1,513 research outputs found
The gravitational wave contribution to CMB anisotropies and the amplitude of mass fluctuations from COBE results
A stochastic background of primordial gravitational waves may substantially
contribute, via the Sachs--Wolfe effect, to the large--scale Cosmic Microwave
Background (CMB) anisotropies recently detected by COBE. This implies a {\it
bias} in any resulting determination of the primordial amplitude of density
fluctuations. We consider the constraints imposed on (``tilted")
power--law fluctuation spectra, taking into account the contribution from both
scalar and tensor waves, as predicted by power--law inflation. The
gravitational--wave contribution to CMB anisotropies generally reduces the
required {\it rms} level of mass fluctuation, thereby increasing the linear
{\it bias parameter}, even in models where the spectral index is close to the
Harrison--Zel'dovich value . This ``gravitational--wave bias" helps to
reconcile the predictions of CDM models with observations on pairwise galaxy
velocity dispersion on small scales.Comment: 11 pages. Two figures available upon reques
On Relativistic Perturbations of Second and Higher Order
We present the results of a study of the gauge dependence of spacetime
perturbations. In particular, we consider gauge invariance in general, we give
a generating formula for gauge transformations to an arbitrary order n, and
explicit transformation rules at second order.Comment: 6 pages, latex, with special style included, Proceedings of the 12th
Italian Conference on General Relativity and Gravitational Physic
Dynamics of Silent Universes
We investigate the local non--linear dynamics of irrotational dust with
vanishing magnetic part of the Weyl tensor, . Once coded in the initial
conditions, this dynamical restriction is respected by the relativistic
evolution equations. Thus, the outcome of the latter are {\it exact solutions}
for special initial conditions with , but with no symmetries: they
describe inhomogeneous triaxial dynamics generalizing that of a fluid element
in a Tolman--Bondi, Kantowski--Sachs or Szekeres geometry. A subset of these
solutions may be seen as (special) perturbations of Friedmann models, in the
sense that there are trajectories in phase--space that pass arbitrarily close
to the isotropic ones. We find that the final fate of ever--expanding
configurations is a spherical void, locally corresponding to a Milne universe.
For collapsing configurations we find a whole family of triaxial attractors,
with vanishing local density parameter . These attractors locally
correspond to Kasner vacuum solutions: there is a single physical configuration
collapsing to a degenerate {\it pancake}, while the generic configuration
collapses to a triaxial {\it spindle} singularity. These {\it silent universe}
models may provide a fair representation of the universe on super horizon
scales. Moreover, one might conjecture that the non--local information carried
by becomes negligible during the late highly non--linear stages of
collapse, so that the attractors we find may give all of the relevant expansion
or collapse configurations of irrotational dust.Comment: 40 pages with 4 figures, compressed and uuencoded PostScript file,
submitted to ApJ, SISSA preprint Ref. 85/94/
The Three--Point Correlation Function of the Cosmic Microwave Background in Inflationary Models
We analyze the temperature three--point correlation function and the skewness
of the Cosmic Microwave Background (CMB), providing general relations in terms
of multipole coefficients. We then focus on applications to large angular scale
anisotropies, such as those measured by the {\em COBE} DMR, calculating the
contribution to these quantities from primordial, inflation generated, scalar
perturbations, via the Sachs--Wolfe effect. Using the techniques of stochastic
inflation we are able to provide a {\it universal} expression for the ensemble
averaged three--point function and for the corresponding skewness, which
accounts for all primordial second--order effects. These general expressions
would moreover apply to any situation where the bispectrum of the primordial
gravitational potential has a {\em hierarchical} form. Our results are then
specialized to a number of relevant models: power--law inflation driven by an
exponential potential, chaotic inflation with a quartic and quadratic potential
and a particular case of hybrid inflation. In all these cases non--Gaussian
effects are small: as an example, the {\em mean} skewness is much smaller than
the cosmic {\em rms} skewness implied by a Gaussian temperature fluctuation
field.Comment: 18 pages; LaTeX; 4 PostScript figures included at the end of the
file; SISSA REF.193/93/A and DFPD 93/A/8
Disformal invariance of continuous media with linear equation of state
We show that the effective theory describing single component continuous
media with a linear and constant equation of state of the form is
invariant under a 1-parameter family of continuous disformal transformations.
In the special case of (ultrarelativistic gas), such a family reduces
to conformal transformations. As examples, perfect fluids, homogeneous and
isotropic solids are discussed.Comment: latex, 7 page
Large-scale bias in the Universe: bispectrum method
Evidence that the Universe may be close to the critical density, required for
its expansion eventually to be halted, comes principally from dynamical studies
of large-scale structure. These studies either use the observed peculiar
velocity field of galaxies directly, or indirectly by quantifying its
anisotropic effect on galaxy clustering in redshift surveys. A potential
difficulty with both such approaches is that the density parameter
is obtained only in the combination , if linear
perturbation theory is used. The determination of the density parameter
is therefore compromised by the lack of a good measurement of the
bias parameter , which relates the clustering of sample galaxies to the
clustering of mass.
In this paper, we develop an idea of Fry (1994), using second-order
perturbation theory to investigate how to measure the bias parameter on large
scales. The use of higher-order statistics allows the degeneracy between
and to be lifted, and an unambiguous determination of
then becomes possible. We apply a likelihood approach to the bispectrum, the
three-point function in Fourier space. This paper is the first step in turning
the idea into a practical proposition for redshift surveys, and is principally
concerned with noise properties of the bispectrum, which are non-trivial. The
calculation of the required bispectrum covariances involves the six-point
function, including many noise terms, for which we have developed a generating
functional approach which will be of value in calculating high-order statistics
in general.Comment: 12 pages, latex, 7 postscript figures included. Accepted by MNRAS.
(Minor numerical typesetting errors corrected: results unchanged
The nonlinear redshift-space power spectrum of galaxies
We study the power spectrum of galaxies in redshift space, with third order
perturbation theory to include corrections that are absent in linear theory. We
assume a local bias for the galaxies: i.e. the galaxy density is sampled from
some local function of the underlying mass distribution. We find that the
effect of the nonlinear bias in real space is to introduce two new features:
first, there is a contribution to the power which is constant with wavenumber,
whose nature we reveal as essentially a shot-noise term. In principle this
contribution can mask the primordial power spectrum, and could limit the
accuracy with which the latter might be measured on very large scales.
Secondly, the effect of second- and third-order bias is to modify the effective
bias (defined as the square root of the ratio of galaxy power spectrum to
matter power spectrum). The effective bias is almost scale-independent over a
wide range of scales. These general conclusions also hold in redshift space. In
addition, we have investigated the distortion of the power spectrum by peculiar
velocities, which may be used to constrain the density of the Universe. We look
at the quadrupole-to-monopole ratio, and find that higher-order terms can mimic
linear theory bias, but the bias implied is neither the linear bias, nor the
effective bias referred to above. We test the theory with biased N-body
simulations, and find excellent agreement in both real and redshift space,
providing the local biasing is applied on a scale whose fractional r.m.s.
density fluctuations are .Comment: 13 pages, 7 figures. Accepted by MNRA
Super Heavy Dark Matter in light of BICEP2, Planck and Ultra High Energy Cosmic Rays Observations
The announcement by BICEP2 of the detection of B-mode polarization consistent
with primordial gravitational waves with a tensor-to-scalar ratio,
, challenged predictions from most inflationary models
of a lower value for . More recent results by Planck on polarized dust
emission show that the observed tensor modes signal is compatible with pure
foreground emission. A more significant constraint on was then obtained by
a joint analysis of Planck, BICEP2 and Keck Array data showing an upper limit
to the tensor to scalar ratio , excluding the case with low
statistical significance. Forthcoming measurements by BICEP3, the Keck Array,
and other CMB polarization experiments, open the possibility for making the
fundamental measurement of . Here we discuss how sets the scale for
models where the dark matter is created at the inflationary epoch, the
generically called super-heavy dark matter models. We also consider the
constraints on such scenarios given by recent data from ultrahigh energy cosmic
ray observatories which set the limit on super-heavy dark matter particles
lifetime. We discuss how super-heavy dark matter can be discovered by a precise
measurement of combined with future observations of ultra high energy
cosmic rays.Comment: 17 pages, 14 eps figures, accepted for publication in JCA
Comments on Backreaction and Cosmic Acceleration
In this brief WEB note we comment on recent papers related to our paper "On
Acceleration Without Dark Energy".Comment: 5 pages WEB not
Primordial Black Holes from Inflation and non-Gaussianity
Primordial black holes may owe their origin to the small-scale enhancement of
the comoving curvature perturbation generated during inflation. Their mass
fraction at formation is markedly sensitive to possible non-Gaussianities in
such large, but rare fluctuations. We discuss a path-integral formulation which
provides the exact mass fraction of primordial black holes at formation in the
presence of non-Gaussianity. Through a couple of classes of models, one based
on single-field inflation and the other on spectator fields, we show that
restricting to a Gaussian statistics may lead to severe inaccuracies in the
estimate of the mass fraction as well as on the clustering properties of the
primordial black holes.Comment: 21 pages, 2 figures, v2: matching published versio
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