242 research outputs found
The Cosmic Linear Anisotropy Solving System (CLASS) III: Comparision with CAMB for LambdaCDM
By confronting the two independent Boltzmann codes CLASS and CAMB, we
establish that for concordance cosmology and for a given recombination history,
lensed CMB and matter power spectra can be computed by current codes with an
accuracy of 0.01%. We list a few tiny changes in CAMB which are necessary in
order to reach such a level. Using the common limit of the two codes as a set
of reference spectra, we derive precision settings corresponding to fixed
levels of error in the computation of a CMB likelihood. We find that for a
given precision level, CLASS is about 2.5 times faster than CAMB for computing
the lensed CMB spectra of a LambdaCDM model. The nature of the main
improvements in CLASS (which may each contribute to these performances) is
discussed in companion papers.Comment: 16 pages, 4 figures, 1 table. Typos corrected, comparison extended to
lower precision settings. Code available at http://class-code.ne
TASI Lectures on Cosmological Perturbations
We present a self-contained summary of the theory of linear cosmological
perturbations. We emphasize the effect of the six parameters of the minimal
cosmological model, first, on the spectrum of Cosmic Microwave Background
temperature anisotropies, and second, on the linear matter power spectrum. We
briefly review at the end the possible impact of a few non-minimal dark matter
and dark energy models.Comment: TASI 2013 lecture note
The Cosmic Linear Anisotropy Solving System (CLASS) I: Overview
The Cosmic Linear Anisotropy Solving System (CLASS) is a new accurate
Boltzmann code, designed to offer a more user-friendly and flexible coding
environment to cosmologists. CLASS is very structured, easy to modify, and
offers a rigorous way to control the accuracy of output quantities. It is also
incidentally a bit faster than other codes. In this overview, we present the
general principles of CLASS and its basic structure. We insist on the
friendliness and flexibility aspects, while accuracy, physical approximations
and performances are discussed in a series of companion papers.Comment: 19 pages, typos corrected. Code available at http://class-code.ne
Optimal polarisation equations in FLRW universes
This paper presents the linearised Boltzmann equation for photons for scalar,
vector and tensor perturbations in flat, open and closed FLRW cosmologies. We
show that E- and B-mode polarisation for all types can be computed using only a
single hierarchy. This was previously shown explicitly for tensor modes in flat
cosmologies but not for vectors, and not for non-flat cosmologies.Comment: 27 pages, prepared for submission to JCAP. Matches published versio
Massive neutrinos and cosmology
The present experimental results on neutrino flavour oscillations provide
evidence for non-zero neutrino masses, but give no hint on their absolute mass
scale, which is the target of beta decay and neutrinoless double-beta decay
experiments. Crucial complementary information on neutrino masses can be
obtained from the analysis of data on cosmological observables, such as the
anisotropies of the cosmic microwave background or the distribution of
large-scale structure. In this review we describe in detail how free-streaming
massive neutrinos affect the evolution of cosmological perturbations. We
summarize the current bounds on the sum of neutrino masses that can be derived
from various combinations of cosmological data, including the most recent
analysis by the WMAP team. We also discuss how future cosmological experiments
are expected to be sensitive to neutrino masses well into the sub-eV range.Comment: 122 pages, 23 figures, misprints corrected and references added.
Review article to be published in Physics Report
Dark goo: Bulk viscosity as an alternative to dark energy
We present a simple (microscopic) model in which bulk viscosity plays a role
in explaining the present acceleration of the universe. The effect of bulk
viscosity on the Friedmann equations is to turn the pressure into an
"effective" pressure containing the bulk viscosity. For a sufficiently large
bulk viscosity, the effective pressure becomes negative and could mimic a dark
energy equation of state. Our microscopic model includes self-interacting
spin-zero particles (for which the bulk viscosity is known) that are added to
the usual energy content of the universe. We study both background equations
and linear perturbations in this model. We show that a dark energy behavior is
obtained for reasonable values of the two parameters of the model (i.e. the
mass and coupling of the spin-zero particles) and that linear perturbations are
well-behaved. There is no apparent fine tuning involved. We also discuss the
conditions under which hydrodynamics holds, in particular that the spin-zero
particles must be in local equilibrium today for viscous effects to be
important.Comment: 21 pages, 14 figures. References added, typos corrected, figure 2
corrected, a few comments added, results unchange
How to constrain inflationary parameter space with minimal priors
We update constraints on the Hubble function H(phi) during inflation, using
the most recent cosmic microwave background (CMB) and large scale structure
(LSS) data. Our main focus is on a comparison between various commonly used
methods of calculating the primordial power spectrum via analytical
approximations and the results obtained by integrating the exact equations
numerically. In each case, we impose naive, minimally restrictive priors on the
duration of inflation. We find that the choice of priors has an impact on the
results: the bounds on inflationary parameters can vary by up to a factor two.
Nevertheless, it should be noted that within the region allowed by the minimal
prior of the exact method, the accuracy of the approximations is sufficient for
current data. We caution however that a careless minimal implementation of the
approximative methods allows models for which the assumptions behind the
analytical approximations fail, and recommend using the exact numerical method
for a self-consistent analysis of cosmological data.Comment: 16 pages, 3 figure
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