15 research outputs found
On the new string theory inspired mechanism of generation of cosmological perturbations
Recently a non-inflationary mechanism of generation of scale-free
cosmological perturbations of metric was proposed by Brandenberger, Nayeri, and
Vafa in the context of the string gas cosmology. We discuss various problems of
their model and argue that the cosmological perturbations of metric produced in
this model have blue spectrum with a spectral index n = 5, which strongly
disagrees with observations. We conclude that this model in its present form is
not a viable alternative to inflationary cosmology.Comment: 11 pages, 1 figur
Suppressing the lower Multipoles in the CMB Anisotropies
The Cosmic Microwave Background (CMB) anisotropy power on the largest angular
scales observed both by WMAP and COBE DMR appears to be lower than the one
predicted by the standard model of cosmology with almost scale free primordial
perturbations arising from a period of inflation
\cite{cobe,Bennett:2003bz,Spergel,Peiris}. One can either interpret this as a
manifestation of cosmic variance or as a physical effect that requires an
explanation. We discuss various mechanisms that could be responsible for the
suppression of such low multipoles. Features in the late time evolution
of metric fluctuations may do this via the integral Sachs-Wolfe effect. Another
possibility is a suppression of power at large scales in the primordial
spectrum induced by a fast rolling stage in the evolution of the inflaton field
at the beginning of the last 65 e-folds of inflation. We illustrate this effect
in a simple model of inflation and fit the resulting CMB spectrum to the
observed temperature-temperature (TT) power spectrum. We find that the WMAP
observations suggest a cutoff at Mpc at 68% confidence, while only an upper limit of Mpc at 95%. Thus, although it improves the fit of the
data, the presence of a cutoff in power spectrum is only required at a level
close to . This is obtained with a prior which corresponds to equal
distribution wrt . We discuss how other choices (such as an equal
distribution wrt which is natural in the context of inflation) can
affect the statistical interpretation.Comment: 11 pages, 4 figures, replaced with published version, comparison with
recent papers is extende
Optimal Location of Two Laser-interferometric Detectors for Gravitational Wave Backgrounds at 100 MHz
Recently, observational searches for gravitational wave background (GWB) have
been developed and given constraints on the energy density of GWB in a broad
range of frequencies. These constraints have already resulted in the rejection
of some theoretical models of relatively large GWB spectra. However, at 100
MHz, there is no strict upper limit from direct observation, though an indirect
limit exists due to He4 abundance due to big-bang nucleosynthesis. In our
previous paper, we investigated the detector designs that can effectively
respond to GW at high frequencies, where the wavelength of GW is comparable to
the size of a detector, and found that the configuration, a so-called
synchronous-recycling interferometer is best at these sensitivity. In this
paper, we investigated the optimal location of two synchronous-recycling
interferometers and derived their cross-correlation sensitivity to GWB. We
found that the sensitivity is nearly optimized and hardly changed if two
coaligned detectors are located within a range 0.2 m, and that the sensitivity
achievable in an experiment is far below compared with the constraint
previously obtained in experiments.Comment: 17 pages, 6 figure
Radiative Corrections to the Inflaton Potential as an Explanation of Suppressed Large Scale Power in Density Perturbations and the Cosmic Microwave Background
The Wilkinson Microwave Anisotropy Probe microwave background data suggest
that the primordial spectrum of scalar curvature fluctuations is suppressed at
small wavenumbers. We propose a UV/IR mixing effect in small-field inflationary
models that can explain the observable deviation in WMAP data from the
concordance model. Specifically, in inflationary models where the inflaton
couples to an asymptotically free gauge theory, the radiative corrections to
the effective inflaton potential can be anomalously large. This occurs for
small values of the inflaton field which are of the order of the gauge theory
strong coupling scale. Radiative corrections cause the inflaton potential to
blow up at small values of the inflaton field. As a result, these corrections
can violate the slow-roll condition at the initial stage of the inflation and
suppress the production of scalar density perturbations.Comment: 20 pages, 2 figures, v2: refs added, v3: JCAP versio
Tachyonic Universes in Patch Cosmologies with
In this article we study closed inflationary universe models by means of a
tachyonic field. We described a general treatment for created a universe with
in patch cosmology, which is able to represent General Relativity,
Gauss-Bonnet or Randall-Sundrum patches. We use recent data from astronomical
observations to constrain the parameters appearing in our model.Comment: 15 pages, 2 figures. Accepted by Modern Physics Letters
Multidimensional cosmological models: cosmological and astrophysical implications and constraints
We investigate four-dimensional effective theories which are obtained by
dimensional reduction of multidimensional cosmological models with factorizable
geometry and consider the interaction between conformal excitations of the
internal space (geometrical moduli excitations) and Abelian gauge fields. It is
assumed that the internal space background can be stabilized by minima of an
effective potential. The conformal excitations over such a background have the
form of massive scalar fields (gravitational excitons) propagating in the
external spacetime. We discuss cosmological and astrophysical implications of
the interaction between gravexcitons and four-dimensional photons as well as
constraints arising on multidimensional models of the type considered in our
paper. In particular, we show that due to the experimental bounds on the
variation of the fine structure constant, gravexcitons should decay before
nucleosynthesis starts. For a successful nucleosynthesis the masses of the
decaying gravexcitons should be m>10^4 GeV. Furthermore, we discuss the
possible contribution of gravexcitons to UHECR. It is shown that, at energies
of about 10^{20}eV, the decay length of gravexcitons with masses m>10^4 GeV is
very small, but that for m <10^2 GeV it becomes much larger than the
Greisen-Zatsepin-Kuzmin cut-off distance. Finally, we investigate the
possibility for gravexciton-photon oscillations in strong magnetic fields of
astrophysical objects. The corresponding estimates indicate that even the high
magnetic field strengths of magnetars are not sufficient for an efficient and
copious production of gravexcitons.Comment: 16 pages, LaTeX2e, minor changes, improved references, to appear in
PR
Local non-Gaussianity from inflation
The non-Gaussian distribution of primordial perturbations has the potential
to reveal the physical processes at work in the very early Universe. Local
models provide a well-defined class of non-Gaussian distributions that arise
naturally from the non-linear evolution of density perturbations on
super-Hubble scales starting from Gaussian field fluctuations during inflation.
I describe the delta-N formalism used to calculate the primordial density
perturbation on large scales and then review several models for the origin of
local primordial non-Gaussianity, including the cuvaton, modulated reheating
and ekpyrotic scenarios. I include an appendix with a table of sign conventions
used in specific papers.Comment: 21 pages, 1 figure, invited review to appear in Classical and Quantum
Gravity special issue on non-linear and non-Gaussian cosmological
perturbation
Thermal Inflation and the Gravitational Wave Background
We consider the impact of thermal inflation -- a short, secondary period of
inflation that can arise in supersymmetric scenarios -- on the stochastic
gravitational wave background. We show that while the primordial inflationary
gravitational wave background is essentially unchanged at CMB scales, it is
massively diluted at solar system scales and would be unobservable by a BBO
style experiment. Conversely, bubble collisions at the end of thermal inflation
can generate a new stochastic background. We calculate the likely properties of
the bubbles created during this phase transition, and show that the expected
amplitude and frequency of this signal would fall within the BBO range.Comment: 21 pages, 4 figures; accepted for JCAP; a reference added; table
reformatte
Observational Signatures and Non-Gaussianities of General Single Field Inflation
We perform a general study of primordial scalar non-Gaussianities in single
field inflationary models in Einstein gravity. We consider models where the
inflaton Lagrangian is an arbitrary function of the scalar field and its first
derivative, and the sound speed is arbitrary. We find that under reasonable
assumptions, the non-Gaussianity is completely determined by 5 parameters. In
special limits of the parameter space, one finds distinctive ``shapes'' of the
non-Gaussianity. In models with a small sound speed, several of these shapes
would become potentially observable in the near future. Different limits of our
formulae recover various previously known results.Comment: 53 pages, 5 figures; v3, minor revision, JCAP version; v4, numerical
coefficients corrected in Appendix B, discussion on consistency condition
revise
Observational Signatures and Non-Gaussianities of General Single Field Inflation
We perform a general study of primordial scalar non-Gaussianities in single
field inflationary models in Einstein gravity. We consider models where the
inflaton Lagrangian is an arbitrary function of the scalar field and its first
derivative, and the sound speed is arbitrary. We find that under reasonable
assumptions, the non-Gaussianity is completely determined by 5 parameters. In
special limits of the parameter space, one finds distinctive ``shapes'' of the
non-Gaussianity. In models with a small sound speed, several of these shapes
would become potentially observable in the near future. Different limits of our
formulae recover various previously known results.Comment: 53 pages, 5 figures; v3, minor revision, JCAP version; v4, numerical
coefficients corrected in Appendix B, discussion on consistency condition
revise