4,582 research outputs found
Fading Gravity and Self-Inflation
We study the cosmology of a toy modified theory of gravity in which gravity
shuts off at short distances, as in the fat graviton scenario of Sundrum. In
the weak-field limit, the theory is perturbatively local, ghost-free and
unitary, although likely suffers from non-perturbative instabilities. We derive
novel self-inflationary solutions from the vacuum equations of the theory,
without invoking scalar fields or other forms of stress energy. The modified
perturbation equation expressed in terms of the Newtonian potential closely
resembles its counterpart for inflaton fluctuations. The resulting scalar
spectrum is therefore slightly red, akin to the simplest scalar-driven
inflationary models. A key difference, however, is that the gravitational wave
spectrum is generically not scale invariant. In particular the tensor spectrum
can have a blue tilt, a distinguishing feature from standard inflation.Comment: 35 pages, 4 figures. v3: version to appear in Phys. Rev.
New Ekpyrotic Cosmology
In this paper, we present a new scenario of the early Universe that contains
a pre big bang Ekpyrotic phase. By combining this with a ghost condensate, the
theory explicitly violates the null energy condition without developing any
ghost-like instabilities. Thus the contracting universe goes through a
non-singular bounce and evolves smoothly into the expanding post big bang
phase. The curvature perturbation acquires a scale-invariant spectrum well
before the bounce in this scenario. It is sourced by the scale-invariant
entropy perturbation engendered by two ekpyrotic scalar fields, a mechanism
recently proposed by Lehners et al. Since the background geometry is
non-singular at all times, the curvature perturbation remains nearly constant
on super horizon scales. It emerges from the bounce unscathed and imprints a
scale-invariant spectrum of density fluctuations in the matter-radiation fluid
at the onset of the hot big bang phase. The ekpyrotic potential can be chosen
so that the spectrum has a ``red'' tilt, in accordance with the recent data
from WMAP. As in the original Ekpyrotic scenario, the model predicts a
negligible gravity wave signal on all observable scales. As such ``New
Ekpyrotic Cosmology" provides a consistent and distinguishable alternative to
inflation to account for the origin of the seeds of large scale structure.Comment: 41 pages, 4 figures. v2: minor corrections, references added. v3:
small modifications in bounce section, references added. v4: version
published in PR
Supersymmetric Galileons
Galileon theories are of considerable interest since they allow for stable
violations of the null energy condition. Since such violations could have
occurred during a high-energy regime in the history of our universe, we are
motivated to study supersymmetric extensions of these theories. This is carried
out in this paper, where we construct generic classes of N=1 supersymmetric
Galileon Lagrangians. They are shown to admit non-equivalent stress-energy
tensors and, hence, vacua manifesting differing conditions for violating the
null energy condition. The temporal and spatial fluctuations of all component
fields of the supermultiplet are analyzed and shown to be stable on a large
number of such backgrounds. In the process, we uncover a surprising connection
between conformal Galileon and ghost condensate theories, allowing for a deeper
understanding of both types of theories.Comment: 41 pages, v2: added a referenc
Dynamical vs spectator models of (pseudo-)conformal Universe
We discuss two versions of the conformal scenario for generating scalar
cosmological perturbations: a spectator version with a scalar field conformally
coupled to gravity and carrying negligible energy density, and a dynamical
version with a scalar field minimally coupled to gravity and dominating the
cosmological evolution. By making use of the Newtonian gauge, we show that (i)
no UV strong coupling scale is generated below due to mixing with
metric perturbations in the dynamical scenario, and (ii) the dynamical and
spectator models yield identical results to the leading non-linear order. We
argue that these results, which include potentially observable effects like
statistical anisotropy and non-Gaussianity, are characteristic of the entire
class of conformal models. As an example, we reproduce, within the dynamical
scenario and working in comoving gauge, our earlier result on the statistical
anisotropy, which was originally obtained within the spectator approach.Comment: 13 page
Orbital angular momentum exchange in an optical parametric oscillator
We present a study of orbital angular momentum transfer from pump to
down-converted beams in a type-II Optical Parametric Oscillator. Cavity and
anisotropy effects are investigated and demostrated to play a central role in
the transverse mode dynamics. While the idler beam can oscillate in a
Laguerre-Gauss mode, the crystal birefringence induces an astigmatic effect in
the signal beam that prevents the resonance of such mode.Comment: 10 pages, 8 figures, regular articl
Scalar tilt from broken conformal invariance
Within recently proposed scenario which explains flatness of the spectrum of
scalar cosmological perturbations by a combination of conformal and global
symmetries, we discuss the effect of weak breaking of conformal invariance. We
find that the scalar power spectrum obtains a small tilt which depends on both
the strength of conformal symmetry breaking and the law of evolution of the
scale factor
WMAP and Inflation
We assay how inflationary models whose properties are dominated by the
dynamics of a single scalar field are constrained by cosmic microwave
background (CMB) data from the Wilkinson Microwave Anisotropy Probe (WMAP). We
classify inflationary models in a plane defined by the horizon-flow parameters.
Our approach differs from that of the WMAP collaboration in that we analyze
only WMAP data and take the spectral shapes from slow-roll inflation rather
than power-law parameterizations of the spectra. The only other information we
use is the measurement of from the Hubble Space Telescope (HST) Key
Project. We find that the spectral index of primordial density perturbations
lies in the 1\sigma range 0.94<n_s<1.04 with no evidence of running. The ratio
of the amplitudes of tensor and scalar perturbations is smaller than 0.61 and
the inflationary scale is below 2.8\times 10^{16} GeV, both at the 2\sigma C.L.
No class of inflation or ekpyrotic/cyclic models is excluded. The \lambda
\phi^4 potential is excluded at 3\sigma only if the number of e-folds is
assumed to be less than 45.Comment: 7 pages, 3 figures. Version to appear in PL
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