463 research outputs found
Cosmological perturbations in a family of deformations of general relativity
We study linear cosmological perturbations in a previously introduced family
of deformations of general relativity characterized by the absence of new
degrees of freedom. The homogeneous and isotropic background in this class of
theories is unmodified and is described by the usual Friedmann equations. The
theory of cosmological perturbations is modified and the relevant deformation
parameter has the dimension of length. Gravitational perturbations of the
scalar type can be described by a certain relativistic potential related to the
matter perturbations just as in general relativity. A system of differential
equations describing the evolution of this potential and of the stress-energy
density perturbations is obtained. We find that the evolution of scalar
perturbations proceeds with a modified effective time-dependent speed of sound,
which, contrary to the case of general relativity, does not vanish even at the
matter-dominated stage. In a broad range of values of the length parameter
controlling the deformation, a specific transition from the regime of modified
gravity to the regime of general relativity in the evolution of scalar
perturbations takes place during the radiation domination. In this case, the
resulting power spectrum of perturbations in radiation and dark matter is
suppressed on the comoving spatial scales that enter the Hubble radius before
this transition. We estimate the bounds on the deformation parameter for which
this suppression does not lead to observable consequences. Evolution of scalar
perturbations at the inflationary stage is modified but very slightly and the
primordial spectrum generated during inflation is not noticeably different from
the one obtained in general relativity.Comment: 45 pages, version published in JCAP; minor changes, one section moved
to the appendi
The nearly Newtonian regime in Non-Linear Theories of Gravity
The present paper reconsiders the Newtonian limit of models of modified
gravity including higher order terms in the scalar curvature in the
gravitational action. This was studied using the Palatini variational principle
in [Meng X. and Wang P.: Gen. Rel. Grav. {\bf 36}, 1947 (2004)] and
[Dom\'inguez A. E. and Barraco D. E.: Phys. Rev. D {\bf 70}, 043505 (2004)]
with contradicting results. Here a different approach is used, and problems in
the previous attempts are pointed out. It is shown that models with negative
powers of the scalar curvature, like the ones used to explain the present
accelerated expansion, as well as their generalization which include positive
powers, can give the correct Newtonian limit, as long as the coefficients of
these powers are reasonably small. Some consequences of the performed analysis
seem to raise doubts for the way the Newtonian limit was derived in the purely
metric approach of fourth order gravity [Dick R.: Gen. Rel. Grav. {\bf 36}, 217
(2004)]. Finally, we comment on a recent paper [Olmo G. J.: Phys. Rev. D {\bf
72}, 083505 (2005)] in which the problem of the Newtonian limit of both the
purely metric and the Palatini formalism is discussed, using the equivalent
Brans--Dicke theory, and with which our results partly disagree.Comment: typos corrected, replaced to match published versio
Non-Vacuum Bianchi Types I and V in f(R) Gravity
In a recent paper \cite{1}, we have studied the vacuum solutions of Bianchi
types I and V spacetimes in the framework of metric f(R) gravity. Here we
extend this work to perfect fluid solutions. For this purpose, we take stiff
matter to find energy density and pressure of the universe. In particular, we
find two exact solutions in each case which correspond to two models of the
universe. The first solution gives a singular model while the second solution
provides a non-singular model. The physical behavior of these models has been
discussed using some physical quantities. Also, the function of the Ricci
scalar is evaluated.Comment: 15 pages, accepted for publication in Gen. Realtiv. Gravi
Chaotic scalar fields as models for dark energy
We consider stochastically quantized self-interacting scalar fields as
suitable models to generate dark energy in the universe. Second quantization
effects lead to new and unexpected phenomena is the self interaction strength
is strong. The stochastically quantized dynamics can degenerate to a chaotic
dynamics conjugated to a Bernoulli shift in fictitious time, and the right
amount of vacuum energy density can be generated without fine tuning. It is
numerically observed that the scalar field dynamics distinguishes fundamental
parameters such as the electroweak and strong coupling constants as
corresponding to local minima in the dark energy landscape. Chaotic fields can
offer possible solutions to the cosmological coincidence problem, as well as to
the problem of uniqueness of vacua.Comment: 30 pages, 3 figures. Replaced by final version accepted by Phys. Rev.
Accelerated Cosmological Models in First-Order Non-Linear Gravity
The evidence of the acceleration of universe at present time has lead to
investigate modified theories of gravity and alternative theories of gravity,
which are able to explain acceleration from a theoretical viewpoint without the
need of introducing dark energy. In this paper we study alternative
gravitational theories defined by Lagrangians which depend on general functions
of the Ricci scalar invariant in minimal interaction with matter, in view of
their possible cosmological applications. Structural equations for the
spacetimes described by such theories are solved and the corresponding field
equations are investigated in the Palatini formalism, which prevents
instability problems. Particular examples of these theories are also shown to
provide, under suitable hypotheses, a coherent theoretical explanation of
earlier results concerning the present acceleration of the universe and
cosmological inflation. We suggest moreover a new possible Lagrangian,
depending on the inverse of sinh(R), which gives an explanation to the present
acceleration of the universe.Comment: 23 pages, Revtex4 fil
Dark energy from conformal symmetry breaking
The breakdown of conformal symmetry in a conformally invariant scalar-tensor
gravitational model is revisited in the cosmological context. Although the old
scenario of conformal symmetry breaking in cosmology containing scalar field
has already been used in many earlier works, it seems that no special attention
has been paid for the investigation on the possible connection between the
breakdown of conformal symmetry and the existence of dark energy. In this
paper, it is shown that the old scenario of conformal symmetry breaking in
cosmology, if properly interpreted, not only has a potential ability to
describe the origin of dark energy as a symmetry breaking effect, but also may
resolve the coincidence problem.Comment: 11 pages, minor revision, published online in EPJ
Crossing of the w=-1 Barrier in Two-Fluid Viscous Modified Gravity
Singularities in the dark energy late universe are discussed, under the
assumption that the Lagrangian contains the Einstein term R plus a modified
gravity term of the form R^\alpha, where \alpha is a constant. It is found,
similarly as in the case of pure Einstein gravity [I. Brevik and O. Gorbunova,
Gen. Rel. Grav. 37 (2005), 2039], that the fluid can pass from the quintessence
region (w>-1) into the phantom region (w<-1) as a consequence of a bulk
viscosity varying with time. It becomes necessary now, however, to allow for a
two-fluid model, since the viscosities for the two components vary differently
with time. No scalar fields are needed for the description of the passage
through the phantom barrier.Comment: 16 pages latex, no figure
Phantom Field with O(N) Symmetry in Exponential Potential
In this paper, we study the phase space of phantom model with O(\emph{N})
symmetry in exponential potential. Different from the model without O(\emph{N})
symmetry, the introduction of the symmetry leads to a lower bound on the
equation of state for the existence of stable phantom dominated attractor
phase. The reconstruction relation between the potential of O(\textit{N})
phantom system and red shift has been derived.Comment: 5 pages, 3 figures, replaced with the version to appear on Phys. Rev.
Generalized Second Law of Thermodynamics on the Event Horizon for Interacting Dark Energy
Here we are trying to find the conditions for the validity of the generalized
second law of thermodynamics (GSLT) assuming the first law of thermodynamics on
the event horizon in both cases when the FRW universe is filled with
interacting two fluid system- one in the form of cold dark matter and the other
is either holographic dark energy or new age graphic dark energy. Using the
recent observational data we have found that GSLT holds both in quintessence
era as well as in phantom era for new age graphic model while for holographic
dark energy GSLT is valid only in phantom era.Comment: 8 pages, 2 figure
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