Acceleration of the universe in f(R) gravity models

Nonlocal gravity models are constructed to explain the current acceleration of the universe. These models are inspired by the infrared correction appearing in Einstein–Hilbert action. Here, we develop the Hamiltonian formalism of a nonlocal model by considering only terms to quadratic order in Riemann tensor, Ricci tensor and Ricci scalar. We show how to count degrees of freedom using Hamiltonian formalism including Ricci tensor and Ricci scalar terms. In this model, we have also worked out with a choice of a nonlocal action which has only two degrees of freedom equivalent to GR. Finally, we find the existence of additional constraints in Hamiltonian required to remove the ghosts in our full action. We also compare our results with that of obtained using Lagrangian formalism.

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Cosmic voids in modified gravity scenarios

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935949 ◽

2019 ◽

Vol 632 ◽

pp. A52 ◽

Cited By ~ 4

Author(s):

Eder L. D. Perico ◽

Rodrigo Voivodic ◽

Marcos Lima ◽

David F. Mota

Keyword(s):

General Relativity ◽

Modified Gravity ◽

Gravitational Theory ◽

Gravity Models ◽

Consistency Check ◽

Dark Sector ◽

Modified Gravity Theories ◽

Acceleration Of The Universe ◽

Best Fit ◽

The Universe

Modified gravity (MG) theories aim to reproduce the observed acceleration of the Universe by reducing the dark sector while simultaneously recovering General Relativity (GR) within dense environments. Void studies appear to be a suitable scenario to search for imprints of alternative gravity models on cosmological scales. Voids cover an interesting range of density scales where screening mechanisms fade out, which reaches from a density contrast δ ≈ −1 close to their centers to δ ≈ 0 close to their boundaries. We present an analysis of the level of distinction between GR and two modified gravity theories, the Hu–Sawicki f(R) and the symmetron theory. This study relies on the abundance, linear bias, and density profile of voids detected in N-body cosmological simulations. We define voids as connected regions made up of the union of spheres with a mean density given by ρ̅v = 0.2 ρ̅m, but disconnected from any other voids. We find that the height of void walls is considerably affected by the gravitational theory, such that it increases for stronger gravity modifications. Finally, we show that at the level of dark matter N-body simulations, our constraints allow us to distinguish between GR and MG models with |fR0| > 10−6 and zSSB > 1. Differences of best-fit values for MG parameters that are derived independently from multiple void probes may indicate an incorrect MG model. This serves as an important consistency check.

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Reconstruction of some cosmological models from the deceleration parameter

Journal of Physics Conference Series ◽

10.1088/1742-6596/2081/1/012001 ◽

2021 ◽

Vol 2081 (1) ◽

pp. 012001

Author(s):

Aroonkumar Beesham

Keyword(s):

Dark Energy ◽

Deceleration Parameter ◽

Particle Physics ◽

Unsolved Problem ◽

Gravity Models ◽

Reasonable Assumption ◽

Late Time ◽

The Past ◽

Acceleration Of The Universe ◽

The Universe

Abstract Since the discovery of the late-time acceleration of the universe, researchers are still trying to fnd an explanation for it. This is regarded as the most important unsolved problem in cosmology today. The most favoured explanation is dark energy, an unknown or exotic form of matter with negative pressure. One may argue that particle physics may provide the answer in time. Currently, the LambdaCDM model is regarded as the best model. Although this model is reasonably successful and widely accepted, there is growing interest in looking at alternatives. Some of the reasons for this are the fne-tuning, coincidence, infationary paradigm and cosmological constant problems, and whether general relativity is valid on large scales. One focus in trying to understand dark energy is to assume some form of the scale, Hubble or deceleration parameter (or some other reasonable assumption), and then to see how well the model fts in with current observations. This approach is broadly called reconstruction. In this talk, we focus on the deceleration parameter. We provide a brief review of the various forms of the deceleration parameter that have been employed in the past in cosmology, and then focus on some particular forms of interest which have drawn some attention. We note that it is most worthwhile to study alternative dark energy and dark gravity models in order to fully understand the entire space of possibilities.

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DARK ENERGY IN SOME INTEGRABLE AND NONINTEGRABLE FRW COSMOLOGICAL MODELS

International Journal of Modern Physics D ◽

10.1142/s0218271811020445 ◽

2011 ◽

Vol 20 (12) ◽

pp. 2419-2446 ◽

Cited By ~ 11

Author(s):

KURALAY ESMAKHANOVA ◽

NURGISSA MYRZAKULOV ◽

GULGASYL NUGMANOVA ◽

YERLAN MYRZAKULOV ◽

LEONID CHECHIN ◽

...

Keyword(s):

Dark Energy ◽

Cosmological Models ◽

Gravity Models ◽

New Class ◽

Energy Models ◽

Acceleration Of The Universe ◽

Frw Models ◽

The Universe ◽

Friedmann Robertson Walker ◽

Hypergeometric Equations

One of the greatest challenges in today's cosmology to determine the nature of dark energy, the sourse of the observed present acceleration of the universe. Besides the vacuum energy, various dark energy models have been suggested. The Friedmann–Robertson–Walker (FRW) spacetime plays an important role in modern cosmology. In particular, the most popular models of dark energy work in the FRW spacetime. In this work, a new class of integrable FRW cosmological models is presented. These models induced by the well-known Painlevé equations. Some nonintegrable FRW models are also considered. These last models are constructed with the help of Pinney, Schrödinger and hypergeometric equations. Scalar field description and two-dimensional generalizations of some cosmological models are presented. Finally some integrable and nonintegrable F(R) and F(G) gravity models are constructed.

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An Analysis for Conservative and Non-conservative f(R, T) Gravity Models

Asian Journal of Research and Reviews in Physics ◽

10.9734/ajr2p/2021/v5i130152 ◽

2021 ◽

pp. 1-11

Author(s):

Diyadin Can ◽

Ertan Güdekli

Keyword(s):

Energy Condition ◽

Hubble Constant ◽

Momentum Tensor ◽

Cosmic Acceleration ◽

Gravity Models ◽

Energy Conditions ◽

Theory Of Relativity ◽

General Theory Of Relativity ◽

Acceleration Of The Universe ◽

The Universe

As it is known that General Theory of Relativity does not explain the current acceleration of the universe, so there are many attempts to generalize this theory in order to explain the cosmic acceleration without introducing some dark components such as the Dark Energy. Because of the crowd of models in literature, a need to check the models according to some criteria arises. In this study, we analyze two classes of models by means of energy condition restrictions and illustrate the analysis of those classes by graphical simulations. We consider the conservative and non-conservative cases of two classes of models to perform the analysis. The results of the viability of the classes are discussed and it is found that the value of the Hubble constant has no effect on the viability of the models. Focusing on some general classes for the models, we restrict them by means of the so-called energy conditions the energy-momentum tensor on physical grounds. Besides, we find numerical values for coefficients of those classes of models.

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Chameleon field and the late time acceleration of the Universe

Pramana ◽

10.1007/s12043-010-0044-5 ◽

2010 ◽

Vol 74 (3) ◽

pp. 481-489 ◽

Cited By ~ 19

Author(s):

Narayan Banerjee ◽

Sudipta Das ◽

Koyel Ganguly

Keyword(s):

Late Time ◽

Late Time Acceleration ◽

Acceleration Of The Universe ◽

The Universe

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Cosmological constant and late transient acceleration of the universe in the Horava–Witten heterotic M-theory on S1/Z2

Physics Letters B ◽

10.1016/j.physletb.2008.04.003 ◽

2008 ◽

Vol 663 (3) ◽

pp. 147-151 ◽

Cited By ~ 19

Author(s):

Yungui Gong ◽

Anzhong Wang ◽

Qiang Wu

Keyword(s):

Cosmological Constant ◽

Acceleration Of The Universe ◽

M Theory ◽

The Universe

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INTERACTION BETWEEN DARK ENERGY AND DARK MATTER: OBSERVATIONAL CONSTRAINTS FROM OHD, BAO, CMB AND SNe Ia

International Journal of Modern Physics D ◽

10.1142/s021827181350082x ◽

2013 ◽

Vol 22 (14) ◽

pp. 1350082 ◽

Cited By ~ 26

Author(s):

SHUO CAO ◽

NAN LIANG

Keyword(s):

Dark Matter ◽

Dark Energy ◽

Observational Constraints ◽

Coincidence Problem ◽

Cosmological Constraints ◽

Coincidence Index ◽

Acceleration Of The Universe ◽

Interaction Term ◽

Negative Coupling ◽

The Universe

In order to test if there is energy transfer between dark energy (DE) and dark matter (DM), we investigate cosmological constraints on two forms of nontrivial interaction between the DM sector and the sector responsible for the acceleration of the universe, in light of the newly revised observations including OHD, CMB, BAO and SNe Ia. More precisely, we find the same tendencies for both phenomenological forms of the interaction term Q = 3γHρ, i.e. the parameter γ to be a small number, |γ| ≈ 10-2. However, concerning the sign of the interaction parameter, we observe that γ > 0 when the interaction between dark sectors is proportional to the energy density of dust matter, whereas the negative coupling (γ < 0) is preferred by observations when the interaction term is proportional to DE density. We further discuss two possible explanations to this incompatibility and apply a quantitative criteria to judge the severity of the coincidence problem. Results suggest that the γm IDE model with a positive coupling may alleviate the coincidence problem, since its coincidence index C is smaller than that for the γd IDE model, the interacting quintessence and phantom models by four orders of magnitude.

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On generalized Lemaitre–Tolman–Bondi metric: Fractal matter at the end of matter–antimatter recombination

International Journal of Modern Physics D ◽

10.1142/s0218271821500863 ◽

2021 ◽

pp. 2150086

Author(s):

Sergio L. Cacciatori ◽

Alessio Marrani ◽

Federico Re

Keyword(s):

Dark Matter ◽

Relativistic Effects ◽

Partial Explanation ◽

First Order ◽

Acceleration Of The Universe ◽

Particular Solution ◽

Ancient Times ◽

The Universe ◽

Recombination Epoch ◽

Ltb Metric

Many recent researches have investigated the deviations from the Friedmannian cosmological model, as well as their consequences on unexplained cosmological phenomena, such as dark matter and the acceleration of the Universe. On one hand, a first-order perturbative study of matter inhomogeneity returned a partial explanation of dark matter and dark energy, as relativistic effects due to the retarded potentials of far objects. On the other hand, the fractal cosmology, now approximated by a Lemaitre–Tolman–Bondi (LTB) metric, results in distortions of the luminosity distances of SNe Ia, explaining the acceleration as apparent. In this work, we extend the LTB metric to ancient times. The origin of the fractal distribution of matter is explained as the matter remnant after the matter–antimatter recombination epoch. We show that the evolution of such a inhomogeneity necessarily requires a dynamical generalization of LTB, and we propose a particular solution.

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Cosmological dynamics of f(R) models in dynamical system analysis

International Journal of Modern Physics A ◽

10.1142/s0217751x20501249 ◽

2020 ◽

Vol 35 (22) ◽

pp. 2050124

Author(s):

Parth Shah ◽

Gauranga C. Samanta

Keyword(s):

Dynamical System ◽

Asymptotic Behavior ◽

Critical Points ◽

System Analysis ◽

Late Time ◽

Field Equations ◽

Acceleration Phase ◽

First Case ◽

Acceleration Of The Universe ◽

The Universe

In this work we try to understand the late-time acceleration of the universe by assuming some modification in the geometry of the space and using dynamical system analysis. This technique allows to understand the behavior of the universe without analytically solving the field equations. We study the acceleration phase of the universe and stability properties of the critical points which could be compared with observational results. We consider an asymptotic behavior of two particular models [Formula: see text] and [Formula: see text] with [Formula: see text], [Formula: see text], [Formula: see text] for the study. As a first case we fix the value of [Formula: see text] and analyze for all [Formula: see text]. Later as second case, we fix the value of [Formula: see text] and calculation are done for all [Formula: see text]. At the end all the calculations for the generalized case have been shown and results have been discussed in detail.

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