805 research outputs found
New Models of f(R) Theories of Gravity
We introduce new models of f(R) theories of gravity that are generalization
of Horava-Lifshitz gravity.Comment: 16 pages, typos corrected, v2:minor changes, references adde
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
Fractal universe and quantum gravity
We propose a field theory which lives in fractal spacetime and is argued to
be Lorentz invariant, power-counting renormalizable, ultraviolet finite, and
causal. The system flows from an ultraviolet fixed point, where spacetime has
Hausdorff dimension 2, to an infrared limit coinciding with a standard
four-dimensional field theory. Classically, the fractal world where fields live
exchanges energy momentum with the bulk with integer topological dimension.
However, the total energy momentum is conserved. We consider the dynamics and
the propagator of a scalar field. Implications for quantum gravity, cosmology,
and the cosmological constant are discussed.Comment: 4 pages. v2: typos corrected; v3: discussion improved, intuitive
introduction added, matches the published versio
Observational test of inflation in loop quantum cosmology
We study in detail the power spectra of scalar and tensor perturbations generated during inflation in loop quantum cosmology (LQC). After clarifying in a novel quantitative way how inverse-volume corrections arise in inhomogeneous settings, we show that they can generate large running spectral indices, which generally lead to an enhancement of power at large scales. We provide explicit formulas for the scalar/tensor power spectra under the slow-roll approximation, by taking into account corrections of order higher than the runnings. We place observational bounds on the inverse-volume quantum correction \delta ~ a^{- \sigma} (\sigma >0, is the scale factor) and the slow-roll parameter \epsilon_V for power-law potentials as well as exponential potentials by using the data of WMAP 7yr combined with other observations. We derive the constraints on \delta for two pivot wavenumbers k_0 for several values of \delta. The quadratic potential can be compatible with the data even in the presence of the LQC corrections, but the quartic potential is in tension with observations. We also find that the upper bounds on \delta (k_0) for given \sigma and k_0 are insensitive to the choice of the inflaton potentials
Degeneracy of consistency equations in braneworld inflation
In a Randall-Sundrum type II inflationary scenario we compute perturbation
amplitudes and spectral indices up to next-to-lowest order in the slow-roll
parameters, starting from the well-known lowest-order result for a de Sitter
brane. Using two different prescriptions for the tensor amplitude, we show that
the braneworld consistency equations are not degenerate with respect to the
standard relations and we explore their observational consequences. It is then
shown that, while the degeneracy between high- and low-energy regimes can come
from suitable values of the cosmological observables, exact functional matching
between consistency expressions is plausibly discarded. This result is then
extended to the Gauss-Bonnet case.Comment: 16 pages, 3 figures. v3: major revision. Changed title, updated
references, rearranged material, new prescription for the tensor spectrum,
new figures, extended and more robust conclusion
Atrioventricular canal defect and genetic syndromes: the unifying role of sonic hedgehog
The atrioventricular canal defect (AVCD) is a congenital heart defect (CHD) frequently associated with extracardiac anomalies (75%). Previous observations from a personal series of patients with AVCD and "polydactyly syndromes" showed that the distinct morphology and combination of AVCD features in some of these syndromes is reminiscent of the cardiac phenotype found in heterotaxy, a malformation complex previously associated with functional cilia abnormalities and aberrant Hedgehog (Hh) signaling. Hh signaling coordinates multiple aspects of left-right lateralization and cardiovascular growth. Being active at the venous pole the secondary heart field (SHF) is essential for normal development of dorsal mesenchymal protrusion and AVCD formation and septation. Experimental data show that perturbations of different components of the Hh pathway can lead to developmental errors presenting with partially overlapping manifestations and AVCD as a common denominator. We review the potential role of Hh signaling in the pathogenesis of AVCD in different genetic disorders. AVCD can be viewed as part of a "developmental field," according to the concept that malformations can be due to defects in signal transduction cascades or pathways, as morphogenetic units which may be altered by Mendelian mutations, aneuploidies, and environmental causes
Cosmological tachyon from cubic string field theory
The classical dynamics of the tachyon scalar field of cubic string field
theory is considered on a cosmological background. Starting from a nonlocal
action with arbitrary tachyon potential, which encodes the bosonic and several
supersymmetric cases, we study the equations of motion in the Hamilton-Jacobi
formalism and with a generalized Friedmann equation, appliable in braneworld or
modified gravity models. The cases of cubic (bosonic) and quartic
(supersymmetric) tachyon potential in general relativity are automatically
included. We comment the validity of the slow-roll approximation, the stability
of the cosmological perturbations, and the relation between this tachyon and
the Dirac-Born-Infeld one.Comment: 20 pages JHEP style, 1 figure; v4: misprints corrected, matches the
published versio
Cosmological constraints from Gauss-Bonnet braneworld with large-field potentials
We calculate the spectral index and tensor-to-scalar ratio for patch
inflation defined by and ,
using the slow-roll expansion. The patch cosmology arisen from the Gauss-Bonnet
braneworld consists of Gauss-Bonnet (GB), Randall-Sundrum (RS), and 4D general
relativistic (GR) cosmological models. In this work, we choose large-field
potentials of to compare with the observational data. Since
second-order corrections are rather small in the slow-roll limit, the
leading-order calculation is sufficient to compare with the data. Finally, we
show that it is easier to discriminate between quadratic potential and quartic
potential in the GB cosmological model rather than the GR or RS cosmological
models.Comment: 13 pages, title changed, version to appear in JCA
Slow-roll corrections to inflaton fluctuations on a brane
Quantum fluctuations of an inflaton field, slow-rolling during inflation are
coupled to metric fluctuations. In conventional four dimensional cosmology one
can calculate the effect of scalar metric perturbations as slow-roll
corrections to the evolution of a massless free field in de Sitter spacetime.
This gives the well-known first-order corrections to the field perturbations
after horizon-exit. If inflaton fluctuations on a four dimensional brane
embedded in a five dimensional bulk spacetime are studied to first-order in
slow-roll then we recover the usual conserved curvature perturbation on
super-horizon scales. But on small scales, at high energies, we find that the
coupling to the bulk metric perturbations cannot be neglected, leading to a
modified amplitude of vacuum oscillations on small scales. This is a large
effect which casts doubt on the reliability of the usual calculation of
inflaton fluctuations on the brane neglecting their gravitational coupling.Comment: 18 pages, 4 figure
Slow-roll corrections to inflaton fluctuations on a brane
Quantum fluctuations of an inflaton field, slow-rolling during inflation are
coupled to metric fluctuations. In conventional four dimensional cosmology one
can calculate the effect of scalar metric perturbations as slow-roll
corrections to the evolution of a massless free field in de Sitter spacetime.
This gives the well-known first-order corrections to the field perturbations
after horizon-exit. If inflaton fluctuations on a four dimensional brane
embedded in a five dimensional bulk spacetime are studied to first-order in
slow-roll then we recover the usual conserved curvature perturbation on
super-horizon scales. But on small scales, at high energies, we find that the
coupling to the bulk metric perturbations cannot be neglected, leading to a
modified amplitude of vacuum oscillations on small scales. This is a large
effect which casts doubt on the reliability of the usual calculation of
inflaton fluctuations on the brane neglecting their gravitational coupling.Comment: 18 pages, 4 figure
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