scholarly journals Hamiltonian formalism for nonlocal gravity models

2022 ◽  
Author(s):  
Pawan Joshi ◽  
Utkarsh Kumar ◽  
Sukanta Panda
Keyword(s):  
Ricci Tensor ◽  
Degrees Of Freedom ◽  
Hamiltonian Formalism ◽  
Riemann Tensor ◽  
Ricci Scalar ◽  
Gravity Models ◽  
Lagrangian Formalism ◽  
Acceleration Of The Universe ◽  
Nonlocal Action ◽  
The Universe

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.

scholarly journals Models & Searches of CPT Violation: a personal, very partial, list

2018 ◽  
Vol 166 ◽  
pp. 00005 ◽  
Author(s):  
Nick E. Mavromatos
Keyword(s):  
Degrees Of Freedom ◽  
Pauli Exclusion Principle ◽  
Gravity Models ◽  
Exclusion Principle ◽  
Cpt Violation ◽  
Temperature Dependent ◽  
Neutral Mesons ◽  
Quantum Matter ◽  
Partial List ◽  
The Universe

In this talk, first I motivate theoretically, and then I review the phenomenology of, some models entailing CPT Violation (CPTV). The latter is argued to be responsible for the observed matter-antimatter asymmetry in the Cosmos, and may owe its origin to either Lorentz-violating background geometries, whose effects are strong in early epochs of the Universe but very weak today, being temperature dependent in general, or to an ill-defined CPT generator in some quantum gravity models entailing decoherence of quantum matter as a result of quantum degrees of freedom in the gravity sector that are inaccessible to the low-energy observers. In particular, for the latter category of CPTV, I argue that entangled states of neutral mesons (Kaons or B-systems), of central relevance to KLOE-2 experiment, can provide smoking-gun sensitive tests or even falsify some of these models. If CPT is ill-defined one may also encounter violations of the spin-statistics theorem, with possible consequences for the Pauli Exclusion Principle, which I only briefly touch upon.

scholarly journals Hamiltonian analysis of Mimetic gravity with higher derivatives

2021 ◽  
Vol 2021 (1) ◽  
Author(s):  
Yunlong Zheng
Keyword(s):  
Hamiltonian Formalism ◽  
Ricci Scalar ◽  
Gravity Models ◽  
Jordan Frame ◽  
Cosmological Perturbation ◽  
Unitary Gauge ◽  
Dirac Matrix ◽  
Higher Derivatives ◽  
Mimetic Gravity ◽  
Hamiltonian Analysis

Abstract Two types of mimetic gravity models with higher derivatives of the mimetic field are analyzed in the Hamiltonian formalism. For the first type of mimetic gravity, the Ricci scalar only couples to the mimetic field and we demonstrate the number of degrees of freedom (DOFs) is three. Then in both Einstein frame and Jordan frame, we perform the Hamiltonian analysis for the extended mimetic gravity with higher derivatives directly coupled to the Ricci scalar. We show that different from previous studies working at the cosmological perturbation level, where only three propagating DOFs show up, this generalized mimetic model, in general, has four DOFs. To understand this discrepancy, we consider the unitary gauge and find out that the number of DOFs reduces to three. We conclude that the reason why this system looks peculiar is that the Dirac matrix of all secondary constraints becomes singular in the unitary gauge, resulting in extra secondary constraints and thus reducing the number of DOFs. Furthermore, we give a simple example of a dynamic system to illustrate how gauge choice can affect the number of secondary constraints as well as the DOFs when the rank of the Dirac matrix is gauge dependent.

scholarly journals SPATIAL RICCI SCALAR DARK ENERGY MODEL

2011 ◽  
Vol 26 (02) ◽  
pp. 317-329 ◽  
Author(s):  
RONG-JIA YANG ◽  
ZONG-HONG ZHU ◽  
FENGQUAN WU
Keyword(s):  
Dark Energy ◽  
Power Spectra ◽  
Ricci Scalar ◽  
Temperature Anisotropy ◽  
Microwave Background ◽  
Cosmic Microwave Background Temperature ◽  
Acceleration Of The Universe ◽  
Background Temperature ◽  
Best Fit ◽  
The Universe

Inspired by the holographic principle, we suggest that the density of dark energy is proportional to the spatial Ricci scalar curvature (SRDE). Such a model is phenomenologically viable. The best fit values of its parameters at 68% confidence level are found to be Ωm 0= 0.259±0.016 and α = 0.261±0.0122, constrained from the Union+CFA3 sample of 397 SNIa and the BAO measurement. We find that the equation of state of SRDE crosses -1 at z ≃ -0.14. The present value of the deceleration parameter q(z) for SRDE is found to be qz = 0~-0.85. The phase transition from deceleration to acceleration of the Universe for SRDE occurs at the redshift zq = 0~0.4. After studying the perturbation of each component of the Universe, we show that the matter power spectra and cosmic microwave background temperature anisotropy are slightly affected by SRDE, compared with ΛCDM.

scholarly journals Acceleration of the universe in f(R) gravity models

2014 ◽  
Vol 352 (2) ◽  
pp. 893-898 ◽  
Author(s):  
Ankan Mukherjee ◽  
Narayan Banerjee
Keyword(s):  
Gravity Models ◽  
Acceleration Of The Universe ◽  
The Universe

scholarly journals Cosmic voids in modified gravity scenarios

2019 ◽  
Vol 632 ◽  
pp. A52 ◽  
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.

Role of modified scalar variables in the modeling of spherical fluids

2018 ◽  
Vol 15 (08) ◽  
pp. 1850140 ◽  
Author(s):  
A. Akram ◽  
A. Rehman Jami ◽  
S. Ahmad ◽  
M. Sufyan ◽  
R. Munir
Keyword(s):  
Degrees Of Freedom ◽  
Dust Cloud ◽  
Mass Function ◽  
Riemann Tensor ◽  
Ricci Scalar ◽  
Structure Scalars ◽  
Specific Distribution ◽  
Scalar Matter ◽  
Symmetric Geometry

The aim of this work is to analyze the role of shear evolution equation in the modeling of relativistic spheres in [Formula: see text] gravity. We assume that non-static diagonally symmetric geometry is coupled with dissipative anisotropic viscous fluid distributions in the presence of [Formula: see text] dark source terms. A specific distribution of [Formula: see text] cosmic model has been assumed and the spherical mass function through generic formula introduced by Misner-Sharp has been formulated. Some very important relations regarding Weyl scalar, matter variables and mass functions are being computed. After decomposing orthogonally the Riemann tensor, some scalar variables in the presence of [Formula: see text] extra degrees of freedom are calculated. The effects of the polynomial modified structure scalars in the modeling of through Weyl, shear, expansion scalar differential equations are investigated. The energy density irregularity factor has been calculated for both anisotropic radiating viscous with varying Ricci scalar and for dust cloud with present Ricci scalar corrections.

scholarly journals Reconstruction of some cosmological models from the deceleration parameter

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.

Cosmology with fermionic sources and relativistic fluid in Schutz’s formalism: Classical and quantum solutions

2017 ◽  
Vol 32 (40) ◽  
pp. 1750220 ◽  
Author(s):  
Marlos O. Ribas ◽  
Fernando P. Devecchi ◽  
Gilberto M. Kremer
Keyword(s):  
Degrees Of Freedom ◽  
Hamiltonian Formalism ◽  
Expected Value ◽  
Scale Factor ◽  
Inflationary Universe ◽  
Conformal Time ◽  
Classical Analysis ◽  
Relativistic Fluid ◽  
The Universe ◽  
Quantum Solution

In this work, a model for the pre-inflationary universe is developed where the sources of the gravitational field are a relativistic fluid and a self-interacting fermionic field. The inclusion of the relativistic fluid is based on Schutz’s model. From the classical analysis based on the Hamiltonian formalism, it is shown that the fluid degrees of freedom can be embodied by a conformal time variable and an expression for the scale factor as function of the conformal time is obtained. From the Wheeler–DeWitt equation, the expected value for the scale factor as function of the conformal time is determined. It is shown that contrary to the classical solution, the expected value of the scale factor does not have a singularity, since it is preceded by a contracted phase up to a minimum value from which the universe begins to expand. Furthermore, from the plots of the classical and quantum solutions for the scale factor as functions of the conformal time it is shown that a decoherence of the quantum solution occurs for late times and both solutions coincide.

scholarly journals DARK ENERGY IN SOME INTEGRABLE AND NONINTEGRABLE FRW COSMOLOGICAL MODELS

2011 ◽  
Vol 20 (12) ◽  
pp. 2419-2446 ◽  
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.

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

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.

Sign in / Sign up

Export Citation Format

Share Document