scholarly journals FLRW-cosmology in generic gravity theories

2020 ◽  
Vol 80 (11) ◽  
Author(s):  
Metin Gürses ◽  
Yaghoub Heydarzade
Keyword(s):  
Perfect Fluid ◽  
Gravity Theory ◽  
Fluid Type ◽  
Field Equations ◽  
Walker Metric ◽  
Flrw Cosmology ◽  
Arbitrary Dimensions ◽  
Gravity Theories

AbstractWe prove that for the Friedmann–Lemaitre–Robertson–Walker metric, the field equations of any generic gravity theory in arbitrary dimensions are of the perfect fluid type. The cases of general Lovelock and $${\mathcal {F}}(R, {\mathcal {G}})$$ F ( R , G ) theories are given as examples.

scholarly journals Relativistic models of thin disks immersed in a Robertson-Walker type spacetime

2013 ◽  
Vol 9 (18) ◽  
pp. 131-140
Author(s):  
Gonzalo García Reyes ◽  
Edwin García-Quintero
Keyword(s):  
Perfect Fluid ◽  
Time Dependent ◽  
Energy Conditions ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Ricci Collineations ◽  
Walker Metric ◽  
Relativistic Models ◽  
Thin Disks

Using the well known “displace, cut and reflect” method used to generate disks from given solutions of Einstein field equations, we construct somerelativistic models of time dependent thin disks of infinite extension made of a perfect fluid based on the Robertson-Walker metric. Two simple families of models of disks based on Robertson-Walker solutions admitting Matter and Ricci collineations are presented. We obtain disks that are inagreement with all the energy conditions.

Classical and quantum cosmology in f(T)-gravity theory: A Noether symmetry approach

2021 ◽  
Author(s):  
Roshni Bhaumik ◽  
Sourav Dutta ◽  
Subenoy Chakraborty
Keyword(s):  
Quantum Cosmology ◽  
Functional Form ◽  
Gravity Theory ◽  
Noether Symmetry ◽  
Field Equations ◽  
Time Model ◽  
Symmetry Approach ◽  
Walker Metric ◽  
Augmented Space ◽  
Space Time Model

In the framework of [Formula: see text]-gravity theory, classical and quantum cosmology has been studied in this work for Friedmann Lemaitre Robertson Walker Metric (FLRW) space-time model. The Noether symmetry, a point-like symmetry of the Lagrangian, is used to the physical system and a specific functional form of [Formula: see text] is determined. A point transformation in the 2D augmented space restricts one of the variables to be cyclic so that the Lagrangian as well as the field equations are simplified so that they are solvable. Lastly, for quantum cosmology, the WD equation is constructed and a possible solution has been evaluated.

scholarly journals CAUSAL ANOMALIES IN KALUZA–KLEIN GRAVITY THEORIES

1998 ◽  
Vol 13 (18) ◽  
pp. 3181-3191 ◽  
Author(s):  
M. J. REBOUÇAS ◽  
A. F. F. TEIXEIRA
Keyword(s):  
General Relativity ◽  
Perfect Fluid ◽  
Momentum Tensor ◽  
Causality Principle ◽  
Field Equations ◽  
Perfect Fluid Solution ◽  
Dust Solution ◽  
Gravity Theories ◽  
Induced Matter ◽  
Kaluza Klein

Causal anomalies in two Kaluza–Klein gravity theories are examined, particularly as to whether these theories permit solutions in which the causality principle is violated. It is found that similarly to general relativity the field equations of the space–time–mass Kaluza–Klein (STM-KK) gravity theory do not exclude violation of causality of Gödel type, whereas the induced matter Kaluza–Klein (IM-KK) gravity rules out noncausal Gödel-type models. The induced matter version of general relativity is shown to be an efficient therapy for causal anomalies that occurs in a wide class of noncausal geometries. Perfect fluid and dust Gödel-type solutions of the STM-KK field equations are studied. It is shown that every Gödel-type perfect fluid solution is isometric to the unique dust solution of the STM-KK field equations. The question as to whether 5D Gödel-type noncausal geometries induce any physically acceptable 4D energy–momentum tensor is also addressed.

scholarly journals Exact Axially Symmetric Solution inf(T)Gravity Theory

2014 ◽  
Vol 2014 ◽  
pp. 1-9 ◽  
Author(s):  
Gamal G. L. Nashed
Keyword(s):  
Vacuum Solution ◽  
Gravity Theory ◽  
Kerr Metric ◽  
Radial Coordinate ◽  
Lorentz Transformations ◽  
Axially Symmetric ◽  
Field Equations ◽  
Tetrad Field ◽  
Kerr Spacetime ◽  
Euler’S Angles

A general tetrad field with sixteen unknown functions is applied to the field equations off(T)gravity theory. An analytic vacuum solution is derived with two constants of integration and an angleΦthat depends on the angle coordinateϕand radial coordinater. The tetrad field of this solution is axially symmetric and the scalar torsion vanishes. We calculate the associated metric of the derived solution and show that it represents Kerr spacetime. Finally, we show that the derived solution can be described by two local Lorentz transformations in addition to a tetrad field that is the square root of the Kerr metric. One of these local Lorentz transformations is a special case of Euler’s angles and the other represents a boost when the rotation parameter vanishes.

Causality of 3D extended gravity theories

2019 ◽  
Vol 34 (16) ◽  
pp. 1950122
Author(s):  
Meguru Komada
Keyword(s):  
Shock Wave ◽  
Three Dimensional ◽  
Massive Gravity ◽  
Gravity Theory ◽  
Delay Effect ◽  
Fundamental Physics ◽  
Extended Gravity ◽  
3D Gravity ◽  
Gravity Theories ◽  
Time Delay Effect

Causality is one of the most important properties to understand gravity theories. It gives us not only a method to confirm that the gravity theories are really consistent, but also gives implications about the properties which unknown fundamental physics should obey. We investigate the causality of three-dimensional (3D) gravity theories, which are considered to be important, by using the Shapiro time delay effect in the Shock wave geometry. One of such gravity theories is the Zwei-Dreibein Gravity (ZDG) theory, which is a consistent 3D gravity theory. In ZDG theory, the serious problems can be removed that have appeared in another important gravity theory called New Massive Gravity (NMG). We study whether the ZDG theory could preserve the causality without losing the above good properties and how the causality structure is related to the structure of the NMG theory.

scholarly journals Bounds on higher derivative f(R,□R,T) models from energy conditions

2019 ◽  
Vol 34 (11) ◽  
pp. 1950082 ◽  
Author(s):  
M. Ilyas ◽  
Z. Yousaf ◽  
M. Z. Bhatti
Keyword(s):  
Perfect Fluid ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Ricci Scalar ◽  
Energy Conditions ◽  
Higher Derivative ◽  
Derivative Formula ◽  
Cosmological Solutions ◽  
Walker Metric ◽  
Cosmic Parameters

This paper studies the viable regions of some cosmic models in a higher derivative [Formula: see text] theory with the help of energy conditions (where [Formula: see text], [Formula: see text] and [Formula: see text] are the Ricci scalar, d’Alembert’s operator and trace of energy–momentum tensor, respectively). For this purpose, we assume a flat Friedmann–Lemaître–Robertson–Walker metric which is assumed to be filled with perfect fluid configurations. We take two distinct realistic models that might be helpful to explore stable regimes of cosmological solutions. After taking some numerical values of cosmic parameters, like crackle, snap, jerk (etc.) as well as viable constraints from energy conditions, the viable zones for the under observed [Formula: see text] models are examined.

scholarly journals Anisotropic evolution of D-dimensional FRW spacetime

2019 ◽  
Vol 79 (12) ◽  
Author(s):  
Chad Middleton ◽  
Bret A. Brouse ◽  
Scott D. Jackson
Keyword(s):  
Early Time ◽  
Scale Factor ◽  
Accelerated Expansion ◽  
Late Time ◽  
Time Behavior ◽  
Einstein Field Equations ◽  
Fluid Regime ◽  
Field Equations ◽  
Higher Dimensional ◽  
Walker Metric

AbstractWe examine the time evolution of the $$D=d+4$$D=d+4 dimensional Einstein field equations subjected to a flat Robertson-Walker metric where the 3D and higher-dimensional scale factors are allowed to evolve at different rates. We find the exact solution to these equations for a single fluid component, which yields two limiting regimes offering the 3D scale factor as a function of the time. The fluid regime solution closely mimics that described by 4D FRW cosmology, offering a late-time behavior for the 3D scale factor after becoming valid in the early universe, and can give rise to a late-time accelerated expansion driven by vacuum energy. This is shown to be preceded by an earlier volume regime solution, which offers a very early-time epoch of accelerated expansion for a radiation-dominated universe for $$d=1$$d=1. The time scales describing these phenomena, including the transition from volume to fluid regime, are shown to fall within a small fraction of the first second when the fundamental constants of the theory are aligned with the Planck time. This model potentially offers a higher-dimensional alternative to scalar-field inflationary theory and a consistent cosmological theory, yielding a unified description of early- and late-time accelerated expansions via a 5D spacetime scenario.

scholarly journals Nonminimal kinetic coupled gravity: Inflation on the warped DGP brane

2016 ◽  
Vol 31 (10) ◽  
pp. 1650047
Author(s):  
F. Darabi ◽  
A. Parsiya ◽  
K. Atazadeh
Keyword(s):  
Scalar Field ◽  
Coupling Constants ◽  
Kinetic Term ◽  
Field Potentials ◽  
Coupling Constant ◽  
Field Equations ◽  
Einstein Tensor ◽  
Brane Model ◽  
Walker Metric ◽  
Friedmann Robertson Walker

We consider the nonminimally kinetic coupled version of DGP brane model, where the kinetic term of the scalar field is coupled to the metric and Einstein tensor on the brane by a coupling constant [Formula: see text]. We obtain the corresponding field equations, using the Friedmann–Robertson–Walker metric and the perfect fluid, and study the inflationary scenario to confront the numerical analysis of six typical scalar field potentials with the current observational results. We find that among the suggested potentials and coupling constants, subject to the e-folding [Formula: see text], the potentials [Formula: see text], [Formula: see text] and [Formula: see text] provide the best fits with both Planck+WP+highL data and Planck+WP+highL+BICEP2 data.

scholarly journals NEW GEOMETRIC FORMALISM FOR GRAVITY EQUATION IN EMPTY SPACE

2005 ◽  
Vol 14 (06) ◽  
pp. 1009-1022 ◽  
Author(s):  
XIN-BING HUANG
Keyword(s):  
Differential Equations ◽  
Gauge Field ◽  
Empty Space ◽  
Space Time ◽  
Gravity Theory ◽  
Spin Connection ◽  
Square Root ◽  
Field Equations ◽  
Complex Spin ◽  
Set Up

In this paper, a complex daor field which can be regarded as the square root of space–time metric is proposed to represent gravity. The locally complexified geometry is set up, and the complex spin connection constructs a bridge between gravity and SU(1, 3) gauge field. Daor field equations in empty space are acquired, which are one-order differential equations and do not conflict with Einstein's gravity theory.

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