231 research outputs found
Nonminimal Scalar-Tensor Theories and Quantum Gravity
Recentely, it is shown that the quantum effects of matter determine the
conformal degree of freedom of the space-time metric. This was done in the
framework of a scalar-tensor theory with one scalar field. A point with that
theory is that the form of quantum potential is preassumed. Here we present a
scalar-tensor theory with two scalar fields, and no assumption on the form of
quantum potential. It is shown that using the equations of motion one gets the
correct form of quantum potential plus some corrections.Comment: 15 page
Black hole solutions in the warped DGP braneworld
We study the static, analytical solution of black holes in the warped DGP
braneworld scenario. We show that the linearized field equations and matching
conditions lead to solutions that are not compatible with
Schwarzschild-(A)dS solutions on the brane. This incompatibility is
similar to vDVZ discontinuity in massive gravity theory. Following the standard
procedure to remove this discontinuity, which firstly was proposed by
Vainshtein, we keep some appropriate nonlinear terms in the field equations.
This strategy has its origin in the fact that the spatial extrinsic curvature
of the brane plays a crucial role in the nonlinear nature of the solutions and
also in recovering the well-measured predictions of General Relativity (GR) at
small scales. Using this feature, we obtained an interesting black string
solution in the bulk when it is compatible with 4D GR solutions on the brane.Comment: 15 pages, no figure
Universe Reheating after Inflation
We study the problem of scalar particle production after inflation by a
rapidly oscillating inflaton field. We use the framework of the chaotic
inflation scenario with quartic and quadratic inflaton potentials. Particular
attention is paid to parametric resonance phenomena which take place in the
presence of the quickly oscillating inflaton field. We have found that in the
region of applicability of perturbation theory the effects of parametric
resonance are crucial, and estimates based on first order Born approximation
often underestimate the particle production. In the case of the quartic
inflaton potential , the particle production
process is very efficient even for small values of coupling constants. The
reheating temperature of the universe in this case is times larger than the corresponding estimates based
on first order Born approximation. In the case of the quadratic inflaton
potential the reheating process depends crucially on the type of coupling
between the inflaton and the other scalar field and on the magnitudes of the
coupling constants. If the inflaton coupling to fermions and its linear (in
inflaton field) coupling to scalar fields are suppressed, then, as previously
discussed by Kofman, Linde and Starobinsky (see e.g. Ref. 13), the inflaton
field will eventually decouple from the rest of the matter, and the residual
inflaton oscillations may provide the (cold) dark matter of the universe. In
the case of the quadratic inflaton potential we obtain the lowest and the
highest possible bounds on the effective energy density of the inflaton field
when it freezes out.Comment: 40 pages, Preprint BROWN-HET-957 (revised version, some mistakes
corrected), uses phyzz
Quadratic reheating
The reheating process for the inflationary scenario is investigated
phenomenologically. The decay of the oscillating massive inflaton field into
light bosons is modeled after an out of equilibrium mixture of interacting
fluids within the framework of irreversible thermodynamics. Self-consistent,
analytic results for the evolution of the main macroscopic magnitudes like
temperature and particle number densities are obtained. The models for linear
and quadratic decay rates are investigated in the quasiperfect regime. The
linear model is shown to reheat very slowly while the quadratic one is shown to
yield explosive particle and entropy production. The maximum reheating
temperature is reached much faster and its magnitude is comparable with the
inflaton mass.Comment: 21 pages, LaTeX 2.09, 4 figures. To be published in International
Journal of Modern Physics
Arrow of time in dissipationless cosmology
© 2015 IOP Publishing Ltd. It is generally believed that a cosmological arrow of time must be associated with entropy production. Indeed, in his seminal work on cyclic cosmology, Tolman introduced a viscous fluid in order to make successive expansion/contraction cycles larger than previous ones, thereby generating an arrow of time. However, as we demonstrate in this letter, the production of entropy is not the only means by which a cosmological arrow of time may emerge. Remarkably, systems which are dissipationless may nevertheless demonstrate a preferred direction of time provided they possess attractors. An example of a system with well defined attractors is scalar-field driven cosmology. In this case, for a wide class of potentials (especially those responsible for inflation), the attractor equation of state during expansion can have the form Ï â -Ï, and during contraction Ï â Ï. If the resulting cosmology is cyclic, then the presence of cosmological hysteresis, Ï pdV â 0 during successive cycles, causes an arrow of time to emerge in a system which is formally dissipationless. An important analogy is drawn between the arrow of time in cyclic cosmology and an arrow of time in an N-body system of gravitationally interacting particles. We find that, like the N-body system, a cyclic Universe can evolve from a single past into two futures with oppositely directed arrows of time
Primordial Black Hole Formation from Inflaton
Measurements of the distances to SNe Ia have produced strong evidence that
the Universe is really accelarating, implying the existence of a nearly uniform
component of dark energy with the simplest explanation as a cosmological
constant. In this paper a small changing cosmological term is proposed, which
is a function of a slow-rolling scalar field, by which the de Sitter primordial
black holes' properties, for both charged and uncharged cases, are carefully
examined and the relationship between the black hole formation and the energy
transfer of the inflaton within this cosmological term is eluciatedComment: 6 pages, Late
Induced cosmological constant and other features of asymmetric brane embedding
We investigate the cosmological properties of an "induced gravity" brane
scenario in the absence of mirror symmetry with respect to the brane. We find
that brane evolution can proceed along one of four distinct branches. By
contrast, when mirror symmetry is imposed, only two branches exist, one of
which represents the self-accelerating brane, while the other is the so-called
normal branch. This model incorporates many of the well-known possibilities of
brane cosmology including phantom acceleration (w < -1), self-acceleration,
transient acceleration, quiescent singularities, and cosmic mimicry.
Significantly, the absence of mirror symmetry also provides an interesting way
of inducing a sufficiently small cosmological constant on the brane. A small
(positive) Lambda-term in this case is induced by a small asymmetry in the
values of bulk fundamental constants on the two sides of the brane.Comment: 17 pages, 4 figures. New results and two figures discussing transient
acceleration are included. Version accepted for publication in JCA
On generation of metric perturbations during preheating
We consider the generation of the scalar mode of the metric perturbations
during preheating stage in a two field model with the potential . We discuss two possible
sources of such perturbations: a) due to the coupling between the perturbation
of the matter field and the background part of the matter field
, b) due to non-linear fluctuations in a condensate of
``particles'' of the field . Both types of the metric perturbations are
assumed to be small, and estimated using the linear theory of the metric
perturbations. We estimate analytically the upper limit of the amplitude of the
metric perturbations for all scales in the limit of so-called broad resonance,
and show that the large scale metric perturbations are very small, and taking
them into account does not influence the standard picture of the production of
the metric perturbations in inflationary scenario.Comment: This version is to be published in PRD, new references added and
typos correcte
Excitation of a Kaluza-Klein mode by parametric resonance
In this paper we investigate a parametric resonance phenomenon of a
Kaluza-Klein mode in a -dimensional generalized Kaluza-Klein theory. As the
origin of the parametric resonance we consider a small oscillation of a scale
of the compactification around a today's value of it. To make our arguments
definite and for simplicity we consider two classes of models of the
compactification: those by () and those by (, ). For these models we show that
parametric resonance can occur for the Kaluza-Klein mode. After that, we give
formulas of a creation rate and a number of created quanta of the Kaluza-Klein
mode due to the parametric resonance, taking into account the first and the
second resonance band. By using the formulas we calculate those quantities for
each model of the compactification. Finally we give conditions for the
parametric resonance to be efficient and discuss cosmological implications.Comment: 36 pages, Latex file, Accepted for publication in Physical Review
Reheating and thermalization in a simple scalar model
We consider a simple model for the Universe reheating, which consists of a
single self--interacting scalar field in Minkowskian space--time. Making use of
the existence of an additional small parameter proportional to the amplitude of
the initial spatially homogeneous field oscillations, we show that the behavior
of the field can be found reliably. We describe the evolution of the system
from the homogeneous oscillations to the moment when thermalization is
completed. We compare our results with the Hartree--Fock approximation and
argue that some properties found for this model may be the common features of
realistic theories.Comment: Some changes in Introduction and Discussion, comparison with the
Hartree--Fock results added. 37 pages, 2 postscript figures attache
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