62 research outputs found
Scalar-Scalar, Scalar-Tensor, and Tensor-Tensor Correlators from Anisotropic Inflation
We compute the phenomenological signatures of a model (Watanabe et al' 09) of
anisotropic inflation driven by a scalar and a vector field. The action for the
vector is U(1) invariant, and the model is free of ghost instabilities. A
suitable coupling of the scalar to the kinetic term of the vector allows for a
slow roll evolution of the vector vev, and hence for a prolonged anisotropic
expansion; this provides a counter example to the cosmic no hair conjecture. We
compute the nonvanishing two point correlation functions between physical modes
of the system, and express them in terms of power spectra with angular
dependence. The anisotropy parameter g_* for the scalar-scalar spectrum
(defined as in the Ackerman et al '07 parametrization) turns out to be negative
in the simplest realization of the model, which, therefore, cannot account for
the angular dependence emerged in some analyses of the WMAP data. A g_* of
order -0.1 is achieved when the energy of the vector is about 6-7 orders of
magnitude smaller than that of the scalar during inflation. For such values of
the parameters, the scalar-tensor correlation (which is in principle a
distinctive signature of anisotropic spaces) is smaller than the tensor-tensor
correlation
The nonperturbative decay of SUSY flat directions
We compute the nonperturbative decay of supersymmetric flat directions due to
their D-term potential. Flat directions can develop large vacuum expectation
values (vevs) during inflation, and, if they are long-lived, this can strongly
affect the reheating and thermalization stages after the inflation. We study a
generic system of two U(1) or SU(2) flat directions which are cosmologically
evolving after inflation. After proper gauge fixing, we show that the
excitations of the fields around this background can undergo exponential
amplification, at the expense of the energy density of the flat directions. We
compute this effect for several values of the masses and the initial vevs of
the two flat directions, through a combination of analytical methods and
extensive numerical simulations. For a wide range of parameters the flat
directions decay within their first few rotations.Comment: 29 pages, 7 .ps figure
Cosmological solutions of massive gravity on de Sitter
In the framework of the recently proposed models of massive gravity, defined
with respect to a de Sitter reference metric, we obtain new homogeneous and
isotropic solutions for arbitrary cosmological matter and arbitrary spatial
curvature. These solutions can be classified into three branches. In the first
two, the massive gravity terms behave like a cosmological constant. In the
third branch, the massive gravity effects can be described by a time evolving
effective fluid with rather remarkable features, including the property to
behave as a cosmological constant at late time.Comment: 6 pages, 1 figure; discussion extended, a few references added,
improved analysis in Section
Gravity Waves Signatures from Anisotropic pre-Inflation
We show that expanding or contracting Kasner universes are unstable due to
the amplification of gravitational waves (GW). As an application of this
general relativity effect, we consider a pre-inflationary anisotropic geometry
characterized by a Kasner-like expansion, which is driven dynamically towards
inflation by a scalar field. We investigate the evolution of linear metric
fluctuations around this background, and calculate the amplification of the
long-wavelength GW of a certain polarization during the anisotropic expansion
(this effect is absent for another GW polarization, and for scalar
fluctuations). These GW are superimposed to the usual tensor modes of quantum
origin from inflation, and are potentially observable if the total number of
inflationary e-folds exceeds the minimum required to homogenize the observable
universe only by a small margin. Their contribution to the temperature
anisotropy angular power spectrum decreases with the multipole l as l^(-p),
where p depends on the slope of the initial GW power-spectrum. Constraints on
the long-wavelength GW can be translated into limits on the total duration of
inflation and the initial GW amplitude. The instability of classical GW (and
zero-vacuum fluctuations of gravitons) during Kasner-like expansion (or
contraction) may have other interesting applications. In particular, if GW
become non-linear, they can significantly alter the geometry before the onset
of inflation
Instability of the ACW model, and problems with massive vectors during inflation
We prove that the anisotropic inflationary background of the
Ackerman-Carroll-Wise model, characterized by a fixed-norm vector field, is
unstable. We found the instability by explicitly solving the linearized
equations for the most general set of perturbations around this background, and
by noticing that the solutions diverge close to horizon crossing. This happens
because one perturbation becomes a ghost at that moment. A simplified
computation, with only the perturbations of the vector field included, shows
the same instability, clarifying the origin of the problem. We then discuss
several other models, with a particular emphasis on the case of a nonminimal
coupling to the curvature, in which vector fields are used either to support an
anisotropic expansion, or to generate cosmological perturbations on an
isotropic background. In many cases, the mass term of the vector needs to have
the ``wrong'' sign; we show that, as a consequence, the longitudinal vector
mode is a ghost (a field with negative kinetic term, and negative energy; not
simply a tachyon). We comment on problems that arise at the quantum level. In
particular, the presence of a ghost can be a serious difficulty for the UV
completion that such models require in the sub-horizon regime.Comment: 22 pages, 4 figure
Gravitational Field Equations on and off a 3-Brane World
The effective gravitational field equations on and off a 3-brane world
possessing a Z_{2} mirror symmetry and embedded in a five-dimensional bulk
spacetime with cosmological constant were derived by Shiromizu, Maeda and
Sasaki (SMS) in the framework of the Gauss-Codazzi projective approach with the
subsequent specialization to the Gaussian normal coordinates in the
neighborhood of the brane. However, the Gaussian normal coordinates imply a
very special slicing of spacetime and clearly, the consistent analysis of the
brane dynamics would benefit from complete freedom in the slicing of spacetime,
pushing the layer surfaces in the fifth dimension at any rates of evolution and
in arbitrary positions. We generalize the SMS effective field equations on and
off a 3-brane to the case where there is an arbitrary energy-momentum tensor in
the bulk. We use a more general setting to allow for acceleration of the
normals to the brane surface through the lapse function and the shift vector in
the spirit of Arnowitt, Deser and Misner. We show that the gravitational
influence of the bulk spacetime on the brane may be described by a traceless
second-rank tensor W_{ij}, constructed from the "electric" part of the bulk
Riemann tensor. We also present the evolution equations for the tensor W_{ij},
as well as for the corresponding "magnetic" part of the bulk curvature. These
equations involve the terms determined by both the nonvanishing acceleration of
normals in the nongeodesic slicing of spacetime and the presence of other
fields in the bulk.Comment: 22 pages, REVTEX
Effects of anisotropic dynamics on cosmic strings
The dynamics of cosmic strings is considered in anisotropic backgrounds. In
particular, the behaviour of infinitely long straight cosmic strings and of
cosmic string loops is determined. Small perturbations of a straight cosmic
string are calculated. The relevance of these results is discussed with respect
to the possible observational imprints of an anisotropic phase on the behaviour
of a cosmic string network.Comment: 16 pages, 9 figures; matches version published in JCA
Searching for planar signatures in WMAP
We search for planar deviations of statistical isotropy in the Wilkinson
Microwave Anisotropy Probe (WMAP) data by applying a recently introduced
angular-planar statistics both to full-sky and to masked temperature maps,
including in our analysis the effect of the residual foreground contamination
and systematics in the foreground removing process as sources of error. We
confirm earlier findings that full-sky maps exhibit anomalies at the planar
() and angular () scales and , which
seem to be due to unremoved foregrounds since this features are present in the
full-sky map but not in the masked maps. On the other hand, our test detects
slightly anomalous results at the scales and in the
masked maps but not in the full-sky one, indicating that the foreground
cleaning procedure (used to generate the full-sky map) could not only be
creating false anomalies but also hiding existing ones. We also find a
significant trace of an anomaly in the full-sky map at the scale
, which is still present when we consider galactic cuts of
18.3% and 28.4%. As regards the quadrupole (), we find a coherent
over-modulation over the whole celestial sphere, for all full-sky and cut-sky
maps. Overall, our results seem to indicate that current CMB maps derived from
WMAP data do not show significant signs of anisotropies, as measured by our
angular-planar estimator. However, we have detected a curious coherence of
planar modulations at angular scales of the order of the galaxy's plane, which
may be an indication of residual contaminations in the full- and cut-sky maps.Comment: 15 pages with pdf figure
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