20 research outputs found
WMAP constraints on inflationary models with global defects
We use the cosmic microwave background angular power spectra to place upper
limits on the degree to which global defects may have aided cosmic structure
formation. We explore this under the inflationary paradigm, but with the
addition of textures resulting from the breaking of a global O(4) symmetry
during the early stages of the Universe. As a measure of their contribution, we
use the fraction of the temperature power spectrum that is attributed to the
defects at a multipole of 10. However, we find a parameter degeneracy enabling
a fit to the first-year WMAP data to be made even with a significant defect
fraction. This degeneracy involves the baryon fraction and the Hubble constant,
plus the normalization and tilt of the primordial power spectrum. Hence,
constraints on these cosmological parameters are weakened. Combining the WMAP
data with a constraint on the physical baryon fraction from big bang
nucleosynthesis calculations and high-redshift deuterium abundance, limits the
extent of the degeneracy and gives an upper bound on the defect fraction of
0.13 (95% confidence).Comment: 10pp LaTeX/RevTeX, 6 eps figs; matches accepted versio
CMBR Weak Lensing and HI 21-cm Cross-correlation Angular Power Spectrum
Weak gravitational lensing of the CMBR manifests as a secondary anisotropy in
the temperature maps. The effect, quantified through the shear and convergence
fields imprint the underlying large scale structure (LSS), geometry and
evolution history of the Universe. It is hence perceived to be an important
observational probe of cosmology. De-lensing the CMBR temperature maps is also
crucial for detecting the gravitational wave generated B-modes. Future
observations of redshifted 21-cm radiation from the cosmological neutral
hydrogen (HI) distribution hold the potential of probing the LSS over a large
redshift range. We have investigated the correlation between post-reionization
HI signal and weak lensing convergence field. Assuming that the HI follows the
dark matter distribution, the cross-correlation angular power spectrum at a
multipole \ell is found to be proportional to the cold dark matter power
spectrum evaluated at \ell/r, where r denotes the comoving distance to the
redshift where the HI is located. The amplitude of the ross-correlation depends
on quantities specific to the HI distribution, growth of perturbations and also
the underlying cosmological model. In an ideal ituation, we found that a
statistically significant detection of the cross-correlation signal is
possible. If detected, the cross-correlation signal hold the possibility of a
joint estimation of cosmological parameters and also test various CMBR
de-lensing estimators.Comment: 14 pages, 4 figures, publishe
The CMBR ISW and HI 21-cm Cross-correlation Angular Power Spectrum
The late-time growth of large scale structures (LSS) is imprinted in the CMBR
anisotropy through the Integrated Sachs Wolfe (ISW) effect. This is perceived
to be a very important observational probe of dark energy. Future observations
of redshifted 21-cm radiation from the cosmological neutral hydrogen (HI)
distribution hold the potential of probing the LSS over a large redshift range.
We have investigated the possibility of detecting the ISW through
cross-correlations between the CMBR anisotropies and redshifted 21-cm
observations. Assuming that the HI traces the dark matter, we find that the
ISW-HI cross-correlation angular power spectrum at an angular multipole l is
proportional to the dark matter power spectrum evaluated at the comoving wave
number l/r, where r is the comoving distance to the redshift from which the HI
signal originated. The amplitude of the cross-correlation signal depends on
parameters related to the HI distribution and the growth of cosmological
perturbations. However the cross-correlation is extremely weak as compared to
the CMBR anisotropies and the predicted HI signal. As a consequence the
cross-correlation signal is smaller than the cosmic variance, and a
statistically significant detection is not very likely.Comment: 13 pages, 4 eps figures, submitte
Large Angle CMB Fluctuations from Cosmic Strings with a Comological Constant
In this paper, we present results for large-angle CMB anisotropies generated
from high resolution simulations of cosmic string networks in a range of flat
FRW universes with a cosmological constant. Using an ensemble of all-sky maps,
we compare with the COBE data to infer a normalization (or upper bound) on the
string linear energy density . For a flat matter-dominated model
() we find , which is lower
than previous constraints probably because of the more accurate inclusion of
string small-scale structure. For a cosmological constant within an
observationally acceptable range, we find a relatively weak dependence with
less than 10% higher.Comment: 5 pages, 5 figures; replaced with version to appear in Physical
Review
Can inflationary models of cosmic perturbations evade the secondary oscillation test?
We consider the consequences of an observed Cosmic Microwave Background (CMB)
temperature anisotropy spectrum containing no secondary oscillations. While
such a spectrum is generally considered to be a robust signature of active
structure formation, we show that such a spectrum {\em can} be produced by
(very unusual) inflationary models or other passive evolution models. However,
we show that for all these passive models the characteristic oscillations would
show up in other observable spectra. Our work shows that when CMB polarization
and matter power spectra are taken into account secondary oscillations are
indeed a signature of even these very exotic passive models. We construct a
measure of the observability of secondary oscillations in a given experiment,
and show that even with foregrounds both the MAP and \pk satellites should be
able to distinguish between models with and without oscillations. Thus we
conclude that inflationary and other passive models can {\em not} evade the
secondary oscillation test.Comment: Final version accepted for publication in PRD. Minor improvements
have been made to the discussion and new data has been included. The
conclusions are unchagne
The Formation of Cosmic Structures in a Light Gravitino Dominated Universe
We analyse the formation of cosmic structures in models where the dark matter
is dominated by light gravitinos with mass of eV -- 1 keV, as predicted
by gauge-mediated supersymmetry (SUSY) breaking models. After evaluating the
number of degrees of freedom at the gravitinos decoupling (), we compute
the transfer function for matter fluctuations and show that gravitinos behave
like warm dark matter (WDM) with free-streaming scale comparable to the galaxy
mass scale. We consider different low-density variants of the WDM model, both
with and without cosmological constant, and compare the predictions on the
abundances of neutral hydrogen within high-redshift damped Ly-- systems
and on the number density of local galaxy clusters with the corresponding
observational constraints. We find that none of the models satisfies both
constraints at the same time, unless a rather small value (\mincir
0.4) and a rather large Hubble parameter (\magcir 0.9) is assumed.
Furthermore, in a model with warm + hot dark matter, with hot component
provided by massive neutrinos, the strong suppression of fluctuation on scales
of \sim 1\hm precludes the formation of high-redshift objects, when the
low-- cluster abundance is required. We conclude that all different variants
of a light gravitino DM dominated model show strong difficulties for what
concerns cosmic structure formation.
This gives a severe cosmological constraint on the gauge-mediated SUSY
breaking scheme.Comment: 28 pages,Latex, submitted for publication to Phys.Rev.
Numerical simulations of the Warm-Hot Intergalactic Medium
In this paper we review the current predictions of numerical simulations for
the origin and observability of the warm hot intergalactic medium (WHIM), the
diffuse gas that contains up to 50 per cent of the baryons at z~0. During
structure formation, gravitational accretion shocks emerging from collapsing
regions gradually heat the intergalactic medium (IGM) to temperatures in the
range T~10^5-10^7 K. The WHIM is predicted to radiate most of its energy in the
ultraviolet (UV) and X-ray bands and to contribute a significant fraction of
the soft X-ray background emission. While O VI and C IV absorption systems
arising in the cooler fraction of the WHIM with T~10^5-10^5.5 K are seen in
FUSE and HST observations, models agree that current X-ray telescopes such as
Chandra and XMM-Newton do not have enough sensitivity to detect the hotter
WHIM. However, future missions such as Constellation-X and XEUS might be able
to detect both emission lines and absorption systems from highly ionised atoms
such as O VII, O VIII and Fe XVII.Comment: 18 pages, 5 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 14; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
Reionization by active sources and its effects on the cosmic microwave background
We investigate the possible effects of reionization by active sources on the
cosmic microwave background. We concentrate on the sources themselves as the
origin of reionization, rather than early object formation, introducing an
extra period of heating motivated by the active character of the perturbations.
Using reasonable parameters, this leads to four possibilities depending on the
time and duration of the energy input: delayed last scattering, double last
scattering, shifted last scattering and total reionization. We show that these
possibilities are only very weakly constrained by the limits on spectral
distortions from the COBE FIRAS measurements. We illustrate the effects of
these reionization possibilities on the angular power spectrum of temperature
anisotropies and polarization for simple passive isocurvature models and simple
coherent sources, observing the difference between passive and active models.
Finally, we comment on the implications of this work for more realistic active
sources, such as causal white noise and topological defect models. We show for
these models that non-standard ionization histories can shift the peak in the
CMB power to larger angular scales.Comment: 21 pages LaTeX with 11 eps figures; replaced with final version
accepted for publication in Phys. Rev.
Weak lensing, dark matter and dark energy
Weak gravitational lensing is rapidly becoming one of the principal probes of
dark matter and dark energy in the universe. In this brief review we outline
how weak lensing helps determine the structure of dark matter halos, measure
the expansion rate of the universe, and distinguish between modified gravity
and dark energy explanations for the acceleration of the universe. We also
discuss requirements on the control of systematic errors so that the
systematics do not appreciably degrade the power of weak lensing as a
cosmological probe.Comment: Invited review article for the GRG special issue on gravitational
lensing (P. Jetzer, Y. Mellier and V. Perlick Eds.). V3: subsection on
three-point function and some references added. Matches the published versio