567 research outputs found
Cosmic Microwave Background Anisotropies from Scaling Seeds: Fit to Observational Data
We compute cosmic microwave background angular power spectra for scaling seed
models of structure formation. A generic parameterization of the energy
momentum tensor of the seeds is employed. We concentrate on two regions of
parameter space inspired by global topological defects: O(4) texture models and
the large-N limit of O(N) models. We use fitting to compare these
models to recent flat-band power measurements of the cosmic microwave
background. Only scalar perturbations are considered.Comment: LaTeX file 4 pages, 4 postscript figs. revised version, to appear in
PR
Vector and Tensor Contributions to the Luminosity Distance
We compute the vector and tensor contributions to the luminosity distance
fluctuations in first order perturbation theory and we expand them in spherical
harmonics. This work presents the formalism with a first application to a
stochastic background of primordial gravitational waves.Comment: 14 pages, 3 figure
Effects of biasing on the galaxy power spectrum at large scales
n this paper we study the effect of biasing on the power spectrum at large
scales. We show that even though non-linear biasing does introduce a white
noise contribution on large scales, the behavior of the
matter power spectrum on large scales may still be visible and above the white
noise for about one decade. We show, that the Kaiser biasing scheme which leads
to linear bias of the correlation function on {\em large} scales, also
generates a linear bias of the {\rm power spectrum} on rather small scales.
This is a consequence of the divergence on small scales of the pure
Harrison-Zeldovich spectrum. However, biasing becomes k-dependent when we damp
the underlying power spectrum on small scales. We also discuss the effect of
biasing on the baryon acoustic oscillations.Comment: 9 pages, 4 figures. One figure and comments clarifying the linear
biasing on small scales and references added. V3 version accepted in PR
Crossing the Phantom Divide
We consider fluid perturbations close to the "phantom divide" characterised
by p = -rho and discuss the conditions under which divergencies in the
perturbations can be avoided. We find that the behaviour of the perturbations
depends crucially on the prescription for the pressure perturbation delta-p.
The pressure perturbation is usually defined using the dark energy rest-frame,
but we show that this frame becomes unphysical at the divide. If the pressure
perturbation is kept finite in any other frame, then the phantom divide can be
crossed. Our findings are important for generalised fluid dark energy used in
data analysis (since current cosmological data sets indicate that the dark
energy is characterised by p ~ -rho so that p < -rho cannot be excluded) as
well as for any models crossing the phantom divide, like some modified gravity,
coupled dark energy and braneworld models. We also illustrate the results by an
explicit calculation for the "Quintom" case with two scalar fields.Comment: 14 pages, 10 figures, v2: updated to agree with published version:
more readable figures, slightly expanded discussion on modified gravity
models and the interpolation across w=-1, results and conclusions unchange
Large Scale Structure Formation with Global Topological Defects. A new Formalism and its implementation by numerical simulations
We investigate cosmological structure formation seeded by topological defects
which may form during a phase transition in the early universe. First we derive
a partially new, local and gauge invariant system of perturbation equations to
treat microwave background and dark matter fluctuations induced by topological
defects or any other type of seeds. We then show that this system is well
suited for numerical analysis of structure formation by applying it to seeds
induced by fluctuations of a global scalar field. Our numerical results are
complementary to previous investigations since we use substantially different
methods. The resulting microwave background fluctuations are compatible with
older simulations. We also obtain a scale invariant spectrum of fluctuations
with about the same amplitude. However, our dark matter results yield a smaller
bias parameter compatible with on a scale of in contrast to
previous work which yielded to large bias factors. Our conclusions are thus
more positive. According to the aspects analyzed in this work, global
topological defect induced fluctuations yield viable scenarios of structure
formation and do better than standard CDM on large scales.Comment: uuencoded, compressed tar-file containing the text in LaTeX and 12
Postscript Figures, 41 page
Cosmic Microwave Background Anisotropies from Scaling Seeds: Global Defect Models
We investigate the global texture model of structure formation in cosmogonies
with non-zero cosmological constant for different values of the Hubble
parameter. We find that the absence of significant acoustic peaks and little
power on large scales are robust predictions of these models. However, from a
careful comparison with data we conclude that at present we cannot safely
reject the model on the grounds of present CMB data. Exclusion by means of
galaxy correlation data requires assumptions on biasing and statistics. New,
very stringent constraints come from peculiar velocities.
Investigating the large-N limit, we argue that our main conclusions apply to
all global O(N) models of structure formation.Comment: LaTeX file with RevTex, 27 pages, 23 eps figs., submitted to Phys.
Rev. D. A version with higher quality images can be found at
http://mykonos.unige.ch/~kunz/download/lam.tar.gz for the LaTeX archive and
at http://mykonos.unige.ch/~kunz/download/lam.ps.gz for the compiled
PostScript fil
Non-Gaussianities due to Relativistic Corrections to the Observed Galaxy Bispectrum
High-precision constraints on primordial non-Gaussianity (PNG) will
significantly improve our understanding of the physics of the early universe.
Among all the subtleties in using large scale structure observables to
constrain PNG, accounting for relativistic corrections to the clustering
statistics is particularly important for the upcoming galaxy surveys covering
progressively larger fraction of the sky. We focus on relativistic projection
effects due to the fact that we observe the galaxies through the light that
reaches the telescope on perturbed geodesics. These projection effects can give
rise to an effective that can be misinterpreted as the primordial
non-Gaussianity signal and hence is a systematic to be carefully computed and
accounted for in modelling of the bispectrum. We develop the technique to
properly account for relativistic effects in terms of purely observable
quantities, namely angles and redshifts. We give some examples by applying this
approach to a subset of the contributions to the tree-level bispectrum of the
observed galaxy number counts calculated within perturbation theory and
estimate the corresponding non-Gaussianity parameter, , for the
local, equilateral and orthogonal shapes. For the local shape, we also compute
the local non-Gaussianity resulting from terms obtained using the consistency
relation for observed number counts. Our goal here is not to give a precise
estimate of for each shape but rather we aim to provide a scheme
to compute the non-Gaussian contamination due to relativistic projection
effects. For the terms considered in this work, we obtain contamination of
.Comment: 31 pages, 6 figures, Typos corrected to match the published version
in JCA
A note on perfect scalar fields
We derive a condition on the Lagrangian density describing a generic, single,
non-canonical scalar field, by demanding that the intrinsic, non-adiabatic
pressure perturbation associated with the scalar field vanishes identically.
Based on the analogy with perfect fluids, we refer to such fields as perfect
scalar fields. It is common knowledge that models that depend only on the
kinetic energy of the scalar field (often referred to as pure kinetic models)
possess no non-adiabatic pressure perturbation. While we are able to construct
models that seemingly depend on the scalar field and also do not contain any
non-adiabatic pressure perturbation, we find that all such models that we
construct allow a redefinition of the field under which they reduce to pure
kinetic models. We show that, if a perfect scalar field drives inflation, then,
in such situations, the first slow roll parameter will always be a
monotonically decreasing function of time. We point out that this behavior
implies that these scalar fields can not lead to features in the inflationary,
scalar perturbation spectrum.Comment: v1: 11 pages; v2: 11 pages, minor changes, journal versio
Universal upper limit on inflation energy scale from cosmic magnetic field
Recently observational lower bounds on the strength of cosmic magnetic fields
were reported, based on gamma-ray flux from distant blazars. If inflation is
responsible for the generation of such magnetic fields then the inflation
energy scale is bounded from above as rho_{inf}^{1/4} < 2.5 times 10^{-7}M_{Pl}
times (B_{obs}/10^{-15}G)^{-2} in a wide class of inflationary magnetogenesis
models, where B_{obs} is the observed strength of cosmic magnetic fields. The
tensor-to-scalar ratio is correspondingly constrained as r< 10^{-19} times
(B_{obs}/10^{-15}G)^{-8}. Therefore, if the reported strength B_{obs} \geq
10^{-15}G is confirmed and if any signatures of gravitational waves from
inflation are detected in the near future, then our result indicates some
tensions between inflationary magnetogenesis and observations.Comment: 12pages, v2: several discussions and references added, version
accepted for publication by JCA
Accelerated expansion of the Universe driven by dynamic self-interaction
We establish a new model, which takes into account a dynamic (inertial)
self-interaction of gravitating systems. The model is formulated by
introduction of a new function depending on the square of the covariant
derivative of the velocity four-vector of the system as a whole into the
Lagrangian. This term is meant for description of both self-action of the
system irregularly moving in the gravitational field, and back-reaction of the
motion irregularities on the gravity field. We discuss one example of exact
solution to the extended master equations in the framework of cosmological
model of the FLRW type with vanishing cosmological constant. It is shown that
accelerated expansion of the Universe can be driven by traditional matter with
positive pressure (e.g., dust, ultrarelativistic fluid) due to the
back-reaction of the gravity field induced by irregular motion of the system as
a whole; this back-reaction is shown to be characterized by the negative
effective pressure.Comment: 9 pages, no figures, accepted for publication in Phys.Letters
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