Dust Universe Research Articles

Primordial dust universe in the Hořava–Lifshitz theory

In this paper, we apply quantum cosmology to investigate the early moments of a Friedmann–Lemaître–Robertson–Walker (FLRW) cosmological model, using Hořava–Lifshitz (HL) as the gravitational theory. The matter content of the model is a dust perfect fluid. We start studying the classical model. Then, we write the total Hamiltonian of the model, quantize it and find the appropriate Wheeler–DeWitt equation. In order to avoid factor ordering ambiguities, in the Wheeler–DeWitt equation, we introduce a canonical transformation. We solve that equation using the Wentzel–Kramers–Brillouin (WKB) approximation and compute the tunneling probabilities for the birth of that universe ([Formula: see text]). Since the WKB wave function depends on the dust energy and the free coupling constants coming from the HL theory, we compute the behavior of [Formula: see text] as a function of all these quantities.

Two fluid cosmological models in f(R,T) theory of gravity

This work deals with the two fluid Bianchi type-V cosmological models consisting of matter and radiating source in the [Formula: see text] theory of gravity studied by Harko et al. [Phys. Rev. D 84, 024020 (2011)]. In this paper, we developed a new idea about [Formula: see text] gravity with the help of two fluids: one fluid is matter field modeling material content of the Universe and other fluid is radiation field modeling the cosmic microwave background (CMB). We have determined the solution of the two fluid gravitational field equations with the systematic structure in [Formula: see text] gravity. Here, we have deliberated four types of universe such as dust universe, radiation universe, hard universe and Zel’dovich universe and also extended our work to observe the big rip and big bang singularity. We have also tested the cosmological parameters.

Influence of new general relativity parameters on the Big Rip–Big Crunch Model

In this article, the field equations of new general relativity, constructed by Hayashi and Shirafuji, contain three free parameters. These field equations have been applied to the Friedmann–Robertson–Walker metric in the domain of cosmology. In the application, a family of models, involving two of the parameters characterizing the field equations of new general relativity, is obtained. Conditions are placed on these parameters in order for the model to be compatible with the Big Rip or Big Crunch models. These models refer to original relativistic models of relativity theory if the parameters characterizing the field equations are equal to unity. The exact solutions are obtained under a specific choice of the parameters characterizing the field equations and the quadratic deceleration parameter. Radiation, dust, dark energy, vacuum, and phantom universes are obtained from field equations, and these models are not affected by the field parameters. Energy conditions, as well as the effective potential of the proposed models, are discussed.

BIANCHI TYPE V MAGNETIZED STRING DUST BULK VISCOUS FLUID COSMOLOGICAL MODELS WITH VARIABLE DECELERATION PARAMETER

We have investigated Bianchi type ∨ viscous fluid cosmological models with string dust universe in general relativity. Exact solution of Einstein eld equations have been obtained by choosing deceleration parameter is function of cosmic time. Solutions for exponential and polynomial form are obtained. Some geometrical and physical aspects of models are also discussed.

Phenomenological dark energy model with hybrid dynamic cosmological constant

We investigate dark energy (DE) by associating it with vacuum energy or cosmological constant Λ which is taken to be dynamic in nature. Our approach is phenomenological and falls within the domain of variable-Λ cosmology. However, motivated by quantum theory of metastable vacuum decay, we proposed a new phenomenological decay law of Λ(t) where Λ(t) is a superposition of constant and variable components viz Λ(t) = ΛC + Λv which is indicated by the word ‘hybrid dynamic’ in the title. By taking a simplified two-fluid scenario with the Universe consisting of DE and another major component, we found the solutions for three particular phenomenological expressions and made a parametrization of the model in terms of dilution parameter (the dilution parameter has been defined in the text as the exponent of scale factor in the expression of density of the other major component, representing the dilution of the component with the expansion of Universe in the presence of dynamic DE). For pressureless dust and dynamic DE Universe, we found the present day matter density (Ω m0) and dilution parameter (u) to be Ω m0 = 0.29 ± 0.03, u = 2.90 ± 0.54 at 1σ by analysing 580 supernova from Union 2.1 catalogue. The physical features of the model in regard to scale factor evolution, deceleration parameter, cosmic age has also been studied and parallels have been drawn with ΛCDM model. The status of cosmological problems in the model has also been checked which showed that the model solves the cosmological constant problem but the coincidence problem still exists in the model.

BiGONLight: light propagation with bilocal operators in numerical relativity

BiGONLight, Bilocal Geodesic Operators framework for Numerical Light propagation, is a new tool for light propagation in numerical relativity. The package implements the bilocal geodesic operators formalism, a new framework for light propagation in general relativity. With BiGONLight it is possible to extract observables such as angular diameter distance, luminosity distance, magnification as well as new real-time observables like parallax and redshift drift within the same computation. As a test-bed for our code we consider two exact cosmological models, the ΛCDM and the inhomogeneous Szekeres model, and a simulated dust Universe. All our tests show an excellent agreement with known results.

Matter perturbations in Einstein dark energy model

We consider the growth rate of matter perturbations in the Einstein dark energy theory. The theory consists of the Einstein–Hilbert Lagrangian plus the trace of the energy momentum tensor, coupled non-minimally to a dynamical vector field. We will show that the theory has three fixed points corresponding to the dust, radiation and de Sitter universes. Due to the present of trace of the energy–momentum tensor in the Lagrangian, the fixed points occurs in different locations compared to Lambda CDM theory. We will analyze the theory with and without cosmological constant. We will fit the model parameters using two independent data sets corresponding to the Hubble parameter H and also fsigma _8. The theory is then shown to be consistent with observational data.

Study of cosmic models in f(R,T) gravity with tilted observers

In this work, we have studied LRS Bianchi type I cosmological models in [Formula: see text] gravity with tilted observers, where [Formula: see text] is the Ricci scalar and [Formula: see text] is the trace of the stress energy tensor. We have explored a tilted model and determined the solutions of the field equations by assuming special law of variation of Hubble’s parameter, proposed by Berman (1983) that yields constant deceleration parameter. In this scenario, we have used the equation of state [Formula: see text] and power law of velocity to describe the different anisotropic physical models such as Dust Universe, Radiation Universe, Hard Universe and Zedovich Universe. We have discussed graphical presentation of all parameters of the derived models with the help of MATLAB. Some physical and geometrical aspects of the models are also discussed.

Growth of matter fluctuations in [formula omitted] gravity

In this work, I present for the first time the analysis concerning the growth of matter fluctuations in the framework of f(R,T) modified gravity where I presume f(R,T)=R+λT, where R denote the Ricci scalar, T the trace of energy momentum tensor and λ a constant. I first solve the Friedman equations assuming a dust universe (ω=0) for the Hubble parameter H(z) and then employ it in the equation of matter density fluctuations δ(z) to solve for δ(z) and the growth rate f(z). Next, I proceed to show the behavior of f(z) and δ(z) with redshift for some values of λ with observational constraints. Finally, following the prescription of [31], I present an analytical expression for the growth index γ which is redshift dependent and the expression reduces to 3/5 for λ=0, which is the growth index for a dust universe.

Dynamics of some cosmological solutions in modified f(R) gravity

This paper is devoted to investigate the modified f(R) theory of gravity, where R represents the Ricci scalar respectively. For our current work, we consider the Friedmann-Robertson-Walker (FRW) space-time for finding solutions of field equations. Furthermore, some numerical solutions are examined by taking the Klein-Gordon Equation and using distinct values of the equation of state (EoS) parameter. In this way, we have discussed the solutions for acceleration expansion of the Universe, sub-relativistic Universe, radiation Universe, ultra-relativistic Universe, dust Universe, and stiff fluid Universe respectively. Moreover, their behaviours are examined by using power-law and exponential law techniques. The bouncing scenario is also discussed by choosing some particular values of the model parameters and observed the energy conditions, which are satisfied for a successful bouncing model. It is also concluded that some solution in f(R) theory of gravity supports the concept of exotic matter and accelerated expansion of the Universe due to a large amount of negative pressure.

Big bang nucleosynthesis and entropy evolution in f(R, T) gravity

The present article is devoted to constrain the model parameter $\chi$ for the $f(R,T)= R + \chi T$ gravity model by employing the constraints coming from big bang nucleosynthesis. We solve the field equations and constrain $\chi$ in the range $-0.14 \kappa^{2} \leq \chi \leq 0.84 \kappa^{2}$ (where $\kappa^{2} = \frac{8 \pi G}{c^{4}}$) from the primordial abundances of light elements such as helium-4, deuterium and lithium-7. We found the abundances of helium-4 and deuterium agrees with theoretical predictions, however the lithium problem persists for the $f(R,T)$ gravity model. We also investigate the evolution of entropy for the constrained parameter space of $\chi$ for the radiation and dust universe. We report that entropy is constant when $\chi = 0$ for the radiation dominated universe, whereas for the dust universe, entropy increases with time. We finally use the constraints to show that $\chi$ has negligible influence on the cold dark matter annihilation cross section.

Temporally varying universal gravitational “constant” and speed of light in energy momentum squared gravity

Energy Momentum Squared Gravity (EMSG) is a cosmological model where the scale factor is non vanishing at all times and hence does not favor big bang cosmology. However, the profile of density in the radiation dominated universe shows that EMSG supports inflationary cosmology. Inflationary cosmological models are successful in providing convincing answers to major cosmological issues like horizon problem, flatness problem and small value of cosmological constant but hitherto no model of inflation has been observationally confirmed. Owing to this, Varying Speed of Light (VSL) were introduced which are a class of cosmological models which disfavor inflation and propose an alternative route to solve these cosmological issues by just allowing the speed of light (and Newtonian Gravitational constant) to vary. VSL theories were motivated to address the shortcomings of inflation but do not address the shortcomings related to the initial big bang singularity. In this spirit, we present here a novel cosmological model which is free from both the "initial big bang singularity" and "inflation" by incorporating a mutually varying speed of light $c(t)$ and Newtonian gravitational constant $G(t)$ in the framework of EMSG. We report that in EMSG, for a dust universe ($\omega=0$), cosmological models for a time varying $c(t)$ and $G(t)$ and constant $c$ and $G$ are indistinguishable, whereas for a radiation dominated universe ($\omega = 1/3$), a mutually varying $c(t)$ and $G(t)$ provides an exiting alternative to inflationary cosmology which is also free from initial big bang singularity. We further report that for an ansatz of scale factor representing a bouncing cosmological model, the VSL theory can be applied to a quadratic $T$ gravity model to get rid of "inflation" and "big bang singularity" and concurrently solve the above mentioned cosmological enigmas.

Does spatial flatness forbid the turnaround epoch of collapsing structures?

Cosmological observational analysis frequently assumes that the Universe is spatially flat. We aim to non-perturbatively check the conditions under which a flat or nearly flat expanding dust universe, including the Λ-cold-dark-matter (ΛCDM) model if interpreted as strictly flat, forbids the gravitational collapse of structure. We quantify spatial curvature at turnaround. We use the Hamiltonian constraint to determine the pointwise conditions required for an overdensity to reach its turnaround epoch in an exactly flat spatial domain. We illustrate this with a plane-symmetric, exact, cosmological solution of the Einstein equation, extending earlier work. More generally, for a standard initial power spectrum, we use the relativistic Zel'dovich approximation implemented in INHOMOG to numerically estimate how much positive spatial curvature is required to allow turnaround at typical epochs/length scales in almost-Einstein-de Sitter (EdS)/ΛCDM models with inhomogeneous curvature. We find that gravitational collapse in a spatially exactly flat, irrotational, expanding, dust universe is relativistically forbidden pointwise. In the spatially flat plane-symmetric model considered here, pancake collapse is excluded both pointwise and in averaged domains. In an almost-EdS/ΛCDM model, the per-domain average curvature in collapsing domains almost always becomes strongly positive prior to turnaround, with the expansion-normalised curvature functional reaching Ωℛ\U0001d49f∼−5. We show analytically that a special case gives Ωℛ\U0001d49f=−5 exactly (if normalised using the EdS expansion rate) at turnaround. An interpretation of ΛCDM as literally 3-Ricci flat would forbid structure formation. The difference between relativistic cosmology and a strictly flat ΛCDM model is fundamental in principle, but we find that the geometrical effect is weak.

Investigating cosmology with equation of state

The aim of this paper is to investigate the field equations of modified [Formula: see text] theory of gravity, where R and [Formula: see text] represent the Ricci scalar and scalar potential, respectively. We consider the Friedmann–Robertson–Walker space–time for finding some exact solutions by using different values of equation of state parameter. In this regard, different possibilities of the exact solutions have been discussed for dust universe, radiation universe, ultra-relativistic universe, sub-relativistic universe, stiff universe, and dark energy universe. Mainly power law and exponential forms of the scale factor are chosen for the analysis.

Signatures of primordial gravitational waves in matter power spectrum

We simulate the evolution of a dust universe from z=1089 to z=0 by numerically integrating the Einstein’s equation for a spatially flat Friedmann–Lemaire–Robertson–Walker (FLRW) background spacetime with scalar perturbations which are derived from the matter power spectrum produced with the Code for Anisotropies in the Microwave Background (CAMB). To investigate the effects of primordial gravitational waves (GWs) on the inhomogeneity of the universe, we add an additional decaying, divergenceless and traceless primordial tensor perturbation with its initial amplitude being 3times 10^{-4} to the above metric. We find that this primordial tensor perturbation suppresses the matter power spectrum by about 0.01% at z=0 for modes with wave number similar to its. This suppression may be a possible probe of a GWs background in the future.

Tachyon with an inverse power-law potential in a braneworld cosmology

We study a tachyon cosmological model based on the dynamics of a 3-brane in the bulk of the second Randall–Sundrum model extended to more general warp functions. A well known prototype of such a generalization is the bulk with a selfinteracting scalar field. As a consequence of a generalized bulk geometry the cosmology on the observer brane is modified by the scale dependent four-dimensional gravitational constant. In particular, we study a power law warp factor which generates an inverse power-law potential of the tachyon field φ. We find a critical power ncr that divides two subclasses with distinct asymptotic behaviors: a dust universe for and a quasi de Sitter universe for .

Accelerating anisotropic cosmological model in f(R,T) theory of gravity

Field equations in a modified theory of gravitation proposed by Harko et al. (Phys Rev D 84:024020, 8) are obtained with the aid of a spatially homogeneous and anisotropic LRS Bianchi type-I metric. Cosmological models corresponding to false vacuum and dust universe are obtained. Some physical and kinematical properties of each of the models are discussed. These models may be physically significant for discussion at an early stage of evolution of the universe.

Cosmic acceleration as an optical illusion

We consider light propagation in an inhomogeneous irrotational dust universe with vanishing cosmological constant, with initial conditions as in standard linear perturbation theory. A non-perturbative approach to the dynamics of such a universe is combined with a distance formula based on the Sachs optical equations. Then a numerical study implies a redshift–distance relation that roughly agrees with observations. Interpreted in the standard homogeneous setup, our results would appear to imply the currently accepted values for the Hubble rate and the deceleration parameter; furthermore there is consistency with density perturbations at last scattering. The determination of these three quantities relies only on a single parameter related to a cutoff scale. Discrepancies with the existing literature are related to subtleties of higher order perturbation theory which make both the reliability of the present approach and the magnitude of perturbative effects beyond second order hard to assess.

Vector dark energy models with quadratic terms in the Maxwell tensor derivatives

We consider a vector–tensor gravitational model with terms quadratic in the Maxwell tensor derivatives, called the Bopp–Podolsky term. The gravitational field equations of the model and the equations describing the evolution of the vector field are obtained and their Newtonian limit is investigated. The cosmological implications of a Bopp–Podolsky type dark energy term are investigated for a Bianchi type I homogeneous and anisotropic geometry for two models, corresponding to the absence and presence of the self-interacting potential of the field, respectively. The time evolutions of the Hubble function, of the matter energy density, of the shear scalar, of the mean anisotropy parameter, and of the deceleration parameter, respectively, as well as the field potentials are obtained for both cases by numerically integrating the cosmological evolution equations. In the presence of the vector type dark energy with quadratic terms in the Maxwell tensor derivatives, depending on the numerical values of the model parameters, the Bianchi type I Universe experiences a complex dynamical evolution, with the dust Universes ending in an isotropic phase. The presence of the self-interacting potential of the vector field significantly shortens the time interval necessary for the full isotropization of the Universe.

Inverse construction of the ΛLTB model from a distance-redshift relation

Spherically symmetric dust universe models with a positive cosmological constant Λ, known as Λ-Lemaȋtre-Tolman-Bondi (ΛLTB) models, are considered. We report a method to construct the ΛLTB model from a given distance-redshift relation observed at the symmetry center. The spherical inhomogeneity is assumed to be composed of growing modes. We derive a set of ordinary differential equations for three functions of the redshift, which specify the spherical inhomogeneity. Once a distance-redshift relation is given, with careful treatment of possible singular points, we can uniquely determine the model by solving the differential equations for each value of Λ. As a demonstration, we fix the distance-redshift relation as that of the flat ΛCDM model with (Ωdism0, ΩdisΛ0)=(0.3,0.7), where Ωdism0 and ΩdisΛ0 are the normalized matter density and the cosmological constant, respectively. Then, we construct the ΛLTB model for several values of ΩΛ0:=Λ/(3H02), where H0 is the present Hubble parameter observed at the symmetry center. We obtain void (over dense) structure around the symmetry center for ΩΛ0 < ΩdisΛ0(ΩΛ0 > ΩdisΛ0). We show the relation between the ratio ΩΛ0/ΩdisΛ0 and the amplitude of the inhomogeneity.