Continuous matter creation and the acceleration of the universe: the growth of density fluctuations

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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Hamiltonian formalism for nonlocal gravity models

International Journal of Geometric Methods in Modern Physics ◽

10.1142/s0219887822500360 ◽

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.

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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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Cosmic acceleration with tachyon field

Modern Physics Letters A ◽

10.1142/s0217732318501997 ◽

2018 ◽

Vol 33 (34) ◽

pp. 1850199 ◽

Cited By ~ 1

Author(s):

A. I. Keskin

Keyword(s):

Scalar Field ◽

Energy Momentum Tensor ◽

Momentum Tensor ◽

Cosmic Acceleration ◽

Late Time ◽

De Sitter ◽

Tachyon Field ◽

Minimal Coupling ◽

Acceleration Of The Universe ◽

The Universe

In this study, we examine two models of the scalar field, that is, a normal scalar field and a tachyon scalar field in [Formula: see text] gravity to describe cosmic acceleration of the universe, where [Formula: see text], [Formula: see text] and [Formula: see text] are Ricci curvature scalar, trace of energy–momentum tensor and kinetic energy of scalar field [Formula: see text], respectively. Using the minimal-coupling Lagrangian [Formula: see text], for both the scalar models we obtain a viable cosmological system, where [Formula: see text] and [Formula: see text] are real constants. While a normal scalar field gives a system describing expansion from the deceleration to the late-time acceleration, tachyon field together with [Formula: see text] in the system produces a quintessential expansion which is very close to de Sitter point, where we find a new condition [Formula: see text] for inflation.

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The Acceleration of the Universe: Measurements of Cosmological Parameters from Type Ia Supernovae

Particle Physics and the Universe ◽

10.1142/9789812810434_0005 ◽

2001 ◽

Author(s):

A. Goobar ◽

S. Perlmutter ◽

G. Aldering ◽

G. Goldhaber ◽

R. A. Knop ◽

...

Keyword(s):

Cosmological Parameters ◽

Type Ia Supernovae ◽

Type Ia ◽

Acceleration Of The Universe ◽

Ia Supernovae ◽

The Universe

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Large Scale Structure of the Universe

Symposium - International Astronomical Union ◽

10.1017/s0074180900128906 ◽

1998 ◽

Vol 179 ◽

pp. 317-328 ◽

Cited By ~ 2

Author(s):

N.A. Bahcall

Keyword(s):

Large Scale ◽

Initial Conditions ◽

Large Scale Structure ◽

Density Fluctuations ◽

Scale Structure ◽

Sloan Digital Sky Survey ◽

Sky Survey ◽

Initial Spectrum ◽

Formation And Evolution ◽

The Universe

How is the universe organized on large scales? How did this structure evolve from the unknown initial conditions of a rather smooth early universe to the present time? The answers to these questions will shed light on the cosmology we live in, the amount, composition and distribution of matter in the universe, the initial spectrum of density fluctuations that gave rise to this structure, and the formation and evolution of galaxies, lusters of galaxies, and larger scale structures.To address these fundamental questions, large and accurate sky surveys are needed—in various wavelengths and to various depths. In this presentation I review current observational studies of large scale structure, present the constraints these observations place on cosmological models and on the amount of dark matter in the universe, and highlight some of the main unsolved problems in the field of large-scale structure that could be solved over the next decade with the aid of current and future surveys. I briefly discuss some of these surveys, including the Sloan Digital Sky Survey that will provide a complete imaging and spectroscopic survey of the high-latitude northern sky, with redshifts for the brightest ∼ 106 galaxies, 105 quasars, and 103.5 rich clusters of galaxies. The potentialities of the SDSS survey, as well as of cross-wavelength surveys, for resolving some of the unsolved problems in large-scale structure and cosmology are discussed.

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