1,272 research outputs found
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
Tests for primordial non-Gaussianity
We investigate the relative sensitivities of several tests for deviations
from Gaussianity in the primordial distribution of density perturbations. We
consider models for non-Gaussianity that mimic that which comes from inflation
as well as that which comes from topological defects. The tests we consider
involve the cosmic microwave background (CMB), large-scale structure (LSS),
high-redshift galaxies, and the abundances and properties of clusters. We find
that the CMB is superior at finding non-Gaussianity in the primordial
gravitational potential (as inflation would produce), while observations of
high-redshift galaxies are much better suited to find non-Gaussianity that
resembles that expected from topological defects. We derive a simple expression
that relates the abundance of high-redshift objects in non-Gaussian models to
the primordial skewness.Comment: 6 pages, 2 figures, MNRAS in press (minor changes to match the
accepted version
Detecting X-ray filaments in the low redshift Universe with XEUS and Constellation-X
We propose a possible way to detect baryons at low redshifts from the
analysis of X-ray absorption spectra of bright AGN pairs. A simple
semi-analytical model to simulate the spectra is presented. We model the
diffuse warm-hot intergalactic medium (WHIM) component, responsible for the
X-ray absorption, using inputs from high-resolution hydro-dynamical simulations
and analytical prescriptions. We show that the number of OVII absorbers per
unit redshift with column density larger than cm -
corresponding to an equivalent width of 1 km/s - which will be possibly
detectable by {\it XEUS}, is \magcir 30 per unit redshift. {\it
Constellation-X} will detect OVII absorptions per unit redshift with
an equivalent width of 10 km/s. Our results show that, in a CDM
Universe, the characteristic size of these absorbers at is
Mpc. The filamentary structure of WHIM can be probed by finding
coincident absorption lines in the spectra of background AGN pairs. We estimate
that at least 20 AGN pairs at separation \mincir 20 arcmin are needed to
detect this filamentary structure at a 3 level. Assuming observations
of distant sources using {\it XEUS} for exposure times of 500 ksec, we find
that the minimum source flux to probe the filamentary structure is erg cm s, in the 0.1-2.4 keV energy band. Thus,
most pairs of these extragalactic X-ray bright sources have already been
identified in the {\it ROSAT} All-Sky Survey. Re-observation of these objects
by future missions could be a powerful way to search for baryons in the low
redshift Universe.Comment: 18 pages, 10 Figures. Two figures added, Sections 2 and 3 expanded.
More optimistic results for Constellation-X. Accepted by MNRA
Scale Dependence of Halo Bispectrum from Non-Gaussian Initial Conditions in Cosmological N-body Simulations
We study the halo bispectrum from non-Gaussian initial conditions. Based on a
set of large -body simulations starting from initial density fields with
local type non-Gaussianity, we find that the halo bispectrum exhibits a strong
dependence on the shape and scale of Fourier space triangles near squeezed
configurations at large scales. The amplitude of the halo bispectrum roughly
scales as . The resultant scaling on the triangular shape is consistent
with that predicted by Jeong & Komatsu based on perturbation theory. We
systematically investigate this dependence with varying redshifts and halo mass
thresholds. It is shown that the dependence of the halo bispectrum is
stronger for more massive haloes at higher redshifts. This feature can be a
useful discriminator of inflation scenarios in future deep and wide galaxy
redshift surveys.Comment: 27 pages, 10 figures; revised argument in section 6, added appendix
C, JCAP accepted versio
General Relativistic Dynamics of Irrotational Dust: Cosmological Implications
The non--linear dynamics of cosmological perturbations of an irrotational
collisionless fluid is analyzed within General Relativity. Relativistic and
Newtonian solutions are compared, stressing the different role of boundary
conditions in the two theories. Cosmological implications of relativistic
effects, already present at second order in perturbation theory, are studied
and the dynamical role of the magnetic part of the Weyl tensor is elucidated.Comment: 12 pages , DFPD 93/A/6
Large-k Limit of Multi-Point Propagators in the RG Formalism
Renormalized versions of cosmological perturbation theory have been very
successful in recent years in describing the evolution of structure formation
in the weakly non-linear regime. The concept of multi-point propagators has
been introduced as a tool to quantify the relation between the initial matter
distribution and the final one and to push the validity of the approaches to
smaller scales. We generalize the n-point propagators that have been considered
until now to include a new class of multi-point propagators that are relevant
in the framework of the renormalization group formalism. The large-k results
obtained for this general class of multi-point propagators match the results
obtained earlier both in the case of Gaussian and non-Gaussian initial
conditions. We discuss how the large-k results can be used to improve on the
accuracy of the calculations of the power spectrum and bispectrum in the
presence of initial non-Gaussianities.Comment: 30 page
The Power Spectrum, Bias Evolution, and the Spatial Three-Point Correlation Function
We calculate perturbatively the normalized spatial skewness, , and full
three-point correlation function (3PCF), , induced by gravitational
instability of Gaussian primordial fluctuations for a biased tracer-mass
distribution in flat and open cold-dark-matter (CDM) models. We take into
account the dependence on the shape and evolution of the CDM power spectrum,
and allow the bias to be nonlinear and/or evolving in time, using an extension
of Fry's (1996) bias-evolution model. We derive a scale-dependent,
leading-order correction to the standard perturbative expression for in
the case of nonlinear biasing, as defined for the unsmoothed galaxy and
dark-matter fields, and find that this correction becomes large when probing
positive effective power-spectrum indices. This term implies that the inferred
nonlinear-bias parameter, as usually defined in terms of the smoothed density
fields, might depend on the chosen smoothing scale. In general, we find that
the dependence of on the biasing scheme can substantially outweigh that
on the adopted cosmology. We demonstrate that the normalized 3PCF, , is an
ill-behaved quantity, and instead investigate , the variance-normalized
3PCF. The configuration dependence of shows similarly strong
sensitivities to the bias scheme as , but also exhibits significant
dependence on the form of the CDM power spectrum. Though the degeneracy of
with respect to the cosmological parameters and constant linear- and
nonlinear-bias parameters can be broken by the full configuration dependence of
, neither statistic can distinguish well between evolving and non-evolving
bias scenarios. We show that this can be resolved, in principle, by considering
the redshift dependence of .Comment: 41 pages, including 12 Figures. To appear in The Astrophysical
Journal, Vol. 521, #
Perturbations of spacetime: gauge transformations and gauge invariance at second order and beyond
We consider in detail the problem of gauge dependence that exists in
relativistic perturbation theory, going beyond the linear approximation and
treating second and higher order perturbations. We first derive some
mathematical results concerning the Taylor expansion of tensor fields under the
action of one-parameter families (not necessarily groups) of diffeomorphisms.
Second, we define gauge invariance to an arbitrary order . Finally, we give
a generating formula for the gauge transformation to an arbitrary order and
explicit rules to second and third order. This formalism can be used in any
field of applied general relativity, such as cosmological and black hole
perturbations, as well as in other spacetime theories. As a specific example,
we consider here second order perturbations in cosmology, assuming a flat
Robertson-Walker background, giving explicit second order transformations
between the synchronous and the Poisson (generalized longitudinal) gauges.Comment: slightly revised version, accepted for publication in Classical and
Quantum Gravity. 27 pages including 4 figures, latex using 2 CQG style files:
ioplppt.sty, iopl10.st
Signatures of Primordial non-Gaussianities in the Matter Power-Spectrum and Bispectrum: the Time-RG Approach
We apply the time-renormalization group approach to study the effect of
primordial non-Gaussianities in the non-linear evolution of cosmological dark
matter density perturbations. This method improves the standard perturbation
approach by solving renormalization group-like equations governing the dynamics
of gravitational instability. The primordial bispectra constructed from the
dark matter density contrast and the velocity fields represent initial
conditions for the renormalization group flow. We consider local, equilateral
and folded shapes for the initial non-Gaussianity and analyze as well the case
in which the non-linear parameter f_{NL} parametrizing the strength of the
non-Gaussianity depends on the momenta in Fourier space through a power-law
relation, the so-called running non-Gaussianity. For the local model of
non-Gaussianity we compare our findings for the power-spectrum with those of
recent N-body simulations and find that they accurately fit the N-body data up
to wave-numbers k \sim 0.25 h/Mpc at z=0. We also present predictions for the
(reduced) matter bispectra for the various shapes of non-Gaussianity.Comment: 27 pages, 12 figures. Results and discussion for a particular case
added. One figure and one reference added. Matches with the version accepted
for publication in the JCAP
Light-cone averages in a swiss-cheese universe
We analyze a toy swiss-cheese cosmological model to study the averaging
problem. In our model, the cheese is the EdS model and the holes are
constructed from a LTB solution. We study the propagation of photons in the
swiss-cheese model, and find a phenomenological homogeneous model to describe
observables. Following a fitting procedure based on light-cone averages, we
find that the the expansion scalar is unaffected by the inhomogeneities. This
is because of spherical symmetry. However, the light-cone average of the
density as a function of redshift is affected by inhomogeneities. The effect
arises because, as the universe evolves, a photon spends more and more time in
the (large) voids than in the (thin) high-density structures. The
phenomenological homogeneous model describing the light-cone average of the
density is similar to the concordance model. Although the sole source in the
swiss-cheese model is matter, the phenomenological homogeneous model behaves as
if it has a dark-energy component. Finally, we study how the equation of state
of the phenomenological model depends on the size of the inhomogeneities, and
find that the equation-of-state parameters w_0 and w_a follow a power-law
dependence with a scaling exponent equal to unity. That is, the equation of
state depends linearly on the distance the photon travels through voids. We
conclude that within our toy model, the holes must have a present size of about
250 Mpc to be able to mimic the concordance model.Comment: 20 pages, 14 figures; replaced to fit the version accepted for
publication in Phys. Rev.
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