937 research outputs found
Redshift remapping and cosmic acceleration in dark-matter-dominated cosmological models
The standard relation between the cosmological redshift and cosmic scale
factor underlies cosmological inference from virtually all kinds of
cosmological observations, leading to the emergence of the LambdaCDM
cosmological model. This relation is not a fundamental theory and thus
observational determination of this function (redshift remapping) should be
regarded as an insightful alternative to holding its standard form in analyses
of cosmological data. Here we present non-parametric reconstructions of
redshift remapping in dark-matter-dominated models and constraints on
cosmological parameters from a joint analysis of all primary cosmological
probes including the local measurement of the Hubble constant, Type Ia
supernovae, baryonic acoustic oscillations (BAO), Planck observations of the
cosmic microwave background (CMB) radiation (temperature power spectrum) and
cosmic chronometers. The reconstructed redshift remapping points to an
additional boost of redshift operating in late epoch of cosmic evolution, but
affecting both low-redshift observations and the CMB. The model predicts a
significant difference between the actual Hubble constant, h=0.48+/-0.02, and
its local determination, h_obs=0.73+/-0.02. The ratio of these two values
coincides closely with the maximum expansion rate inside voids formed in the
corresponding open cosmological model with Omega_m=0.87+/-0.03, whereas the
actual value of the Hubble constant implies the age of the Universe that is
compatible with the Planck LambdaCDM cosmology. The new dark-matter-dominated
model with redshift remapping provides excellent fits to all data and
eliminates recently reported tensions between the Planck LambdaCDM cosmology,
the local determination of the Hubble constant and the BAO measurements from
the Ly-alpha forest of high-redshift quasars.Comment: 21 pages, 11 figures, 4 tables; accepted for publication in MNRA
New Parametrizations of Non-Gaussian Line-of-sight Velocity Distribution
A five-parameter fitting formula for the line-of-sight stellar velocity
distributions of steady state systems is proposed. It can faithfully reproduce
velocity distributions of theoretical models ranging from nearly Gaussian
profiles to strongly skewed or mildly double-peaked profiles. In contrast to
van der Marel and Franx (1993) and Kuijken and Merrifield (1993), the line
profiles are required to have neither multi-peaks nor negative velocity wings.
Information of the profile is mostly specified by five physically meaningful
and nearly orthogonal fitting parameters.Comment: submitted to MNRAS; 22 pages with 3 tables and 8 figures in uuencoded
compressed PS file. Also available at
ftp://ibm-1.mpa-garching.mpg.de/pub/hsz/profile.u
Effects of long-wavelength fluctuations in large galaxy surveys
In order to capture as much information as possible large galaxy surveys have
been increasing their volume and redshift depth. To face this challenge theory
has responded by making cosmological simulations of huge computational volumes
with equally increasing the number of dark matter particles and supercomputing
resources. Thus, it is taken for granted that the ideal situation is when a
single computational box encompasses the whole effective volume of the
observational survey, e.g., ~50 Gpch^3 for the DESI and Euclid surveys. Here we
study the effects of missing long-waves in a finite volume using several
relevant statistics: the abundance of dark matter halos, the PDF, the
correlation function and power spectrum, and covariance matrices. Finite volume
effects can substantially modify the results if the computational volumes are
less than ~(500Mpch)^3. However, the effects become extremely small and
practically can be ignored when the box-size exceeds ~1Gpch^3. We find that the
average power spectra of dark matter fluctuations show remarkable lack of
dependence on the computational box-size with less than 0.1% differences
between 1Gpch and 4Gpch boxes. No measurable differences are expected for the
halo mass functions for these volumes. The covariance matrices are scaled
trivially with volume, and small corrections due to super-sample modes can be
added. We conclude that there is no need to make those extremely large
simulations when a box-size of 1-1.5Gpch is sufficient to fulfil most of the
survey science requirements.Comment: 15 pages, 14 figures, accepted to MNRA
Testing the mapping between redshift and cosmic scale factor
The canonical redshift-scale factor relation, 1/a=1+z, is a key element in
the standard LambdaCDM model of the big bang cosmology. Despite its fundamental
role, this relation has not yet undergone any observational tests since
Lemaitre and Hubble established the expansion of the Universe. It is strictly
based on the assumption of the Friedmann-Lemaitre-Robertson-Walker metric
describing a locally homogeneous and isotropic universe and that photons move
on null geodesics of the metric. Thus any violation of this assumption, within
general relativity or modified gravity, can yield a different mapping between
the model redshift z=1/a-1 and the actually observed redshift z_obs, i.e. z_obs
neq z. Here we perform a simple test of consistency for the standard
redshift-scale factor relation by determining simultaneous observational
constraints on the concordance LambdaCDM cosmological parameters and a
generalized redshift mapping z=f(z_obs). Using current baryon acoustic
oscillations (BAO) and Type Ia supernova (SN) data we demonstrate that the
generalized redshift mapping is strongly degenerated with dark energy.
Marginalization over a class of monotonic functions f(z_obs) changes
substantially degeneracy between matter and dark energy density: the density
parameters become anti correlated with nearly vertical axis of degeneracy.
Furthermore, we show that current SN and BAO data, analysed in a framework with
the generalized redshift mapping, do not constrain dark energy unless the BAO
data include the measurements from the Ly-alpha forest of high-redshift
quasars.Comment: 11 pages, 5 figures, 3 tables; accepted for publication in MNRA
Modelling Baryon Acoustic Oscillations with Perturbation Theory and Stochastic Halo Biasing
In this work we investigate the generation of mock halo catalogues based on
perturbation theory and nonlinear stochastic biasing with the novel
PATCHY-code. In particular, we use Augmented Lagrangian Perturbation Theory
(ALPT) to generate a dark matter density field on a mesh starting from Gaussian
fluctuations and to compute the peculiar velocity field. ALPT is based on a
combination of second order LPT (2LPT) on large scales and the spherical
collapse model on smaller scales. We account for the systematic deviation of
perturbative approaches from N-body simulations together with halo biasing
adopting an exponential bias model. We then account for stochastic biasing by
defining three regimes: a low, an intermediate and a high density regime, using
a Poisson distribution in the intermediate regime and the negative binomial
distribution to model over-dispersion in the high density regime. Since we
focus in this study on massive halos, we suppress the generation of halos in
the low density regime. The various nonlinear and stochastic biasing
parameters, and density thresholds (five) are calibrated with the large
BigMultiDark N-body simulation to match the power spectrum of the corresponding
halo population. Our mock catalogues show power spectra, both in real- and
redshift-space, which are compatible with N-body simulations within about 2% up
to k ~ 1 h Mpc^-1 at z = 0.577 for a sample of halos with the typical BOSS
CMASS galaxy number density. The corresponding correlation functions are
compatible down to a few Mpc. We also find that neglecting over-dispersion in
high density regions produces power spectra with deviations of 10% at k ~ 0.4 h
Mpc^-1. These results indicate the need to account for an accurate statistical
description of the galaxy clustering for precise studies of large-scale
surveys.Comment: 5 pages, 4 figure
Suppressing cosmic variance with paired-and-fixed cosmological simulations: average properties and covariances of dark matter clustering statistics
Making cosmological inferences from the observed galaxy clustering requires
accurate predictions for the mean clustering statistics and their covariances.
Those are affected by cosmic variance -- the statistical noise due to the
finite number of harmonics. The cosmic variance can be suppressed by fixing the
amplitudes of the harmonics instead of drawing them from a Gaussian
distribution predicted by the inflation models. Initial realizations also can
be generated in pairs with 180 degrees flipped phases to further reduce the
variance. Here, we compare the consequences of using paired-and-fixed vs
Gaussian initial conditions on the average dark matter clustering and
covariance matrices predicted from N-body simulations. As in previous studies,
we find no measurable differences between paired-and-fixed and Gaussian
simulations for the average density distribution function, power spectrum and
bispectrum. Yet, the covariances from paired-and-fixed simulations are
suppressed in a complicated scale- and redshift-dependent way. The situation is
particularly problematic on the scales of Baryon Acoustic Oscillations where
the covariance matrix of the power spectrum is lower by only 20% compared to
the Gaussian realizations, implying that there is not much of a reduction of
the cosmic variance. The non-trivial suppression, combined with the fact that
paired-and-fixed covariances are noisier than from Gaussian simulations,
suggests that there is no path towards obtaining accurate covariance matrices
from paired-and-fixed simulations. Because the covariances are crucial for the
observational estimates of galaxy clustering statistics and cosmological
parameters, paired-and-fixed simulations, though useful for some applications,
cannot be used for the production of mock galaxy catalogs.Comment: Submitted to MNRA
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