter problem without necessitating the existence of hypothetic particles of nonbaryonic matter.

PALATINI f(R) COSMOLOGY

2008 ◽  
Vol 23 (17n20) ◽  
pp. 1388-1396 ◽  
Author(s):  
SEOKCHEON LEE
Keyword(s):  
Linear Growth ◽  
Matter Density ◽  
Modified Theories Of Gravity ◽  
Cosmic Expansion ◽  
Energy Models ◽  
Modified Gravity Theories ◽  
Theories Of Gravity ◽  
The Stability ◽  
Cosmic Expansion History ◽  
General Gauge

We investigate the modified gravity theories in terms of the effective dark energy models. We compare the cosmic expansion history and the linear growth in different models. We also study the evolution of linear cosmological perturbations in modified theories of gravity assuming the Palatini formalism. We find the stability of the superhorizon metric evolution depends on models. We also study the matter density fluctuation in the general gauge and show the differential equations in super and sub-horizon scales.

scholarly journals Phenomenological Aspects of Quantum Gravity and Modified Theories of Gravity

2017 ◽  
Vol 2017 ◽  
pp. 1-2
Author(s):  
Ahmed Farag Ali ◽  
Giulia Gubitosi ◽  
Mir Faizal ◽  
Barun Majumder
Keyword(s):  
Quantum Gravity ◽  
Modified Theories Of Gravity ◽  
Theories Of Gravity

scholarly journals SOME ASPECTS OF GENERALIZED MODIFIED GRAVITY MODELS

2013 ◽  
Vol 22 (08) ◽  
pp. 1330017 ◽  
Author(s):  
R. MYRZAKULOV ◽  
L. SEBASTIANI ◽  
S. ZERBINI
Keyword(s):  
Modified Gravity ◽  
Correction Term ◽  
High Redshift ◽  
Early Time ◽  
Gravity Models ◽  
High Frequency Oscillation ◽  
Field Equations ◽  
Modified Gravity Theories ◽  
Gravity Theories ◽  
General Aspects

In this paper, we review some general aspects of modified gravity theories, investigating mathematical and physical properties and, more specifically, the feature of viable and realistic models able to reproduce the dark energy (DE) epoch and the early-time inflation. We will discuss the black hole (BH) solutions in generalized theories of gravity: it is of fundamental interest to understand how properties and laws of BHs in General Relativity (GR) can be addressed in the framework of modified theories. In particular, we will discuss the energy issue and the possibility to derive the First Law of thermodynamics from the field equations. Then, in the analysis of cosmological solutions, we will pay particular attention to the occurrence of finite-time future singularities and to the possibility to avoid them in [Formula: see text]-gravity. Furthermore, realistic models of F(R)-gravity will be analyzed in detail. A general feature occurring in matter era will be shown, namely, the high derivatives of Hubble parameter may be influenced by the high frequency oscillation of the DE and some correction term may be required in order to stabilize the theory at high redshift. The inflationary scenario is also carefully analyzed and a unified description of the universe is evolved. In the final part of the work, we will look at the last developments in modified gravity, namely, we will investigate cosmological and BH solutions in a covariant field theory of gravity and we will introduce the extended "teleparallel" F(T)-gravity theories. A nice application to the dark matter (DM) problem will be presented.

scholarly journals Logarithmic correction of the BTZ black hole and adaptive model of graphene

2018 ◽  
Vol 27 (12) ◽  
pp. 1850118 ◽  
Author(s):  
Behnam Pourhassan ◽  
Mir Faizal ◽  
S. Ahmad Ketabi
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Quantum Gravity ◽  
Correction Term ◽  
Thermal Fluctuations ◽  
Logarithmic Correction ◽  
Adaptive Model ◽  
Btz Black Hole ◽  
The Real ◽  
Almost All

It is known that almost all approaches to quantum gravity produce a logarithmic correction term to the entropy of a black hole, but the exact coefficient of such a term varies between the different approach to quantum gravity. Such logarithmic terms can also occur due to thermal fluctuations in both analogous and real black holes so that we will analyze the effects of logarithmic corrections term with variable coefficient on properties of analogous black hole. As these properties can be experimentally tested, they can be used to obtain the correct coefficient for such terms for an analogous black hole. We will argue that as even the real black holes can be considered as thermodynamical objects in Jacobson formalism, so such analogous black holes can be used to obtain the correct coefficient for the real black holes, and this in turn can be used to select the correct approach to quantum gravity. In that case, we use an adaptive model of graphene, which is still far from real graphene, to investigate some thermodynamics quantities of BTZ black hole.

scholarly journals Causal hierarchy in modified gravity

2020 ◽  
Vol 2020 (12) ◽  
Author(s):  
Raúl Carballo-Rubio ◽  
Francesco Di Filippo ◽  
Stefano Liberati ◽  
Matt Visser
Keyword(s):  
Modified Gravity ◽  
Modified Theories Of Gravity ◽  
Global Hyperbolicity ◽  
Signal Velocity ◽  
Superluminal Propagation ◽  
Strong Causality ◽  
Theories Of Gravity ◽  
Stable Causality ◽  
Multiple Metrics ◽  
Extra Structure

Abstract We investigate the causal hierarchy in various modified theories of gravity. In general relativity the standard causal hierarchy, (key elements of which are chronology, causality, strong causality, stable causality, and global hyperbolicity), is well-established. In modified theories of gravity there is typically considerable extra structure, (such as: multiple metrics, aether fields, modified dispersion relations, Hořava-like gravity, parabolic propagation, etcetera), requiring a reassessment and rephrasing of the usual causal hierarchy. We shall show that in this extended framework suitable causal hierarchies can indeed be established, and discuss the implications for the interplay between “superluminal” propagation and causality. The key distinguishing feature is whether the signal velocity is finite or infinite. Preserving even minimal notions of causality in the presence of infinite signal velocity requires the aether field to be both unique and hypersurface orthogonal, leading us to introduce the notion of global parabolicity.

Particle production in modified gravity in the early and present day universe

2020 ◽  
Vol 35 (36) ◽  
pp. 2044027
Author(s):  
E. V. Arbuzova
Keyword(s):  
Dark Matter ◽  
Energy Spectrum ◽  
Cosmic Rays ◽  
Modified Gravity ◽  
Particle Production ◽  
Equations Of Motion ◽  
Modified Theories Of Gravity ◽  
Oscillating Solutions ◽  
Theories Of Gravity ◽  
Gravitational Equations

Gravitational equations of motion in modified theories of gravity have oscillating solutions, both in the early and in the present day universe. Particle production by such oscillations is analyzed and possible observational consequences are considered. This phenomenon has impact on energy spectrum of cosmic rays and abundance of dark matter particles.

scholarly journals Giant low-surface-brightness dwarf galaxy as a test bench for MOdified Gravity

2020 ◽  
Vol 493 (2) ◽  
pp. 2373-2376 ◽  
Author(s):  
Ivan de Martino
Keyword(s):  
Dark Matter ◽  
Modified Gravity ◽  
Dwarf Galaxy ◽  
Weak Field ◽  
Newtonian Potential ◽  
Modified Theories Of Gravity ◽  
Weak Field Limit ◽  
Low Surface Brightness ◽  
Standard Cosmological Model ◽  
Theories Of Gravity

ABSTRACT The lack of detection of supersymmetric particles is leading to look at alternative avenues for explaining dark matter’s effects. Among them, modified theories of gravity may play an important role accounting even for both dark components needed in the standard cosmological model. Scalar–tensor–vector gravity theory has been proposed to resolve the dark matter puzzle. Such a modified gravity model introduces, in its weak field limit, a Yukawa-like correction to the Newtonian potential, and is capable to explain most of the phenomenology related to dark matter at scale of galaxies and galaxy clusters. Nevertheless, some inconsistencies appear when studying systems that are supposed to be dark matter dominated such as dwarf galaxies. In this sense, Antlia II, an extremely diffuse galaxy which has been recently discovered in Gaia’s second data release, may serve to probe the aforementioned theory against the need for invoking dark matter. Our analysis shows several inconsistencies and leads to argue that MOdified Gravity may not be able to shed light on the intriguing nature of dark matter.

A primer on modified gravity

2019 ◽  
Vol 28 (05) ◽  
pp. 1942004 ◽  
Author(s):  
Radouane Gannouji
Keyword(s):  
General Relativity ◽  
Modified Gravity ◽  
Alternative Model ◽  
Modified Theories Of Gravity ◽  
Observational Constraints ◽  
Special Issue ◽  
Consistent Model ◽  
Theories Of Gravity

In this first chapter of the Special Issue on Modified Theories of Gravity and Constraints Imposed by Recent GW Observations, we review observational constraints on gravity and the possibility to construct an alternative model to general relativity. For that, we motivate the study of theories beyond Einstein’s gravity, some of their phenomenology and the various ingredients to build a consistent model.

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