4,442 research outputs found
Constraining gravity models with disappearing cosmological constant
The gravity models proposed by Hu-Sawicki and Starobinsky are generic
for local gravity constraints to be evaded. The large deviations from these
models either result into violation of local gravity constraints or the
modifications are not distinguishable from cosmological constant. The curvature
singularity in these models is generic but can be avoided provided that proper
fine tuning is imposed on the evolution of scalaron in the high curvature
regime. In principle, the problem can be circumvented by incorporating
quadratic curvature correction in the Lagrangian though it might be quite
challenging to probe the relevant region numerically.Comment: 9 pages and 4 figures, minor clarifications and corrections added,
final version to appear in PR
Galileon gravity and its relevance to late time cosmic acceleration
We consider the covariant galileon gravity taking into account the third
order and fourth order scalar field Lagrangians L_3(\pi) and L_4(\pi)
consisting of three and four 's with four and five derivatives acting on
them respectively. The background dynamical equations are set up for the system
under consideration and the stability of the self accelerating solution is
demonstrated in general setting. We extended this study to the general case of
the fifth order theory. For spherically symmetric static background, we spell
out conditions for suppression of fifth force effects mediated by the galileon
field . We study the field perturbations in the fixed background and
investigate conditions for their causal propagation. We also briefly discuss
metric fluctuations and derive evolution equation for matter perturbations in
galileon gravity.Comment: 11 pages, no figure, minor clarifications and few refs added, to
appear in pr
Observational constraints on braneworld inflation: the effect of a Gauss-Bonnet term
High-energy modifications to general relativity introduce changes to the
perturbations generated during inflation, and the latest high-precision
cosmological data can be used to place constraints on such modified inflation
models. Recently it was shown that Randall-Sundrum type braneworld inflation
leads to tighter constraints on quadratic and quartic potentials than in
general relativity. We investigate how this changes with a Gauss-Bonnet
correction term, which can be motivated by string theory. Randall-Sundrum
models preserve the standard consistency relation between the tensor spectral
index and the tensor-to-scalar ratio. The Gauss-Bonnet term breaks this
relation, and also modifies the dynamics and perturbation amplitudes at high
energies. We find that the Gauss-Bonnet term tends to soften the
Randall-Sundrum constraints. The observational compatibility of the quadratic
potential is strongly improved. For a broad range of energy scales, the quartic
potential is rescued from marginal rejection. Steep inflation driven by an
exponential potential is excluded in the Randall-Sundrum case, but the
Gauss-Bonnet term leads to marginal compatibility for sufficient e-folds.Comment: 10 pages, 10 figures, version to appear in Physical Review
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Avoidance of future singularities in loop quantum cosmology
We consider the fate of future singularities in the effective dynamics of
loop quantum cosmology. Non-perturbative quantum geometric effects which lead
to modification of the Friedmann equation at high energies result in
generic resolution of singularities whenever energy density diverges at
future singularities of Friedmann dynamics. Such quantum effects lead to the
avoidance of a Big Rip, which is followed by a recollapsing universe stable
against perturbations. Resolution of sudden singularity, the case when pressure
diverges but energy density approaches a finite value depends on the ratio of
the latter to a critical energy density of the order of Planck. If the value of
this ratio is greater than unity, the universe escapes the sudden future
singularity and becomes oscillatory.Comment: 6 pages, 2 figure
Proposal for an experiment to search for Randall-Sundrum type corrections to Newton's law of gravitation
String theory, as well as the string inspired brane-world models such as the
Randall-Sundrum (RS) one, suggest a modification of Newton's law of gravitation
at small distance scales. Search for modifications of standard gravity is an
active field of research in this context. It is well known that short range
corrections to gravity would violate the Newton-Birkhoff theorem. Based on
calculations of RS type non-Newtonian forces for finite size spherical bodies,
we propose a torsion balance based experiment to search for the effects of
violation of this celebrated theorem valid in Newtonian gravity as well as the
general theory of relativity. We explain the main principle behind the
experiment and provide detailed calculations suggesting optimum values of the
parameters of the experiment. The projected sensitivity is sufficient to probe
the Randall-Sundrum parameter up to 10 microns.Comment: 4 pages and 5 figures, figures improved, minor clarifications and few
references added, final version to appear in PRD (rapid communications
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