Emergent universe with interacting fluids and the generalized second law of thermodynamics (original) (raw)

Cosmological fluids with general equations of state

Physics Letters A, 1994

This paper analyzes the behaviour of cosmological fluids with general equations of state. Such an investigation appears to be necessary when scalar fields are considered. In particular, the relationship between the equations of state and families of scalar field potentials is shown, when the fluid is constituted by a scalar field or when it is multicomponent.

Inflationary Cosmology with Two-component Fluid and Thermodynamics

General Relativity and Gravitation, 2000

We present a simple and self-consistent cosmology with a phenomenological model of quantum creation of radiation and matter due to the decay of the cosmological constant Λ. The decay drives a non-isentropic inflationary epoch, which exits smoothly to the radiation-dominated era, without reheating, and then evolves to the dust era. The initial vacuum for radiation and matter is a regular Minkowski vacuum. The created radiation and matter obeys standard thermodynamic laws, and the total entropy produced is consistent with the accepted value. This paper is an extension of the model with the decaying cosmological constant considered in [1]. We compare our model with the quantum field theory approach to creation of particles in curved space.

Multiple fluid theory of cosmic evolution and its thermodynamic analysis

Indian Journal of Physics, 2022

In this paper we have discussed the modified gravity and scalar field DE model specifically DBI essence model with the analysis of thermodynamics during cosmological evolution. We have used the modified gravity with the form (,) = + 2 in our calculations. The basic aim behind this paper is to discuss a theory that unifies modified gravity with DE models including the solutions of some cosmological problems like thermodynamics energy conditions violation problems, finite time future singularity problems, initial singularity problem, Cosmic inflation problem, decelerated expansion problem, graceful exit problem, reheating problem, bouncing nature problem, phase transformation-spontaneous symmetry breaking problem, negative heat capacity paradox problem and obviously the present day universe problems (Continuously decreasing temperature problem, present day DE dominated expansion problem). We have established the viscous effects (both positive and negative viscosity) in our calculation and discussed the negative-positive viscosity by introducing two special type of energy cycles. Finally, we have discussed the stability conditions for universe evolution through cosmic perturbation and resolved the instability problems. We have also shown the state finder trajectories for both with and without viscous fluid on the basis of our calculations to compare our research with the results of other independent DE models.

Generalized equilibrium of cosmological fluids in second-order thermodynamics

Classical and Quantum Gravity, 1999

Combining the second-order entropy flow vector of the causal Israel-Stewart theory with the conformal Killing-vector property of u i /T , where u i is the fourvelocity of the medium and T its equilibrium temperature, we investigate generalized equilibrium states for cosmological fluids with nonconserved particle number. We calculate the corresponding equilibrium particle production rate and show that this quantity is reduced compared with the results of the previously studied first-order theory. Generalized equilibrium for massive particles turns out to be compatible with a dependence ρ ∝ a −2 of the fluid energy density ρ on the scale factor a of the Robertson-Walker metric and may be regarded as a realization of so-called K-matter.

Multifluid cosmology: An illustration of fundamental principles

Physical Review D, 2012

Our current understanding of the Universe depends on the interplay of several distinct matter components, which interact mainly through gravity, and electromagnetic radiation. The nature of the different components, and possible interactions, tends to be based on the notion of coupled perfect fluids (or scalar fields). This approach is somewhat naive, especially if one wants to be able to consider issues involving heat flow, dissipative mechanisms, or Bose-Einstein condensation of dark matter. We argue that a more natural starting point would be the multipurpose variational relativistic multifluid system that has so far mainly been applied to neutron star astrophysics. As an illustration of the fundamental principles involved, we develop the formalism for determining the nonlinear cosmological solutions to the Einstein equations for a general relativistic two-fluid model for a coupled system of matter (nonzero rest mass) and radiation (zero rest mass). The two fluids are allowed to interpenetrate and exhibit a relative flow with respect to each other, implying, in general, an anisotropic Universe. We use initial conditions such that the massless fluid flux dominates early on so that the situation is effectively that of a single-fluid and one has the usual Friedmann-Lemaître-Robertson-Walker spacetime. We find that there is a Bianchi I transition epoch out of which the matter flux dominates. The situation is then effectively that of a single fluid and the spacetime evolves towards the Friedmann-Lemaître-Robertson-Walker form. Such a transition opens up the possibility of imprinting observable consequences at the specific scale corresponding to the transition time.

A transient phase in cosmological evolution : A multi-fluid approximation for a quasi-thermodynamics equilibrium

2019

In this article, we examine the dynamics of a multi-fluid system in which (i) different fluids interact, and (ii) there exists a limit in which the multiple fluids evolve into a mixture that satisfies single-fluid approximation. We consider the potential application of this modelling to studies of a cosmological transition which is marked by one material domination replaced by another. The thermodynamical implications of such fluid dynamics are explored where we find that the second law of thermodynamics holds given an emergent Rindler horizon. We argue that such transient periods exist in cosmological evolutions and are best modelled using multi-fluid approximation. Our application of the modelling to an interacting dark-sector leads to the modification of the equation of state of a third non-interacting constituent in a 3-species multi-fluid system

Perfect fluid cosmological Universes: One equation of state and the most general solution

Pramana, 2018

Considering a homogeneous and isotropic universe characterized by the Friedmann-Lemaître-Robertson-Walker (FLRW) line element, in this work, we have prescribed a general formalism for the cosmological solutions when the equation of state of the cosmic substance follows a general structure φ(p, ρ) = 0, where p, ρ are respectively the pressure and the energy density of the cosmic substance. Using the general formalism we recover some well known solutions, namely, when the cosmic substance obeys the linear equation of state, a Chaplygin type equation of state, or a nonlinear equation of state. Thus, the current work offers a new technique to solve the cosmological solutions without any prior relation between p and ρ.

Fluids in Cosmology

Environmental Science and Engineering, 2014

We review the role of fluids in cosmology by first introducing them in General Relativity and then by applying them to a FRW Universe's model. We describe how relativistic and non-relativistic components evolve in the background dynamics. We also introduce scalar fields to show that they are able to yield an inflationary dynamics at very early times (inflation) and late times (quintessence). Then, we proceed to study the thermodynamical properties of the fluids and, lastly, its perturbed kinematics. We make emphasis in the constrictions of parameters by recent cosmological probes.

Cosmological Two-Fluid Thermodynamics

General Relativity and Gravitation, 2001

We reveal unifying thermodynamic aspects of so different phenomena as the cosmological electron-positron annihilation, the evaporation of primordial black holes with a narrow mass range, and the "deflationary" transition from an initial de Sitter phase to a subsequent standard Friedmann-Lemaître-Robertson-Walker (FLRW) behavior.

A transient phase in cosmological evolution : A multi-fluid approximation for a quasi-thermodynamical equilibrium

General Relativity and Gravitation, 2020

This article presents the study of a multi-species fluid characterised by a freeze-out or breakaway of one or more species. Whereas single-fluid approximation suffices for modelling of the pre freeze-out period, multi-fluid approximation is required for the post freeze-out period. Embedded in this is a transition period where neither one of the two approximations is singly appropriate. We examine the thermodynamics of this fluid and find that the second law holds before, during and after the freeze-out. Our application to cosmological modelling involving interacting dark-section species indicates that the species' equations of state are modified.