46 research outputs found
How much of the inflaton potential do we see?
We discuss the latest constraints on a Taylor-expanded scalar inflaton
potential, obtained focusing on its observable part only. This is in contrast
with other works in which an extrapolation of the potential is applied using
the slow-roll hierarchy. We find significant differences. The results discussed
here apply to a broader range of models, since no assumption about the
invisible e-folds of inflation has to be made, thereby remaining conservative.Comment: 5 pages, 2 figures. Talk given at Cargese Summer School: Cosmology
and Particle Physics Beyond the Standard Models. To appear in Po
On the accuracy of N-body simulations at very large scales
We examine the deviation of Cold Dark Matter particle trajectories from the
Newtonian result as the size of the region under study becomes comparable to or
exceeds the particle horizon. To first order in the gravitational potential,
the general relativistic result coincides with the Zel'dovich approximation and
hence the Newtonian prediction on all scales. At second order, General
Relativity predicts corrections which overtake the corresponding second order
Newtonian terms above a certain scale of the order of the Hubble radius.
However, since second order corrections are very much suppressed on such
scales, we conclude that simulations which exceed the particle horizon but use
Newtonian equations to evolve the particles, reproduce the correct trajectories
very well. The dominant relativistic corrections to the power spectrum on
scales close to the horizon are at most of the order of at
and at . The differences in the positions of real
space features are affected at a level below at both redshifts. Our
analysis also clarifies the relation of N-body results to relativistic
considerations.Comment: 7 pages, 2 figures; v2: 7 pages, 3 figures, matches version published
in MNRA
Accurate initial conditions in mixed Dark Matter--Baryon simulations
We quantify the error in the results of mixed baryon--dark-matter
hydrodynamic simulations, stemming from outdated approximations for the
generation of initial conditions. The error at redshift 0 in contemporary large
simulations, is of the order of few to ten percent in the power spectra of
baryons and dark matter, and their combined total-matter power spectrum. After
describing how to properly assign initial displacements and peculiar velocities
to multiple species, we review several approximations: (1) {using the
total-matter power spectrum to compute displacements and peculiar velocities of
both fluids}, (2) scaling the linear redshift-zero power spectrum back to the
initial power spectrum using the Newtonian growth factor ignoring homogeneous
radiation, (3) using longitudinal-gauge velocities with synchronous-gauge
densities, and (4) ignoring the phase-difference in the Fourier modes for the
offset baryon grid, relative to the dark-matter grid. Three of these
approximations do not take into account that dark matter and baryons experience
a scale-dependent growth after photon decoupling, which results in directions
of velocity which are not the same as their direction of displacement. We
compare the outcome of hydrodynamic simulations with these four approximations
to our reference simulation, all setup with the same random seed and simulated
using Gadget-III.Comment: 10 pages, 5 figure
Swiss Cheese and a Cheesy CMB
It has been argued that the Swiss-Cheese cosmology can mimic Dark Energy,
when it comes to the observed luminosity distance-redshift relation. Besides
the fact that this effect tends to disappear on average over random directions,
we show in this work that based on the Rees-Sciama effect on the cosmic
microwave background (CMB), the Swiss-Cheese model can be ruled out if all
holes have a radius larger than about 35 Mpc. We also show that for smaller
holes, the CMB is not observably affected, and that the small holes can still
mimic Dark Energy, albeit in special directions, as opposed to previous
conclusions in the literature. However, in this limit, the probability of
looking in a special direction where the luminosity of supernovae is
sufficiently supressed becomes very small, at least in the case of a lattice of
spherical holes considered in this paper.Comment: 23 pages, 10 figures. Matches published versio
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
Intrinsic uncertainty on the nature of dark energy
We argue that there is an intrinsic noise on measurements of the equation of
state parameter 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 CDM cosmology causes a variation of -- 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