38 research outputs found
Covariant conservation of energy momentum in modified gravities
An explicit proof of the vanishing of the covariant divergence of the
energy-momentum tensor in modified theories of gravity is presented. The
gravitational action is written in arbitrary dimensions and allowed to depend
nonlinearly on the curvature scalar and its couplings with a scalar field. Also
the case of a function of the curvature scalar multiplying a matter Lagrangian
is considered. The proof is given both in the metric and in the first-order
formalism, i.e. under the Palatini variational principle. It is found that the
covariant conservation of energy-momentum is built-in to the field equations.
This crucial result, called the generalized Bianchi identity, can also be
deduced directly from the covariance of the extended gravitational action.
Furthermore, we demonstrate that in all of these cases, the freely falling
world lines are determined by the field equations alone and turn out to be the
geodesics associated with the metric compatible connection. The independent
connection in the Palatini formulation of these generalized theories does not
have a similar direct physical interpretation. However, in the conformal
Einstein frame a certain bi-metricity emerges into the structure of these
theories. In the light of our interpretation of the independent connection as
an auxiliary variable we can also reconsider some criticisms of the Palatini
formulation originally raised by Buchdahl.Comment: 8 pages. v2: more discussio
Primordial statistical anisotropy generated at the end of inflation
We present a new mechanism for generating primordial statistical anisotropy
of curvature perturbations. We introduce a vector field which has a non-minimal
kinetic term and couples with a waterfall field in hybrid inflation model. In
such a system, the vector field gives fluctuations of the end of inflation and
hence induces a subcomponent of curvature perturbations. Since the vector has a
preferred direction, the statistical anisotropy could appear in the
fluctuations. We present the explicit formula for the statistical anisotropy in
the primordial power spectrum and the bispectrum of curvature perturbations.
Interestingly, there is the possibility that the statistical anisotropy does
not appear in the power spectrum but does appear in the bispectrum. We also
find that the statistical anisotropy provides the shape dependence to the
bispectrum.Comment: 9 pages, This version supersedes the JCAP version. Minor revision
Constraints on Gauss-Bonnet Gravity in Dark Energy Cosmologies
Models with a scalar field coupled to the Gauss-Bonnet Lagrangian appear
naturally from Kaluza-Klein compactifications of pure higher-dimensional
gravity. We study linear, cosmological perturbations in the limits of weak
coupling and slow-roll, and derive simple expressions for the main observable
sub-horizon quantities: the anisotropic stress factor, the time-dependent
gravitational constant, and the matter perturbation growth factor. Using
present observational data, and assuming slow-roll for the dark energy field,
we find that the fraction of energy density associated with the coupled
Gauss-Bonnet term cannot exceed 15%. The bound should be treated with caution,
as there are significant uncertainies in the data used to obtain it. Even so,
it indicates that the future prospects for constraining the coupled
Gauss-Bonnet term with cosmological observations are encouraging.Comment: 15 pages. v3: extended analysis, conclusions change
Initial Conditions for Vector Inflation
Recently, a model of inflation using non-minimally coupled massive vector
fields has been proposed. For a particular choice of non-minimal coupling
parameter and for a flat FRW model, the model is reduced to the model of
chaotic inflation with massive scalar field. We study the effect of non-zero
curvature of the universe on the onset of vector inflation. We find that in a
curved universe the dynamics of vector inflation can be different from chaotic
inflation, and the fraction of the initial conditions leading to inflationary
solutions is reduced compared with the chaotic inflation case.Comment: 12 pages, 5 figures, version to be published in JCA
The present universe in the Einstein frame, metric-affine R+1/R gravity
We study the present, flat isotropic universe in 1/R-modified gravity. We use
the Palatini (metric-affine) variational principle and the Einstein
(metric-compatible connected) conformal frame. We show that the energy density
scaling deviates from the usual scaling for nonrelativistic matter, and the
largest deviation occurs in the present epoch. We find that the current
deceleration parameter derived from the apparent matter density parameter is
consistent with observations. There is also a small overlap between the
predicted and observed values for the redshift derivative of the deceleration
parameter. The predicted redshift of the deceleration-to-acceleration
transition agrees with that in the \Lambda-CDM model but it is larger than the
value estimated from SNIa observations.Comment: 11 pages; published versio
On compatibility of string effective action with an accelerating universe
In this paper, we fully investigate the cosmological effects of the moduli
dependent one-loop corrections to the gravitational couplings of the string
effective action to explain the cosmic acceleration problem in early (and/or
late) universe. These corrections comprise a Gauss-Bonnet (GB) invariant
multiplied by universal non-trivial functions of the common modulus
and the dilaton . The model exhibits several features of cosmological
interest, including the transition between deceleration and acceleration
phases. By considering some phenomenologically motivated ansatzs for one of the
scalars and/or the scale factor (of the universe), we also construct a number
of interesting inflationary potentials. In all examples under consideration, we
find that the model leads only to a standard inflation () when the
numerical coefficient associated with modulus-GB coupling is positive,
while the model can lead also to a non-standard inflation (), if
is negative. In the absence of (or trivial) coupling between the GB term and
the scalars, there is no crossing between the phases, while
this is possible with non-trivial GB couplings, even for constant dilaton phase
of the standard picture. Within our model, after a sufficient amount of e-folds
of expansion, the rolling of both fields and can be small. In
turn, any possible violation of equivalence principle or deviations from the
standard general relativity may be small enough to easily satisfy all
astrophysical and cosmological constraints.Comment: 30 pages, 8 figures; v2 significant changes in notations, appendix
and refs added; v3 significant revisions, refs added; v4 appendix extended,
new refs, published versio
On cosmic inflation in vector field theories
We investigate the longitudinal ghost issue in Abelian vector inflation. It
turns out that, within the class of Lorentz-invariant vector field theories
with three degrees of freedom and without any extra (scalar) fields, the
possibilities are essentially exhausted by the classical solution due to Larry
Ford with an extremely flat potential which doesn't feel the fast roll of its
argument. And, moreover, one needs to fulfil an extra condition on that
potential in order to avoid severe gradient instability. At the same time, some
Lorentz-violating modifications are worth to be explored.Comment: 10 pages; a few minor typos corrected; published versio