93 research outputs found
Slow-Roll Suppression of Adiabatic Instabilities in Coupled Scalar Field-Dark Matter Models
We study the evolution of linear density perturbations in the context of
interacting scalar field-dark matter cosmologies, where the presence of the
coupling acts as a stabilization mechanism for the runaway behavior of the
scalar self-interaction potential as in the case of the Chameleon model. We
show that in the "adiabatic" background regime of the system the rise of
unstable growing modes of the perturbations is suppressed by the slow-roll
dynamics of the field. Furthermore the coupled system behaves as an
inhomogeneous adiabatic fluid. In contrast instabilities may develop for large
values of the coupling constant, or along non-adiabatic solutions,
characterized by a period of high-frequency dumped oscillations of the scalar
field. In the latter case the dynamical instabilities of the field
fluctuations, which are typical of oscillatory scalar field regimes, are
amplified and transmitted by the coupling to dark matter perturbations.Comment: 6 pages, 3 figures; minor changes, corrected figure 2, added
references, matches PRD published versio
The innocuousness of adiabatic instabilities in coupled scalar field-dark matter models
Non-minimally coupled scalar field models suffer of unstable growing modes at
the linear perturbation level. The nature of these instabilities depends on the
dynamical state of the scalar field. In particular in systems which admit
adiabatic solutions, large scale instabilities are suppressed by the slow-roll
dynamics of the field. Here we review these results and present a preliminary
likelihood data analysis suggesting that along adiabatic solutions coupled
models with coupling of order of gravitational strength can provide viable
cosmological scenarios satisfying constraints from SN Ia, CMB and large scale
structure data.Comment: 6 pages, 4 figures; to be published in the AIP proceedings of the
'Invisible Universe International Conference', UNESCO-Paris, June 29-July 3,
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Statistical Test of Distance--Duality Relation with Type Ia Supernovae and Baryon Acoustic Oscillations
We test the distance--duality relation between cosmological luminosity distance () from the JLA SNe Ia
compilation (arXiv:1401.4064) and angular-diameter distance () based on
Baryon Oscillation Spectroscopic Survey (BOSS; arXiv:1607.03155) and WiggleZ
baryon acoustic oscillation measurements (arXiv:1105.2862, arXiv:1204.3674).
The measurements are matched to redshift by a statistically
consistent compression procedure. With Monte Carlo methods, nontrivial and
correlated distributions of can be explored in a straightforward manner
without resorting to a particular evolution template . Assuming
independent constraints on cosmological parameters that are necessary to obtain
and values, we find 9% constraints consistent with from
the analysis of SNIa + BOSS and an 18% bound results from SNIa + WiggleZ. These
results are contrary to previous claims that has been found close to
or above the level. We discuss the effect of different cosmological
parameter inputs and the use of the apparent deviation from distance--duality
as a proxy of systematic effects on cosmic distance measurements. The results
suggest possible systematic overestimation of SNIa luminosity distances
compared with data when a Planck {\Lambda}CDM cosmological parameter
inference (arXiv:1502.01589) is used to enhance the precision. If interpreted
as an extinction correction due to a gray dust component, the effect is broadly
consistent with independent observational constraints.Comment: v1: Initial analysis; v2: Using BOSS DR12 consensus BAO data w/
expanded analysis; v3: Major revision & expansion; v4: Matching ApJ-accepted
version, 11 pages, 4 tables, 6 figures. Code & data for replication: see
https://doi.org/10.5281/zenodo.1219473 Comments welcom
Structural properties of artificial halos in non-standard dark matter simulations
Artificial fragmentation of the matter density field causes the formation of
spurious groups of particles in N-body simulations of non-standard Dark Matter
(DM) models which are characterized by a small scale cut-off in the linear
matter power spectrum. These spurious halos alter the prediction of the mass
function in a range of masses where differences among DM models are most
relevant to observational tests. Using a suite of high resolution simulations
we show that the contamination of artificial groups of particles significantly
affect the statistics of halo spin, shape and virial state parameters. We find
that spurious halos have systematically larger spin values, are highly
elliptical or prolate and significantly deviate from virial equilibrium. These
characteristics allow us to detect the presence of spurious halos even in
non-standard DM models for which the low-mass end of the mass function remains
well behaved. We show that selecting halos near the virial equilibrium provides
a simple and effective method to remove the bulk of spurious halos from
numerical halo catalogs and consistently recover the halo mass function at low
masses.Comment: 9 pages, 9 figures, 1 table, added analysis from higher resolution
simulation
Phenomenological aspects of dark energy dominated cosmologies
In this thesis we study some observational consequences of dark energy
dominated cosmologies. After reviewing the main characteristics of
quintessential models of dark energy, by using the SN Ia data and the position
of the CMB peaks we constrain a class of parameterized scalar field potentials.
Going beyond constraining specific classes models, we introduce a
parameterization of the dark energy equation of state. Such an approach allows
us to study in a model independent way the imprint dark energy leaves in the
CMB power spectrum. Finally we develop a formalism to model localized CMB
anisotropies and compute analytical formulae for the power spectrum and the
bispectrum.Comment: PhD thesis, University of Sussex, 109 page
Testing Cosmology with Cosmic Sound Waves
WMAP observations have accurately determined the position of the first two peaks and dips in the CMB temperature power spectrum. These encode information on the ratio of the distance to the last scattering surface to the sound horizon at decoupling. However pre-recombination processes can contaminate this distance information. In order to assess the amplitude of these effects we use the WMAP data and evaluate the relative differences of the CMB peaks and dips multipoles. We find that the position of the first peak is largely displaced with the respect to the expected position of the sound horizon scale at decoupling. In contrast the relative spacings of the higher extrema are statistically consistent with those expected from perfect harmonic oscillations. This provides evidence for a scale dependent phase shift of the CMB oscillations which is caused by gravitational driving forces affecting the propagation of sound waves before recombination. By accounting for these effects we have performed a MCMC likelihood analysis of the location of WMAP extrema to constrain in combination with recent BAO data a constant dark energy equation of state parameter w. For a flat universe we find a strong 2 sigma upper limit w<-1.10, and including the HST prior we obtain w<-1.14. On the other hand we infer larger limits for non-flat cosmologies. From the full CMB likelihood analysis we also estimate the values of the shift parameter R and the mu ltipole l_a of the acoustic horizon at decoupling for several cosmologies to test their dependence on model assumptions. Although the analysis of the full CMB spectra should be always preferred, using the position of the CMB peaks and dips provide a simple and consistent method for combining CMB constraints with other datasets
The essence of quintessence and the cost of compression
Standard two-parameter compressions of the infinite dimensional dark energy
model space show crippling limitations even with current SN-Ia data. Firstly
they cannot cope with rapid evolution - the best-fit to the latest SN-Ia data
shows late and very rapid evolution to w_0 = -2.85. However all of the standard
parametrisations (incorrectly) claim that this best-fit is ruled out at more
than 2-sigma, primarily because they track it well only at very low redshifts,
z < 0.2. Further they incorrectly rule out the observationally acceptable
region w 1. Secondly the parametrisations give wildly different
estimates for the redshift of acceleration, which vary from z_{acc}=0.14 to
z_{acc}=0.59. Although these failings are largely cured by including
higher-order terms (3 or 4 parameters) this results in new degeneracies which
open up large regions of previously ruled-out parameter space. Finally we test
the parametrisations against a suite of theoretical quintessence models. The
widely used linear expansion in z is generally the worst, with errors of up to
10% at z=1 and 20% at z > 2. All of this casts serious doubt on the usefulness
of the standard two-parameter compressions in the coming era of high-precision
dark energy cosmology and emphasises the need for decorrelated compressions
with at least three parameters.Comment: 7 pages, 4 colour figures, EmulateApJ; v2: includes Bayesian evidence
analysis and table that were only present in published version, because of
increased interest in Bayesian model comparison (no new material beyond the
one in the published ApJL of 2004
The Linear Point: A cleaner cosmological standard ruler
We show how a characteristic length scale imprinted in the galaxy two-point
correlation function, dubbed the "linear point", can serve as a comoving
cosmological standard ruler. In contrast to the Baryon Acoustic Oscillation
peak location, this scale is constant in redshift and is unaffected by
non-linear effects to within percent precision. We measure the location
of the linear point in the galaxy correlation function of the LOWZ and CMASS
samples from the Twelfth Data Release (DR12) of the Baryon Oscillation
Spectroscopic Survey (BOSS) collaboration. We combine our linear-point
measurement with cosmic-microwave-background constraints from the Planck
satellite to estimate the isotropic-volume distance , without relying
on a model-template or reconstruction method. We find
Mpc and Mpc respectively, consistent with the quoted
values from the BOSS collaboration. This remarkable result suggests that all
the distance information contained in the baryon acoustic oscillations can be
conveniently compressed into the single length associated with the linear
point.Comment: The optimal two-point correlation function bin-size is employed.
Results are updated and the distance constraints are improve
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