40 research outputs found

### Universal Density Profile for Cosmic Voids

We present a simple empirical function for the average density profile of
cosmic voids, identified via the watershed technique in $\Lambda$CDM N-body
simulations. This function is universal across void size and redshift,
accurately describing a large radial range of scales around void centers with
only two free parameters. In analogy to halo density profiles, these parameters
describe the scale radius and the central density of voids. While we initially
start with a more general four-parameter model, we find two of its parameters
to be redundant, as they follow linear trends with the scale radius in two
distinct regimes of the void sample, separated by its compensation scale.
Assuming linear theory, we derive an analytic formula for the velocity profile
of voids and find an excellent agreement with the numerical data as well. In
our companion paper [Sutter et al., Mon. Not. R. Astron. Soc. 442, 462 (2014)]
the presented density profile is shown to be universal even across tracer type,
properly describing voids defined in halo and galaxy distributions of varying
sparsity, allowing us to relate various void populations by simple rescalings.
This provides a powerful framework to match theory and simulations with
observational data, opening up promising perspectives to constrain competing
models of cosmology and gravity.Comment: 5 pages, 3 figures. Matches PRL published version after minor
correction

### Probing cosmology and gravity with redshift-space distortions around voids

Cosmic voids in the large-scale structure of the Universe affect the peculiar
motions of objects in their vicinity. Although these motions are difficult to
observe directly, the clustering pattern of their surrounding tracers in
redshift space is influenced in a unique way. This allows to investigate the
interplay between densities and velocities around voids, which is solely
dictated by the laws of gravity. With the help of $N$-body simulations and
derived mock-galaxy catalogs we calculate the average density fluctuations
around voids identified with a watershed algorithm in redshift space and
compare the results with the expectation from general relativity and the
$\Lambda$CDM model. We find linear theory to work remarkably well in describing
the dynamics of voids. Adopting a Bayesian inference framework, we explore the
full posterior of our model parameters and forecast the achievable accuracy on
measurements of the growth rate of structure and the geometric distortion
through the Alcock-Paczynski effect. Systematic errors in the latter are
reduced from $\sim15\%$ to $\sim5\%$ when peculiar velocities are taken into
account. The relative parameter uncertainties in galaxy surveys with number
densities comparable to the SDSS MAIN (CMASS) sample probing a volume of
$1h^{-3}{\rm Gpc}^3$ yield $\sigma_{f/b}\left/(f/b)\right.\sim2\%$ ($20\%$) and
$\sigma_{D_AH}/D_AH\sim0.2\%$ ($2\%$), respectively. At this level of precision
the linear-theory model becomes systematics dominated, with parameter biases
that fall beyond these values. Nevertheless, the presented method is highly
model independent; its viability lies in the underlying assumption of
statistical isotropy of the Universe.Comment: 38 pages, 14 figures. Published in JCAP. Referee comments
incorporated, typos corrected, references added. Considerably improved
results thanks to consideration of full covariance matrix in the MCMC
analysi

### The bias of cosmic voids in the presence of massive neutrinos

Cosmic voids offer an extraordinary opportunity to study the effects of
massive neutrinos on cosmological scales. Because they are freely streaming,
neutrinos can penetrate the interior of voids more easily than cold dark matter
or baryons, which makes their relative contribution to the mass budget in voids
much higher than elsewhere in the Universe. In simulations it has recently been
shown how various characteristics of voids in the matter distribution are
affected by neutrinos, such as their abundance, density profiles, dynamics, and
clustering properties. However, the tracers used to identify voids in
observations (e.g., galaxies or halos) are affected by neutrinos as well, and
isolating the unique neutrino signatures inherent to voids becomes more
difficult. In this paper we make use of the DEMNUni suite of simulations to
investigate the clustering bias of voids in Fourier space as a function of
their core density and compensation. We find a clear dependence on the sum of
neutrino masses that remains significant even for void statistics extracted
from halos. In particular, we observe that the amplitude of the linear void
bias increases with neutrino mass for voids defined in dark matter, whereas
this trend gets reversed and slightly attenuated when measuring the relative
void-halo bias using voids identified in the halo distribution. Finally, we
argue how the original behaviour can be restored when considering observations
of the total matter distribution (e.g. via weak lensing), and comment on
scale-dependent effects in the void bias that may provide additional
information on neutrinos in the future.Comment: 23 pages, 18 figure

### Dark matter voids in the SDSS galaxy survey

What do we know about voids in the dark matter distribution given the Sloan
Digital Sky Survey (SDSS) and assuming the $\Lambda\mathrm{CDM}$ model? Recent
application of the Bayesian inference algorithm BORG to the SDSS Data Release 7
main galaxy sample has generated detailed Eulerian and Lagrangian
representations of the large-scale structure as well as the possibility to
accurately quantify corresponding uncertainties. Building upon these results,
we present constrained catalogs of voids in the Sloan volume, aiming at a
physical representation of dark matter underdensities and at the alleviation of
the problems due to sparsity and biasing on galaxy void catalogs. To do so, we
generate data-constrained reconstructions of the presently observed large-scale
structure using a fully non-linear gravitational model. We then find and
analyze void candidates using the VIDE toolkit. Our methodology therefore
predicts the properties of voids based on fusing prior information from
simulations and data constraints. For usual void statistics (number function,
ellipticity distribution and radial density profile), all the results obtained
are in agreement with dark matter simulations. Our dark matter void candidates
probe a deeper void hierarchy than voids directly based on the observed
galaxies alone. The use of our catalogs therefore opens the way to
high-precision void cosmology at the level of the dark matter field. We will
make the void catalogs used in this work available at
http://www.cosmicvoids.net.Comment: 15 pages, 6 figures, matches JCAP published version, void catalogs
publicly available at http://www.cosmicvoids.ne

### Going beyond the Kaiser redshift-space distortion formula: a full general relativistic account of the effects and their detectability in galaxy clustering

Kaiser redshift-space distortion formula describes well the clustering of
galaxies in redshift surveys on small scales, but there are numerous additional
terms that arise on large scales. Some of these terms can be described using
Newtonian dynamics and have been discussed in the literature, while the others
require proper general relativistic description that was only recently
developed. Accounting for these terms in galaxy clustering is the first step
toward tests of general relativity on horizon scales. The effects can be
classified as two terms that represent the velocity and the gravitational
potential contributions. Their amplitude is determined by effects such as the
volume and luminosity distance fluctuation effects and the time evolution of
galaxy number density and Hubble parameter. We compare the Newtonian
approximation often used in the redshift-space distortion literature to the
fully general relativistic equation, and show that Newtonian approximation
accounts for most of the terms contributing to velocity effect. We perform a
Fisher matrix analysis of detectability of these terms and show that in a
single tracer survey they are completely undetectable. To detect these terms
one must resort to the recently developed methods to reduce sampling variance
and shot noise. We show that in an all-sky galaxy redshift survey at low
redshift the velocity term can be measured at a few sigma if one can utilize
halos of mass M>10^12 Msun (this can increase to 10-sigma or more in some more
optimistic scenarios), while the gravitational potential term itself can only
be marginally detected. We also demonstrate that the general relativistic
effect is not degenerate with the primordial non-Gaussian signature in galaxy
bias, and the ability to detect primordial non-Gaussianity is little
compromised.Comment: 13 pages, 5 figures, published in PR

### Constraints on Cosmology and Gravity from the Dynamics of Voids

The Universe is mostly composed of large and relatively empty domains known
as cosmic voids, whereas its matter content is predominantly distributed along
their boundaries. The remaining material inside them, either dark or luminous
matter, is attracted to these boundaries and causes voids to expand faster and
to grow emptier over time. Using the distribution of galaxies centered on voids
identified in the Sloan Digital Sky Survey and adopting minimal assumptions on
the statistical motion of these galaxies, we constrain the average matter
content $\Omega_\mathrm{m}=0.281\pm0.031$ in the Universe today, as well as the
linear growth rate of structure $f/b=0.417\pm0.089$ at median redshift
$\bar{z}=0.57$, where $b$ is the galaxy bias ($68\%$ C.L.). These values
originate from a percent-level measurement of the anisotropic distortion in the
void-galaxy cross-correlation function, $\varepsilon = 1.003\pm0.012$, and are
robust to consistency tests with bootstraps of the data and simulated mock
catalogs within an additional systematic uncertainty of half that size. They
surpass (and are complementary to) existing constraints by unlocking
cosmological information on smaller scales through an accurate model of
nonlinear clustering and dynamics in void environments. As such, our analysis
furnishes a powerful probe of deviations from Einstein's general relativity in
the low-density regime which has largely remained untested so far. We find no
evidence for such deviations in the data at hand.Comment: 11 pages, 7 figures. Reflects published version in PRL including
Supplemental Materia