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.

A study of some cylindrically symmetric solutions in f(R, G) gravity

2020 ◽  
Vol 98 (4) ◽  
pp. 364-374
Author(s):  
Saeeda Zia ◽  
M. Farasat Shamir
Keyword(s):  
Exact Solutions ◽  
Modified Gravity ◽  
Complete System ◽  
Energy Condition ◽  
Null Energy Condition ◽  
Energy Conditions ◽  
Model Parameters ◽  
Field Equations ◽  
Cylindrically Symmetric ◽  
Modified Theory

In this paper, we present the cylindrically symmetric solutions in a well-known modified theory, namely f(R, G) gravity. After driving the complete system of field equations, six different families of exact solutions using a viable f(R, G) gravity model have been discussed. Moreover, we have investigated the well-known Levi–Civita solution in modified gravity for the model f(R, G) = R2 + αGn for some suitable values of model parameters n and α. Null energy conditions are also calculated for all the obtained solutions. Some regions are observed where the null energy condition is violated, indicating the existence of cylindrical wormholes.

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 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.

ENTROPY FORCE AND SELF-GRAVITY FACTOR OF MODIFIED GRAVITY THEORIES

2011 ◽  
Vol 26 (14) ◽  
pp. 1025-1034 ◽  
Author(s):  
SHU ZHU ◽  
SHAO-FENG WU ◽  
GUO-HONG YANG
Keyword(s):  
Modified Gravity ◽  
Einstein Gravity ◽  
Modified Gravity Theories ◽  
Gravity Theories ◽  
Self Gravity ◽  
Trapping Horizons

We investigate the entropy force of modified gravity theories on trapping horizons. It is shown that if the entropy force is separated as a uniform term from the variation of entropy, the associated self-gravity factor is not the simple two in Einstein gravity but is different for various gravity theories.

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 Phantom fluid wormhole in f(R, T) gravity

2019 ◽  
Vol 34 (37) ◽  
pp. 1950303 ◽  
Author(s):  
Parbati Sahoo ◽  
Annika Kirschner ◽  
P. K. Sahoo
Keyword(s):  
Modified Gravity ◽  
Energy Condition ◽  
Phantom Energy ◽  
Null Energy Condition ◽  
Energy Conditions ◽  
Spherically Symmetric ◽  
Phantom Dark Energy ◽  
Modified Gravity Theories ◽  
De Formula ◽  
Modified Formula

Wormholes (WHs) are considered as hypothetical shortcuts or tunnels in spacetime. In general relativity (GR), the fundamental ingredient of WH geometry is the presence of exotic matter at the throat, which is responsible for the violation of null energy condition (NEC). However, the modified gravity theories have shown to be able to provide WH solutions satisfying energy conditions (ECs). In this paper, we study the static spherically symmetric WH solutions in modified [Formula: see text] gravity for a phantom fluid case. The exact solutions of this model are obtained through the equation of state (EoS), [Formula: see text], associated with phantom dark energy (DE) [Formula: see text]. We find the existence of spherically symmetric WH solution supported by phantom energy distribution. The shape function of the WH obtained in this model obeys all the WH metric conditions. In modified gravity scenario, the phantom fluid WH violates the NEC in radial case, unlike in the tangential case. Furthermore, using the “volume integral quantifier” (VIQ) method, the total amount of EC violating matter in spacetime is discussed briefly.

scholarly journals Generating functions of wormholes

2019 ◽  
Vol 34 (01) ◽  
pp. 1950010 ◽  
Author(s):  
Farook Rahaman ◽  
Susmita Sarkar ◽  
Ksh. Newton Singh ◽  
Neeraj Pant
Keyword(s):  
Generating Functions ◽  
Shape Function ◽  
Energy Condition ◽  
Null Energy Condition ◽  
Exotic Matter ◽  
Shape Functions ◽  
Einstein Field Equations ◽  
Field Equations

It is known that wormhole geometry could be found solving the Einstein field equations by tolerating the violation of null energy condition (NEC). Violation of NEC is not possible for the physical matter distributions, however, it can be achieved by considering distributions of “exotic matter”. The main purpose of this work is to find generating functions comprising the wormhole-like geometry and discuss the nature of these generating functions. We have used the Herrera et al. [Phys. Rev. D 77, 027502 (2008)] approaches of obtaining generating functions in the background of wormhole spacetime. Here we have adopted two approaches for solving the field equations to find wormhole geometry. In the first method, we have assumed the redshift function f(r) and the shape function b(r) and solve for the generating functions. In another attempt, we assume generating functions and redshift functions and then try to find shape functions of the wormholes.

scholarly journals Off-diagonal cosmological solutions in emergent gravity theories and Grigory Perelman entropy for geometric flows

2021 ◽  
Vol 81 (1) ◽  
Author(s):  
Sergiu I. Vacaru ◽  
Elşen Veli Veliev ◽  
Laurenţiu Bubuianu
Keyword(s):  
Modified Gravity ◽  
Nonlinear Partial Differential Equations ◽  
Matter Field ◽  
Einstein Equations ◽  
Geometric Flows ◽  
Emergent Gravity ◽  
Flow Equations ◽  
Geometric Evolution ◽  
Modified Gravity Theories ◽  
Gravity Theories

AbstractWe develop an approach to the theory of relativistic geometric flows and emergent gravity defined by entropy functionals and related statistical thermodynamics models. Nonholonomic deformations of G. Perelman’s functionals and related entropic values used for deriving relativistic geometric evolution flow equations. For self-similar configurations, such equations describe generalized Ricci solitons defining modified Einstein equations. We analyse possible connections between relativistic models of nonholonomic Ricci flows and emergent modified gravity theories. We prove that corresponding systems of nonlinear partial differential equations, PDEs, for entropic flows and modified gravity posses certain general decoupling and integration properties. There are constructed new classes of exact and parametric solutions for nonstationary configurations and locally anisotropic cosmological metrics in modified gravity theories and general relativity. Such solutions describe scenarios of nonlinear geometric evolution and gravitational and matter field dynamics with pattern-forming and quasiperiodic structure and various space quasicrystal and deformed spacetime crystal models. We analyse new classes of generic off-diagonal solutions for entropic gravity theories and show how such solutions can be used for explaining structure formation in modern cosmology. Finally, we speculate why the approaches with Perelman–Lyapunov type functionals are more general or complementary to the constructions elaborated using the concept of Bekenstein–Hawking entropy.

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.

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