118 research outputs found
Non-Gaussianity in Cosmic Microwave Background Temperature Fluctuations from Cosmic (Super-)Strings
We compute analytically the small-scale temperature fluctuations of the
cosmic microwave background from cosmic (super-)strings and study the
dependence on the string intercommuting probability . We develop an
analytical model which describes the evolution of a string network and
calculate the numbers of string segments and kinks in a horizon volume. Then we
derive the probability distribution function (pdf) which takes account of
finite angular resolution of observation. The resultant pdf consists of a
Gaussian part due to frequent scatterings by long string segments and a
non-Gaussian tail due to close encounters with kinks. The dispersion of the
Gaussian part is reasonably consistent with that obtained by numerical
simulations by Fraisse et al.. On the other hand, the non-Gaussian tail
contains two phenomenological parameters which are determined by comparison
with the numerical results for P=1. Extrapolating the pdf to the cases with
, we predict that the non-Gaussian feature is suppressed for small .Comment: 6 pages, 2 figure, version accepted by JCA
A general proof of the equivalence between the \delta N and covariant formalisms
Recently, the equivalence between the \delta N and covariant formalisms has
been shown (Suyama et al. 2012), but they essentially assumed Einstein gravity
in their proof. They showed that the evolution equation of the curvature
covector in the covariant formalism on uniform energy density slicings
coincides with that of the curvature perturbation in the \delta N formalism
assuming the coincidence of uniform energy and uniform expansion (Hubble)
slicings, which is the case on superhorizon scales in Einstein gravity. In this
short note, we explicitly show the equivalence between the \delta N and
covariant formalisms without specifying the slicing condition and the
associated slicing coincidence, in other words, regardless of the gravity
theory.Comment: 7 pages,a reference added, to be published in EP
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
Cosmological constraints on extended Galileon models
The extended Galileon models possess tracker solutions with de Sitter
attractors along which the dark energy equation of state is constant during the
matter-dominated epoch, i.e. w_DE = -1-s, where s is a positive constant. Even
with this phantom equation of state there are viable parameter spaces in which
the ghosts and Laplacian instabilities are absent. Using the observational data
of the supernovae type Ia, the cosmic microwave background (CMB), and baryon
acoustic oscillations, we place constraints on the tracker solutions at the
background level and find that the parameter s is constrained to be s=0.034
(-0.034,+0.327) (95% CL) in the flat Universe. In order to break the degeneracy
between the models we also study the evolution of cosmological density
perturbations relevant to the large-scale structure (LSS) and the
Integrated-Sachs-Wolfe (ISW) effect in CMB. We show that, depending on the
model parameters, the LSS and the ISW effect is either positively or negatively
correlated. It is then possible to constrain viable parameter spaces further
from the observational data of the ISW-LSS cross-correlation as well as from
the matter power spectrum.Comment: 17 pages, 9 figures, uses RevTeX4-
Conditions for large non-Gaussianity in two-field slow-roll inflation
We study the level of primordial non-Gaussianity in slow-roll two-field
inflation. Using an analytic formula for the nonlinear parameter f_nl in the
case of a sum or product separable potential, we find that it is possible to
generate significant non-Gaussianity even during slow-roll inflation with
Gaussian perturbations at Hubble exit. In this paper we give the general
conditions to obtain large non-Gaussianity and calculate the level of
fine-tuning required to obtain this. We present explicit models in which the
non-Gaussianity at the end of inflation can exceed the current observational
bound of |f_nl|<100.Comment: 16 pages, 6 figures, 1 table, v2: typos corrected and references
added, matches version accepted by JCA
Conservation of the nonlinear curvature perturbation in generic single-field inflation
It is known that the curvature perturbation on uniform energy density (or
comoving or uniform Hubble) slices on superhorizon scales is conserved to full
nonlinear order if the pressure is only a function of the energy density (ie,
if the perturbation is purely adiabatic), independent of the gravitational
theory. Here we explicitly show that the same conservation holds for a universe
dominated by a single scalar field provided that the field is in an attractor
regime, for a very general class of scalar field theories. However, we also
show that if the scalar field equation contains a second time derivative of the
metric, as in the case of the Galileon theory, one has to invoke the
gravitational field equations to show the conservation.Comment: 6 pages, minor revisions made but conclusion unchanged, references
added, to be published in CQG as a fast track communicatio
Large slow-roll corrections to the bispectrum of noncanonical inflation
Nongaussian statistics are a powerful discriminant between inflationary
models, particularly those with noncanonical kinetic terms. Focusing on
theories where the Lagrangian is an arbitrary Lorentz-invariant function of a
scalar field and its first derivatives, we review and extend the calculation of
the observable three-point function. We compute the "next-order" slow-roll
corrections to the bispectrum in closed form, and obtain quantitative estimates
of their magnitude in DBI and power-law k-inflation. In the DBI case our
results enable us to estimate corrections from the shape of the potential and
the warp factor: these can be of order several tens of percent. We track the
possible sources of large logarithms which can spoil ordinary perturbation
theory, and use them to obtain a general formula for the scale dependence of
the bispectrum. Our result satisfies the next-order version of Maldacena's
consistency condition and an equivalent consistency condition for the scale
dependence. We identify a new bispectrum shape available at next-order, which
is similar to a shape encountered in Galileon models. If fNL is sufficiently
large this shape may be independently detectable.Comment: v1: 37 pages, plus tables, figures and appendices. v2: supersedes
version published in JCAP; some clarifications and more detailed comparison
with earlier literature. All results unchanged. v3:improvements to some
plots; text unchange
The Imperfect Fluid behind Kinetic Gravity Braiding
We present a standard hydrodynamical description for non-canonical scalar
field theories with kinetic gravity braiding. In particular, this picture
applies to the simplest galileons and k-essence. The fluid variables not only
have a clear physical meaning but also drastically simplify the analysis of the
system. The fluid carries charges corresponding to shifts in field space. This
shift-charge current contains a spatial part responsible for diffusion of the
charges. Moreover, in the incompressible limit, the equation of motion becomes
the standard diffusion equation. The fluid is indeed imperfect because the
energy flows neither along the field gradient nor along the shift current. The
fluid has zero vorticity and is not dissipative: there is no entropy
production, the energy-momentum is exactly conserved, the temperature vanishes
and there is no shear viscosity. Still, in an expansion around a perfect fluid
one can identify terms which correct the pressure in the manner of bulk
viscosity. We close by formulating the non-trivial conditions for the
thermodynamic equilibrium of this imperfect fluid.Comment: 23 pages plus appendices. New version includes extended discussion on
diffusion and dynamics in alternative frames, as well as additional
references. v3 reflects version accepted for publication in JHEP: minor
comments added regarding suitability to numerical approache
Primordial fluctuations and non-Gaussianities from multifield DBI Galileon inflation
We study a cosmological scenario in which the DBI action governing the motion
of a D3-brane in a higher-dimensional spacetime is supplemented with an induced
gravity term. The latter reduces to the quartic Galileon Lagrangian when the
motion of the brane is non-relativistic and we show that it tends to violate
the null energy condition and to render cosmological fluctuations ghosts. There
nonetheless exists an interesting parameter space in which a stable phase of
quasi-exponential expansion can be achieved while the induced gravity leaves
non trivial imprints. We derive the exact second-order action governing the
dynamics of linear perturbations and we show that it can be simply understood
through a bimetric perspective. In the relativistic regime, we also calculate
the dominant contribution to the primordial bispectrum and demonstrate that
large non-Gaussianities of orthogonal shape can be generated, for the first
time in a concrete model. More generally, we find that the sign and the shape
of the bispectrum offer powerful diagnostics of the precise strength of the
induced gravity.Comment: 34 pages including 9 figures, plus appendices and bibliography.
Wordings changed and references added; matches version published in JCA
Spectral Index and Non-Gaussianity in Supersymmetric Hybrid Inflation
We consider a supersymmetric hybrid inflation model with two inflaton fields.
The superpotential during inflation is dominated by W=(\kappa S+\kappa' S')M^2,
where S, S' are inflatons carrying the same U(1)_R charge, \kappa, \kappa' are
dimensionless couplings, and M (\sim 10^{15-16} GeV) is a dimensionful
parameter associated with a symmetry breaking scale. One light mass eigenstate
drives inflation, while the other heavier mass eigenstate is stuck to the
origin. The smallness of the lighter inflaton mass for the scalar spectral
index n_s\approx 0.96, which is the center value of WMAP7, can be controlled by
the ratio \kappa'/\kappa through the supergravity corrections. We also discuss
the possibility of the two field inflation and large non-Gaussianity in this
setup.Comment: 17 pages, 2 figures, version published in Eur. Phys. J.
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