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 Blackhole in nonlocal gravity: comparing metric from Newmann–Janis algorithm with slowly rotating solution

2020 ◽  
Vol 80 (7) ◽  
Author(s):  
Utkarsh Kumar ◽  
Sukanta Panda ◽  
Avani Patel
Keyword(s):  
General Relativity ◽  
Gravitational Field ◽  
Modified Gravity ◽  
Kerr Metric ◽  
Slow Rotation ◽  
Field Equations ◽  
External Region ◽  
Modified Gravity Theories ◽  
Spacetime Metric ◽  
Gravity Theories

Abstract The strong gravitational field near massive blackhole is an interesting regime to test General Relativity (GR) and modified gravity theories. The knowledge of spacetime metric around a blackhole is a primary step for such tests. Solving field equations for rotating blackhole is extremely challenging task for the most modified gravity theories. Though the derivation of Kerr metric of GR is also demanding job, the magical Newmann–Janis algorithm does it without actually solving Einstein equation for rotating blackhole. Due to this notable success of Newmann–Janis algorithm in the case of Kerr metric, it has been being used to obtain rotating blackhole solution in modified gravity theories. In this work, we derive the spacetime metric for the external region of a rotating blackhole in a nonlocal gravity theory using Newmann–Janis algorithm. We also derive metric for a slowly rotating blackhole by perturbatively solving field equations of the theory. We discuss the applicability of Newmann–Janis algorithm to nonlocal gravity by comparing slow rotation limit of the metric obtained through Newmann–Janis algorithm with slowly rotating solution of the field equation.

scholarly journals Generic off-diagonal solutions and solitonic hierarchies in (modified) gravity

2015 ◽  
Vol 30 (19) ◽  
pp. 1550090 ◽  
Author(s):  
Sergiu I. Vacaru
Keyword(s):  
General Relativity ◽  
Exact Solutions ◽  
Modified Gravity ◽  
Field Equations ◽  
Physical Data ◽  
Modified Gravity Theories ◽  
De Vries ◽  
Einstein Spaces ◽  
Curve Flows ◽  
Gravity Theories

We have summarized our recent results on encoding exact solutions of field equations in Einstein and modified gravity theories into solitonic hierarchies derived for nonholonomic curve flows with associated bi-Hamilton structure. We argue that there is a canonical distinguished connection for which the fundamental geometric/physical equations decouple in general form. This allows us to construct very general classes of generic off-diagonal solutions determined by corresponding types of generating and integration functions depending on all (spacetime) coordinates. If the integral varieties are constrained to zero torsion configurations, we can extract solutions for the general relativity (GR) theory. We conclude that the geometric and physical data for various classes of effective/modified Einstein spaces can be encoded into multi-component versions of the sine-Gordon, or modified Korteweg–de Vries equations.

scholarly journals Probing modified gravity in cosmic filaments

2018 ◽  
Vol 619 ◽  
pp. A122 ◽  
Author(s):  
Alex Ho ◽  
Max Gronke ◽  
Bridget Falck ◽  
David F. Mota
Keyword(s):  
Modified Gravity ◽  
Gravity Models ◽  
Accelerated Expansion ◽  
Radial Density ◽  
Post Process ◽  
Global Properties ◽  
Modified Gravity Theories ◽  
Expansion Of The Universe ◽  
Gravity Theories ◽  
The Universe

Multiple modifications of general relativity (GR) have been proposed in the literature in order to understand the nature of the accelerated expansion of the Universe. However, thus far all the predictions of GR have been confirmed with constantly increasing accuracy. In this work, we study the imprints of a particular class of models – “screened” modified gravity theories – on cosmic filaments. We have utilized the N-body code ISIS/RAMSES to simulate the symmetron model and the Hu–Sawicky f(R) model, and we post-process the output with DisPerSE to identify the filaments of the cosmic web. We investigated how the global properties of the filaments – such as their lengths, masses, and thicknesses – as well as their radial density and speed profiles change under different gravity theories. We find that filaments are, on average, shorter and denser in modified gravity models compared to in ΛCDM. We also find that the speed profiles of the filaments are enhanced, consistent with theoretical expectations. Overall, our results suggest that cosmic filaments can be an effective complementary probe of screened modified gravity theories on Mpc scales.

scholarly journals PHANTOM CROSSING IN VIABLE f(R) THEORIES

2011 ◽  
Vol 20 (08) ◽  
pp. 1339-1345 ◽  
Author(s):  
KAZUHARU BAMBA ◽  
CHAO-QIANG GENG ◽  
CHUNG-CHI LEE
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Modified Gravity ◽  
Gravity Models ◽  
Phantom Divide ◽  
The Past ◽  
Modified Gravity Theories ◽  
Phantom Crossing ◽  
Gravity Theories

We review the equation of state for dark energy in modified gravity theories. In particular, we summarize the generic feature of the phantom divide crossing in the past and future in viable f(R) gravity models.

Formation of wormholes in modified gravity: Exotic matter and stability

2019 ◽  
Vol 34 (25) ◽  
pp. 1950200
Author(s):  
Subhra Bhattacharya ◽  
Shibaji Halder ◽  
Subenoy Chakraborty
Keyword(s):  
Modified Gravity ◽  
Energy Condition ◽  
Null Energy Condition ◽  
Einstein Gravity ◽  
Einstein Equations ◽  
Exotic Matter ◽  
Field Equations ◽  
Normal Fluid ◽  
Modified Gravity Theories ◽  
Gravity Theories

This work addresses the question whether exotic matter is essential for the formation of wormholes in modified gravity theories. The basic property of wormhole geometry is the flaring-out condition at the throat which essentially states the violation of null energy condition for the matter threading the wormhole in Einstein gravity. In modified gravity theories, the field equations can be written as Einstein equations with two non-interacting fluids of which one is the usual fluid under consideration and the second term, called the effective matter, comes from the extra geometric terms of the theory. So it is interesting to examine whether normal fluid with restrictions on geometry satisfies the conditions for the formation of wormholes and their stability.

scholarly journals Does the Cosmological Expansion Change Local Dynamics?

Symmetry ◽  
2021 ◽  
Vol 13 (8) ◽  
pp. 1417
Author(s):  
Marcelo Schiffer
Keyword(s):  
Dark Matter ◽  
Modified Gravity ◽  
Hubble Constant ◽  
Matter Content ◽  
Matter Distribution ◽  
Present Value ◽  
Field Equations ◽  
Local Dynamics ◽  
Cosmological Expansion ◽  
Modified Gravity Theories

It is a well-known fact that the Newtonian description of dynamics within Galaxies for its known matter content is in disagreement with the observations as the acceleration approaches a0≈1.2×10−10 m/s2 (slighter larger for clusters). Both the Dark Matter scenario and Modified Gravity Theories (MGT) fail to explain the existence of such an acceleration scale. Motivated by the closeness of the acceleration scale and the Hubble constant cH0≈10−9 h m/s2, we are led to analyze whether this coincidence might have a Cosmological origin for scalar-tensor and spinor-tensor theories by performing detailed calculations for perturbations that represent the local matter distribution on the top of the cosmological background. Then, we solve the field equations for these perturbations in a power series in the present value of the Hubble constant. As we shall see, for both theories, the power expansion contains only even powers in the Hubble constant, a fact that renders the cosmological expansion irrelevant for the local dynamics.

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.

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