Pressure-free Matter Research Articles

Kaniadakis agegraphic dark energy

In this manuscript, we present a novel dark energy scenario based on Kaniadakis entropy, which is a generalization of Boltzmann–Gibbs entropy that arises from relativistic statistical theory and is characterized by a parameter K which quantifies the deviations from standard expressions, and we use the age of the Universe as infrared cutoff. The cosmological characteristics of the proposed dark energy model, as well as the evolution of the cosmos filled with pressure-free matter and the ensuing dark energy candidates, are investigated. The interaction as well as non-interaction among the two sectors will also be considered. The differential equation for the dark energy density parameter, including the expression of the equation of state and deceleration parameters, are derived. The analysis of deceleration parameter clearly shows the universe to transit from decelerated to accelerated phase around z≈0.6. The most recent 30 H(z) data samples and the 580 Union 2.1 SNIa data set were also used to limit the theoretical results. The squared sound speed is also plotted against redshift z to check the stability behavior of the model for both the cases.

Comparative Analysis of Standard ΛCDM and ΛCS Models

We draw a comparison of time-dependent cosmological parameters calculated in the standard ΛCDM model with those of the model of a homogeneous and isotropic Universe with non-zero cosmological constant filled with a perfect gas of low-velocity cosmic strings (ΛCS model). It is shown that pressure-free matter can obtain the properties of a gas of low-velocity cosmic strings in the epoch, when the global geometry and the total amount of matter in the Universe as a whole obey an additional constraint. This constraint follows from the quantum geometrodynamical approach in the semiclassical approximation. In terms of general relativity, its effective contribution to the field equations can be linked to the time evolution of the equation of state of matter caused by the processes of redistribution of the energy between matter components. In the present article, the exact solutions of the Einstein equations for the ΛCS model are found. It is demonstrated that this model is equivalent to the open de Sitter model. After the scale transformation of the time variable of the ΛCS model, the standard ΛCDM and ΛCS models provide the equivalent descriptions of cosmological parameters as functions of time at equal values of the cosmological constant. The exception is the behavior of the deceleration parameter in the early Universe.

Power-law cosmology in Weyl-type f(Q, T) gravity

Gravity is attributed to the spacetime curvature in classical general relativity (GR). But, other equivalent formulations or representations of GR, such as torsion or non-metricity, have altered the perception. We consider the Weyl-type f(Q, T) gravity, where Q represents the non-metricity and T is the trace of energy momentum tensor, in which the vector field $$\omega _{\mu }$$ determines the non-metricity $$Q_{\mu \nu \alpha }$$ of the spacetime. In this work, we employ the well-motivated $$f(Q, T)= \alpha Q+ \frac{\beta }{6k^{2}} T$$ , where $$\alpha $$ and $$\beta $$ are the model parameters. Furthermore, we assume that the universe is dominated by the pressure-free matter, i.e., the case of dust ( $$p=0$$ ). We obtain the solution of field equations similar to a power-law in Hubble parameter H(z). We investigate the cosmological implications of the model by constraining the model parameter $$\alpha $$ and $$\beta $$ using the recent 57 points Hubble data and 1048 points Pantheon supernovae data. To study various dark energy models, we use statefinder analysis to address the current cosmic acceleration. We also observe the Om diagnostic describing various phases of the universe. Finally, it is seen that the solution which mimics the power-law fits well with the Pantheon data than the Hubble data.

Szekeres universes with homogeneous scalar fields

We consider the existence of an “inflaton” described by an homogeneous scalar field in the Szekeres cosmological metric. The gravitational field equations are reduced to two families of solutions which describe the homogeneous Kantowski–Sachs spacetime and an inhomogeneous FLRW(-like) spacetime with spatial curvature a constant. The main differences with the original Szekeres spacetimes containing only pressure-free matter are discussed. We investigate the stability of the two families of solution by studying the critical points of the field equations. We find that there exist stable solutions which describe accelerating spatially-flat FLRW geometries.

Quantum field theory in de Sitter and quasi–de Sitter spacetimes revisited

It is possible to associate temperatures with the nonextremal horizons of a large class of spherically symmetric spacetimes using periodicity in the Euclidean sector, and this procedure works for the de Sitter spacetime as well. But unlike, e.g., the black hole spacetimes, the de Sitter spacetime also allows a description in Friedmann coordinates. This raises the question of whether the thermality of the de Sitter horizon can be obtained working entirely in the Friedmann coordinates, without reference to the static coordinates or using the symmetries of de Sitter spacetime. We discuss several aspects of this issue for de Sitter and approximately de Sitter spacetimes in the Friedmann coordinates (with a time-dependent background and the associated ambiguities in defining the vacuum states). The different choices for the vacuum states, the behavior of the mode functions and the detector response are studied in both ($1+1$) and ($1+3$) dimensions. We compare and contrast the differences brought about by the different choices. In the last part of the paper, we also describe a general procedure for studying quantum field theory in spacetimes which are approximately de Sitter and, as an example, derive the corrections to the thermal spectrum due to the presence of pressure-free matter.

Shear-free perturbations of Friedmann-Lemaître-Robertson-Walker universes

A surprising exact result for the Einstein Field Equations is that if pressure-free matter is moving in a shear-free way, then it must be either expansion-free or rotation-free. It has been suggested this result is also true for any barotropic perfect fluid, but a proof has remained elusive. We consider the case of barotropic perfect fluid solutions linearized about a Robertson-Walker geometry, and prove that the result remains true except for the case of a specific highly non-linear equation of state. We argue that this equation of state is non-physical, and hence the result is true in the linearized case for all physically realistic barotropic perfect fluids. This result, which is not true in Newtonian cosmology, demonstrates that the linearized solutions, believed to result in standard local Newtonian theory, do not always give the usual behaviour of Newtonian solutions.

Shear free solutions in general relativity theory

The Goldberg-Sachs theorem is an exact result on shear-free null geodesics in a vacuum spacetime. It is compared and contrasted with an exact result for pressure-free matter: shear-free flows cannot both expand and rotate. In both cases, the shear-free condition restricts the way distant matter can influence the local gravitational field. This leads to intriguing discontinuities in the relation of the General Relativity solutions to Newtonian solutions in the timelike case, and of the full theory to the linearised theory in the null case. It is a pleasure to dedicate this paper to Josh Goldberg.

Relativistic approach to nonlinear peculiar velocities and the Zeldovich approximation

We study the peculiar motion of nonrelativistic matter in a fully covariant way. The exact nonlinear equations are derived and then applied to the case of pressure-free matter, moving relatively to a quasi-Newtonian Eulerian frame. Our two-frame formalism facilitates the study of the nonlinear kinematics of the matter, as the latter decouples from the background expansion and starts to ``turn around'' and collapse. Applied to second perturbative order, our equations provide a fully covariant formulation of the Zeldovich approximation, which by construction addresses the mildly nonlinear regime of structure formation. Employing a dynamical system approach, we show that, just like in the Newtonian case, the relativistic treatment also predicts that pancakes are the natural end structures for any generic overdensity.

Two-Fluid Bianchi Type II Cosmological Models

In this paper we present a class of solutions of Einstein's field equations describing two-fluid models of the universe in a locally rotationally symmetric Bianchi type II space-time. In these models one fluid is the radiation distribution which represents the cosmic microwave background and the other fluid is the perfect fluid representing the matter content of the universe. It is found that both the fluids are comoving in the locally rotationally symmetric Bianchi type II space-time. The behaviour of the radiation density, matter density, the ratio of the matter density to the radiation density and the pressure has been discussed. A subclass of solutions is found to describe models of a spatially homogeneous and partially isotropic universe evolving from a radiation dominated era to a pressure free matter dominated era.

Evolution equations for gravitating ideal fluid bodies in general relativity

We consider the Einstein-Euler equations for a simple ideal fluid in the domain where the speed of sound and the specific enthalpy are positive. Using a Lagrangian description of the fluid flow, we obtain evolution equations which are symmetric hyperbolic. Pressure free matter is discussed as a simple subcase.

Physics of rotating cylindrical strings.

A new gauge-theoretic approach to gravity is applied to the study of rotating cylindrically symmetric strings. The interior and exterior equations are reduced to a simple set of first-order differential equations, and suitable matching conditions are obtained. The gauge theory formulation affords a clear understanding of the physical observables in the theory, and provides simple conditions for the properties of the fields on the string axis. In this context some errors in previously published work are exposed. Three situations are discussed: the vacuum region; pressure-free matter; and a (2+1)-dimensional "ideal fluid." In each case a set of analytic solutions is presented. It is shown that if the fluid is rotating rigidly then closed timelike curves are inevitable, despite the fact that the matter satisfies the weak energy condition.

Vacuum-Energy and the Angular-Size/Redshift Diagram for Milliarcsecond Radio Sources

We have considered two compilations of angular-size/redshift data for ultra-compact radio sources, due to Kellerman (1993) and Gurvits (1994) respectively, and a family of homogeneous isotropic universes with two degrees of freedom, represented by pressure-free matter and vacuum-energy. Kellerman's results seem to support the canonical modelq0= 1/2, ω0= 1; the question posed here is: can we produce significant deceleration without dark matter?

Irrotational dust with a purely magnetic Weyl tensor

Considering irrotational pressure free matter, conditions are obtained under which an orthonormal tetrad simultaneously diagonalizes the shear tensor and the electric part of the Weyl tensor. It is shown that such a frame also diagonalises the magnetic part of the Weyl tensor. Using this principal tetrad we show that there are no consistent solutions for irrotational dust with a purely magnetic Weyl tensor.

Rigidly rotating dust with magnetic pressure in general relativity

Rigidly rotating pressure-free matter interacting with an electromagnetic field is studied under the assumption that in the co-rotating frame the gravitational attraction is balanced by the centrifugal force and by magnetic pressure.

Cosmological models with both radiation and matter

A class of flat, homogeneous and isotropic, relativistic cosmological models containing both radiation and pressure-free matter is investigated.

Kaluza-Klein-Bianchi-Kantowski-Sachs cosmologies

Einstein's field equations are solved for the class of higher-dimensional Bianchi-Kantowski-Sachs space-times of the Kaluza-Klein type. The solutions given are cosmological models filled with pressure-free matter, in general with a non-vanishing cosmological constant.

White holes and high energy astrophysics

The role of exploding objects in high-energy astrophysics is discussed. Quantitative results are obtained for the simple case of the canonical white hole which explodes from a singularity and consists of pressure-free matter in the comoving frame of reference. General relativity is used for calculating the dynamical results. Applications to X-ray background, transient X-ray sources, γ-ray bursts and high energy cosmic rays are considered. White holes of more general types are discussed in a qualitative manner.

New types of singularity in General Relativity: The general cylindrically symmetric stationary dust solution

The general solution of Einstein's equations for a stationary cylindrically symmetric distribution of pressure-free matter is obtained. It contains a function which may be freely prescribed. Using this freedom examples are given of new types of singularity in General Relativity.