2,702 research outputs found

### Cosmic Microwave Background Anisotropies from Scaling Seeds: Generic Properties of the Correlation Functions

In this work we present a partially new method to analyze fluctuations which
are induced by causal scaling seeds. We show that the power spectra due to this
kind of seed perturbations are determined by five analytic functions, which we
determine numerically for a special example. We put forward the view that, even
if recent work disfavors the models with cosmic strings and global O(4)
texture, causal scaling seed perturbations merit a more thorough and general
analysis, which we initiate in this paper.Comment: LaTeX file with RevTex, 6 pages, 6 PS figs., submitted to Phys. Rev.
D. A version with higher quality images can be found at
http://mykonos.unige.ch/~kunz

### Dark Energy versus Modified Gravity

There is now strong observational evidence that the expansion of the Universe is accelerating. The standard explanation invokes an unknown dark energy component. But such scenarios are faced with serious theoretical problems, which has led to increased interest in models where instead general relativity is modified in a way that leads to the observed accelerated expansion. The question then arises whether the two scenarios can be distinguished. Here we show that this may not be so easy, demonstrating explicitly that a generalized dark energy model can match the growth rate of the Dvali-Gabadadze-Porrati model and reproduce the 3+1 dimensional metric perturbations. Cosmological observations are then unable to distinguish the two cases

### Microwave Background Anisotropies Induced by Global Scalar Fields: The Large N Limit

We present an analysis of CMB anisotropies induced by global scalar fields in
the large N limit. In this limit, the CMB anisotropy spectrum can be determined
without cumbersome 3D simulations. We determine the source functions and their
unequal time correlation functions and show that they are quite similar to the
corresponding functions in the texture model. This leads us to the conclusion
that the large N limit provides a 'cheap approximation' to the texture model of
structure formation.Comment: 11 pages Latex, 5 postscript figures include

### A Tale of Two Distances

There are two basic ways to measure physical distances in cosmology: One
based on standard candles and one based on standard rulers. Comparing current
data for each method allows us to rule out axion-photon mixing and
dust-extinction as the sources of supernova dimming and generally protects the
case for cosmic acceleration from attacks based on loss of photons. The
combined data constrains the energy densities in a LCDM model to 0.19 < Omega_m
< 0.32 and 0.47 < Omega_Lambda < 0.82 (at 2\sigma) without recourse to any
further data sets. Future data will improve on these limits and allow us to
place constraints on more exotic physics.Comment: 4 pages, 1 figure, uses moriond.sty. To be published in the
Proceedings of the XXXIX Rencontres de Moriond 'Exploring the Universe

### Observational constraints on the inflaton potential combined with flow-equations in inflaton space

Direct observations provide constraints on the first two derivatives of the
inflaton potential in slow roll models. We discuss how present day
observations, combined with the flow equations in slow roll parameter space,
provide a non-trivial constraint on the third derivative of the inflaton
potential. We find a lower bound on the third derivative of the inflaton
potential V'''/V > -0.2. We also show that unless the third derivative of the
inflaton potential is unreasonably large, then one predicts the tensor to
scalar ratio, r, to be bounded from below r > 3 * 10^{-6}.Comment: 4 pages, 2 figures. Important sign mistake corrected. Conclusions,
abstract and discussion change

### The traces of anisotropic dark energy in light of Planck

We study a dark energy model with non-zero anisotropic stress, either linked
to the dark energy density or to the dark matter density. We compute
approximate solutions that allow to characterise the behaviour of the dark
energy model and to assess the stability of the perturbations. We also
determine the current limits on such an anisotropic stress from the cosmic
microwave background data by the Planck satellite, and derive the corresponding
constraints on the modified growth parameters like the growth index, the
effective Newton's constant and the gravitational slip.Comment: 29 pages, 10 figures; matches version accepted for publication in
JCA

### Constraints on dark-matter properties from large-scale structure

We use large-scale cosmological observations to place constraints on the
dark-matter pressure, sound speed and viscosity, and infer a limit on the mass
of warm-dark-matter particles. Measurements of the cosmic microwave background
(CMB) anisotropies constrain the equation of state and sound speed of the dark
matter at last scattering at the per mille level. Since the redshifting of
collisionless particles universally implies that these quantities scale like
$a^{-2}$ absent shell crossing, we infer that today $w_{\rm (DM)}< 10^{-10.0}$,
$c_{\rm s,(DM)}^2 < 10^{-10.7}$ and $c_{\rm vis, (DM)}^{2} < 10^{-10.3}$ at the
$99\%$ confidence level. This very general bound can be translated to
model-dependent constraints on dark-matter models: for warm dark matter these
constraints imply $m> 70$ eV, assuming it decoupled while relativistic around
the same time as the neutrinos; for a cold relic, we show that $m>100$ eV. We
separately constrain the properties of the DM fluid on linear scales at late
times, and find upper bounds $c_{\rm s, (DM)}^2<10^{-5.9}$, $c_{\rm vis,
(DM)}^{2} < 10^{-5.7}$, with no detection of non-dust properties for the DM.Comment: 17 pages, 9 figures: v2 reflects version accepted for publication by
PRD. Added discussion of kinetic decouplin

### Intrinsic uncertainty on the nature of dark energy

We argue that there is an intrinsic noise on measurements of the equation of
state parameter $w=p/\rho$ from large-scale structure around us. The presence
of the large-scale structure leads to an ambiguity in the definition of the
background universe and thus there is a maximal precision with which we can
determine the equation of state of dark energy. To study the uncertainty due to
local structure, we model density perturbations stemming from a standard
inflationary power spectrum by means of the exact Lema\^{i}tre-Tolman-Bondi
solution of Einstein's equation, and show that the usual distribution of matter
inhomogeneities in a $\Lambda$CDM cosmology causes a variation of $w$ -- as
inferred from distance measures -- of several percent. As we observe only one
universe, or equivalently because of the cosmic variance, this uncertainty is
systematic in nature.Comment: 12 pages, 3 figures. Version as accepted for publication in Physics
of the Dark Universe (Open Access

### Cosmic Microwave Background Anisotropies from Global Texture

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 experiments we conclude that at present we cannot
safely reject the model on the grounds of published CMB anisotropy data.
If bias is close to one on large scales, galaxy correlation data rules out
the models. 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: 20 page LaTeX file, 11 postscript figs. included, proceedings to the
EC conference on 3K Cosmology in Rome, Oct. 9

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