Entropy production in inflation from spectator loops

Classical and Quantum Gravity, 1998

We present a simple and thermodynamically consistent cosmology with a phenomenological model of quantum creation of radiation due to vacuum decay. Thermodynamics and Einstein's equations lead to an equation in which H is determined by the particle number N . The model is completed by specifying the particle creation rate Γ =Ṅ /N , which leads to a second-order evolution equation for H. We propose a simple Γ that is naturally defined and that conforms to the thermodynamical conditions: (a) the entropy production rate starts at a maximum; (b) the initial vacuum (for radiation) is a non-singular regular vacuum; and (c) the creation rate is initially higher than the expansion rate H, but then falls below H. The evolution equation for H then has a remarkably simple exact solution, in which a non-adiabatic inflationary era exits smoothly to the radiation era, without a reheating transition. For this solution, we give exact expressions for the cosmic scale factor, energy density of radiation and vacuum, temperature, entropy and super-horizon scalar perturbations.

Gravitation and Cosmology, 2015

We propose a model of cosmological evolution of the early and late Universe which is consistent with observational data and naturally explains the origin of inflation and dark energy (DE). We show that the de Sitter accelerated expansion of the FLRW space with no matter fields (hereinafter "empty space") is its natural state, and the model does not require either a scalar field or cosmological constant or any other hypotheses. Mathematically, this is due to the fact that the de Sitter state is an exact solution of the rigorous, mathematically consistent equations of one-loop quantum gravity for the empty FLRW space that are finite off the mass shell. Physically, this is due to the fact that the natural quantum metric fluctuations have the backreaction effect on the FLRW background, forming a self-polarized de Sitter graviton-ghost condensate which describes the condensation of gravitons on the horizon scale of the nonstationary Universe leading to its exponential expansion. The energy required to maintain the accelerated expansion is drawn from the graviton vacuum. At the start and the end of cosmological evolution, the Universe is assumed to be empty, which means that de Sitter expansion is a natural state of the Universe at the start and end of its cosmological evolution. The emptiness of the Universe at the start of its cosmological evolution automatically generates inflation. With an increase in the energy density of a new-born matter during inflation, the expansion begins to deviate from the de Sitter law, and eventually stops when the energy density of new-born matter reaches the first threshold which is the energy density of instantons. After that, the standard Big Bang cosmology begins. With a decrease in the energy density of matter by the end of the cosmological evolution of the Universe, the expansion first passes through the second threshold which again is the energy density of instantons. Here the quasi-de Sitter expansion (i.e. DE) is born, and then as the Universe empties, approaches the de Sitter law and asymptotically becomes such when the Universe becomes completely empty. This is observed as the DE effect. This scenario seems consistent with observational data. Existence of the first threshold explains the reason why the inflation is stopped. Existence of the second threshold explains why the DE acceleration is happening during the contemporary epoch of matter domination (coincidence problem). The Universe starts and ends with de Sitter expansion but the evolutionary process runs in these cases in opposite directions. It leads to the prediction that the signs of the parameter 1 w + should be opposite in both cases, and this fact is consistent with observations. The fluctuations of the number of gravitons lead to fluctuations of their energy density which in turn leads to the observed CMB temperature anisotropy of the order of 5 10 −  and CMB polarization. In the frame of this scenario, it is not a hypothetical scalar field that generates inflation and relic gravitational waves but on the contrary, the gravitational waves (gravitons) generate DE, inflation, CMB anisotropy and polarization.

Journal of Cosmology and Astroparticle Physics, 2020

At the end of inflation, the inflaton oscillates at the bottom of its potential and these oscillations trigger a parametric instability for scalar fluctuations with wavelength λ comprised in the instability band (3Hm) −1/2 < λ < H −1 , where H is the Hubble parameter and m the curvature of the potential at its minimum. This "metric preheating" instability, which proceeds in the narrow resonance regime, leads to various interesting phenomena such as early structure formation, production of gravitational waves and formation of primordial black holes. In this work we study its fate in the presence of interactions with additional degrees of freedom, in the form of perturbative decay of the inflaton into a perfect fluid. Indeed, in order to ensure a complete transition from inflation to the radiation-dominated era, metric preheating must be considered together with perturbative reheating. We find that the decay of the inflaton does not alter the instability structure until the fluid dominates the universe content. As an application, we discuss the impact of the inflaton decay on the production of primordial black holes from the instability. We stress the difference between scalar field and perfect fluid fluctuations and explain why usual results concerning the formation of primordial black holes from perfect fluid inhomogeneities cannot be used, clarifying some recent statements made in the literature.

Il Nuovo Cimento B

1998

One of the fundamental problems of modern cosmology is to explain the origin of all the matter and radiation in the Universe today. The inflationary model predicts that the oscillations of the scalar field at the end of inflation will convert the coherent energy density of the inflaton into a large number of particles, responsible for the present entropy of the Universe. The transition from the inflationary era to the radiation era was originally called reheating, and we now understand that it may consist of three different stages: preheating, in which the homogeneous inflaton field decays coherently into bosonic waves (scalars and/or vectors) with large occupation numbers; backreaction and rescattering, in which different energy bands get mixed; and finally decoherence and thermalization, in which those waves break up into particles that thermalize and acquire a black body spectrum at a certain temperature. These three stages are non-perturbative, non-linear and out of equilibrium, and we are just beginning to understand them. In this talk I will concentrate on the preheating part, putting emphasis on the differences between preheating in chaotic and in hybrid inflation.

In this study, we use the Friedman-Robertson-Walker (FRW) model with κ = +1 for cyclic universe for understanding the physics of cosmic inflation. According to this model, kinetic energy of the universe is converted into gravitational energy during deceleration (compression) cycle. This builds up a dense assembly of photons close to the singularity of Planck dimensions at temperature T ~ < T PL. Black body radiation approximation is used to analyze the distribution of the photons in the phase space using quantum statistical mechanics. It is shown that the available states in the phase space are completely filled up when the temperature reaches T E. When the temperature increases above T E , the assembly of superheated photons transforms to a non-equilibrium state. We show that an adiabatic transition from the non-equilibrium state to equilibrium state at T E is accompanied by the inflation of space-time and the big bang. The measurements of the red shifts of radiation from very distant galaxies and the measurement of anisotropy in Cosmic Microwave Background using Wilkinson and Planck observatories have confirmed that dark matter and dark energy (or a positive cosmological constant  make contribution to the expansion of the universe. We discuss here the mechanism which builds up large amount of energy E ps 1 in the phase space created during the cosmic inflation. We propose that this energy is the source of the cosmological constant . Other mechanisms which transfer some of the energy released during the inflation to the observable universe are also discussed. The contribution from these sources to  is comparatively small.

2014

Within the framework of the interacting fluid formalism, we provide the numerical solution to the Boltzmann equation describing the evolution of an inflaton field coupled to radiation. We study the behavior of the system during the slow-roll regime, in the case in which an additional stochastic source term is included in the set of equations, and we recover the expression for the cosmological perturbations previously obtained in the Warm inflation scenarios.

Journal of Cosmology and Astroparticle Physics, 2019

Nowadays cosmological inflation is the most accepted mechanism to explain the primordial seeds that led to the structure formation observed in the Universe. Current observations are in well agreement to initial adiabatic conditions, which imply that single-scalar-field inflation may be enough to describe the early Universe. However, there are several scenarios where the existence of more than a single field could be relevant during this period, for instance, the situation where the so-called spectator is present. Within the spectator scenario we can find the possibility that an ultra-light scalar field dark matter candidate could coexist with the inflaton. In this work we study this possibility where the additional scalar field could be free or self-interacting. We use isocurvature observations to constrain the free parameters of the model.

Astrophysics and Space Science, 2007

Though entropy production is forbidden in standard FRW Cosmology, Berman and Som presented a simple inflationary model where entropy production by bulk viscosity, during standard inflation without ad hoc pressure terms can be accommodated with Robertson-Walker's metric, so the requirement that the early Universe be anisotropic is not essential in order to have entropy growth during inflationary phase, as we show. Entropy also grows due to shear viscosity, for the anisotropic case. The intrinsically inflationary metric that we propose can be thought of as defining a polarized vacuum, and leads directly to the desired effects without the need of introducing extra pressure terms.

Physical review letters, 1992

We propose a general definition of nonequilibrium entropy of a classical stochastic field. As an example of particular interest in cosmology we apply this definition to compute the entropy of density perturbations in an inflationary Universe. On the scales of structures in the Universe, the entropy of density perturbations dominates over the statistical fluctuations of the entropy of cosmic microwave photons, indicating the relevance of the entropy of density fluctuations for structure formation.