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20 pages, 903 KiB

Open AccessArticle

Generic Modification of Gravity, Late Time Acceleration and Hubble Tension

by Mayukh R. Gangopadhyay, Shibesh K. Jas Pacif, Mohammad Sami and Mohit K. Sharma

Universe 2023, 9(2), 83; https://doi.org/10.3390/universe9020083 - 3 Feb 2023

Cited by 18 | Viewed by 2295

We consider a scenario of large-scale modification of gravity that does not invoke extra degrees of freedom, but includes coupling between baryonic matter and dark matter in the Einstein frame. The total matter energy density follows the standard conservation, and evolution has the character of deceleration in this frame. The model exhibits interesting features in the Jordan frame realised by virtue of a disformal transformation where individual matter components adhere to standard conservation but gravity is modified. A generic parametrization of disformal transformation leaves thermal history intact and gives rise to late time acceleration in the Jordan frame, which necessarily includes phantom crossing, which, in the standard framework, can be realised using at least two scalar fields. This scenario is embodied by two distinguished features, namely, acceleration in the Jordan frame and deceleration in the Einstein frame, and the possibility of resolution of the Hubble tension thanks to the emergence of the phantom phase at late times. Full article

(This article belongs to the Special Issue Modified Gravity Approaches to the Tensions of ΛCDM)

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15 pages, 283 KiB

Open AccessArticle

Modifications of Gravity Via Differential Transformations of Field Variables

by Anton Sheykin, Dmitry Solovyev, Vladimir Sukhanov and Sergey Paston

Symmetry 2020, 12(2), 240; https://doi.org/10.3390/sym12020240 - 5 Feb 2020

Cited by 12 | Viewed by 2198

Abstract

We discuss field theories appearing as a result of applying field transformations with derivatives (differential field transformations, DFTs) to a known theory. We begin with some simple examples of DFTs to see the basic properties of the procedure. In this process, the dynamics of the theory might either change or be conserved. After that, we concentrate on the theories of gravity which appear as a result of various DFTs applied to general relativity, namely the mimetic gravity and Regge–Teitelboim embedding theory. We review the main results related to the extension of dynamics in these theories, as well as the possibility to write down the action of a theory after DFTs as the action of the original theory before DFTs plus an additional term. Such a term usually contains some constraints with Lagrange multipliers and can be interpreted as an action of additional matter, which might be of use in cosmological applications, e.g., for the explanation of the effects of dark matter. Full article

(This article belongs to the Special Issue Cosmology)

12 pages, 304 KiB

Open AccessArticle

Disformal Transformations in Modified Teleparallel Gravity

by Alexey Golovnev and María José Guzmán

Symmetry 2020, 12(1), 152; https://doi.org/10.3390/sym12010152 - 11 Jan 2020

Cited by 13 | Viewed by 2526

Abstract

In this work, we explore disformal transformations in the context of the teleparallel equivalent of general relativity and modified teleparallel gravity. We present explicit formulas in components for disformal transformations of the main geometric objects in these theories such as torsion tensor, torsion [...] Read more.

f (T)

gravity that disformal transformations from the Jordan frame representation are unable to straightforwardly remove local Lorentz breaking terms that characterize it. However, we have shown that disformal transformations have interesting properties, which can be useful for future applications in scalar-torsion gravity models, among others. Full article

(This article belongs to the Special Issue Selected Papers: 10th Mathematical Physics Meeting)

10 pages, 224 KiB

Open AccessArticle

Disformal Transformations in Scalar–Torsion Gravity

by Manuel Hohmann

Universe 2019, 5(7), 167; https://doi.org/10.3390/universe5070167 - 7 Jul 2019

Cited by 19 | Viewed by 2674

Abstract

We study disformal transformations in the context of scalar extensions to teleparallel gravity, in which the gravitational interaction is mediated by the torsion of a flat, metric compatible connection. We find a generic class of scalar–torsion actions which is invariant under disformal transformations, and which possesses different invariant subclasses. For the most simple of these subclasses we explicitly derive all terms that may appear in the action. We propose to study actions from this class as possible teleparallel analogues of healthy beyond Horndeski theories. Full article

(This article belongs to the Special Issue Selected Papers from Teleparallel Universes in Salamanca)

16 pages, 303 KiB

Open AccessArticle

Thermodynamics of f(R) Gravity with Disformal Transformation

by Chao-Qiang Geng, Wei-Cheng Hsu, Jhih-Rong Lu and Ling-Wei Luo

Entropy 2019, 21(2), 172; https://doi.org/10.3390/e21020172 - 13 Feb 2019

Cited by 8 | Viewed by 3197

Abstract

We study thermodynamics in

f (R)

gravity with the disformal transformation. The transformation applied to the matter Lagrangian has the form of [...] Read more.

We study thermodynamics in

f (R)

gravity with the disformal transformation. The transformation applied to the matter Lagrangian has the form of

γ_{μ ν} = A (ϕ, X) g_{μ ν} + B (ϕ, X) \partial_{μ} ϕ \partial_{ν} ϕ

with the assumption of the Minkowski matter metric

γ_{μ ν} = η_{μ ν}

, where

ϕ

is the disformal scalar and X is the corresponding kinetic term of

ϕ

. We verify the generalized first and second laws of thermodynamics in this disformal type of

f (R)

gravity in the Friedmann-Lemaître-Robertson-Walker (FLRW) universe. In addition, we show that the Hubble parameter contains the disformally induced terms, which define the effectively varying equations of state for matter. Full article

(This article belongs to the Special Issue Modified Gravity: From Black Holes Entropy to Current Cosmology II)

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