574 research outputs found
Halo assembly bias and the tidal anisotropy of the local halo environment
We study the role of the local tidal environment in determining the assembly
bias of dark matter haloes. Previous results suggest that the anisotropy of a
halo's environment (i.e, whether it lies in a filament or in a more isotropic
region) can play a significant role in determining the eventual mass and age of
the halo. We statistically isolate this effect using correlations between the
large-scale and small-scale environments of simulated haloes at with
masses between . We
probe the large-scale environment using a novel halo-by-halo estimator of
linear bias. For the small-scale environment, we identify a variable
that captures the in a region of radius
around the halo and correlates strongly with halo bias
at fixed mass. Segregating haloes by reveals two distinct
populations. Haloes in highly isotropic local environments
() behave as expected from the simplest, spherically
averaged analytical models of structure formation, showing a
correlation between their concentration and large-scale
bias at masses. In contrast, haloes in anisotropic,
filament-like environments () tend to show a
correlation between bias and concentration at any mass. Our
multi-scale analysis cleanly demonstrates how the overall assembly bias trend
across halo mass emerges as an average over these different halo populations,
and provides valuable insights towards building analytical models that
correctly incorporate assembly bias. We also discuss potential implications for
the nature and detectability of galaxy assembly bias.Comment: 19 pages, 15 figures; v2: revised in response to referee comments,
added references and discussion, conclusions unchanged. Accepted in MNRA
Spontaneous breaking of conformal invariance, solitons and gravitational waves in theories of conformally invariant gravitation
We study conformal gravity as an alternative theory of gravitation. For
conformal gravity to be phenomenologically viable requires that the conformal
symmetry is not manifest at the energy scales of the other known physical
forces. Hence we require a mechanism for the spontaneous breaking of conformal
invariance. In this paper we study the possibility that conformal invariance is
spontaneously broken due to interactions with conformally coupled matter
fields. The vacuum of the theory admits conformally non-invariant solutions
corresponding to maximally symmetric space-times and variants thereof. These
are either de Sitter space-time or anti-de Sitter space-time in the full four
space-time dimensions or in a lower dimensional sub-space. We consider in
particular normalizable, linearized gravitational perturbations around the
anti-de Sitter background. Exploiting the conformal flatness of this
space-time, we show to second order, that these gravitational fluctuations,
that are taken to be fourier decomposable, carry zero energy-momentum. This
squares well with the theorem that asymptotically flat space-times conformal
gravity contain zero energy and momentum \cite{bhs}. We also show the
possibility of domain wall solitons interpolating between the ground states of
spontaneously broken conformal symmetry that we have found. These solitons
necessarily require the vanishing of the scalar field, repudiating the recent
suggestion \cite{f} that the conformal symmetry could be quarantined to a
sterile sector of the theory by choosing an appropriate field redefinition.Comment: 21 pages, 2 figures, colour viewing helpful, version to be published
in PR
Spontaneous Symmetry Breaking and the Renormalization of the Chern-Simons Term
We calculate the one-loop perturbative correction to the coefficient of the
\cs term in non-abelian gauge theory in the presence of Higgs fields, with a
variety of symmetry-breaking structures. In the case of a residual
symmetry, radiative corrections do not change the coefficient of the \cs term.
In the case of an unbroken non-abelian subgroup, the coefficient of the
relevant \cs term (suitably normalized) attains an integral correction, as
required for consistency of the quantum theory. Interestingly, this coefficient
arises purely from the unbroken non-abelian sector in question; the orthogonal
sector makes no contribution. This implies that the coefficient of the \cs term
is a discontinuous function over the phase diagram of the theory.Comment: Version to be published in Phys Lett B., minor additional change
One step beyond: The excursion set approach with correlated steps
We provide a simple formula that accurately approximates the first crossing
distribution of barriers having a wide variety of shapes, by random walks with
a wide range of correlations between steps. Special cases of it are useful for
estimating halo abundances, evolution, and bias, as well as the nonlinear
counts in cells distribution. We discuss how it can be extended to allow for
the dependence of the barrier on quantities other than overdensity, to
construct an excursion set model for peaks, and to show why assembly and scale
dependent bias are generic even at the linear level.Comment: 5 pages, 1 figure. Uses mn2e class styl
Self maps of homogeneous spaces
This article does not have an abstract
The external field effect in cold dark matter models
In general relativity (GR), the internal dynamics of a self-gravitating
system under free-fall in an external gravitational field should not depend on
the external field strength. Recent work has claimed a statistical detection of
an `external field effect' (EFE) using galaxy rotation curve data. We show that
large uncertainties in rotation curve analyses and inaccuracies in published
simulation-based external field estimates compromise the significance of the
claimed EFE detection. We further show analytically that a qualitatively
similar statistical signal is, in fact, expected in a -cold dark
matter (CDM) universe without any violation of the strong equivalence
principle. Rather, such a signal arises simply because of the inherent
correlations between galaxy clustering strength and intrinsic galaxy
properties. We explicitly demonstrate the effect in a baryonified mock catalog
of a CDM universe. Although the detection of an EFE-like signal is
not, by itself, evidence for physics beyond GR, our work shows that the
of the EFE-like correlation between the external field strength
and the shape of the radial acceleration relation can be used to probe new
physics: e.g., in MOND, the predicted sign is opposite to that in our
CDM mocks.Comment: 10 pages, 6 figures, submitted to MNRA
Model-agnostic cosmological constraints from the baryon acoustic oscillation feature in redshift space
We develop a framework for self-consistently extracting cosmological
information from the clustering of tracers in redshift space,
relying on model-dependent templates to describe the baryon
acoustic oscillation (BAO) feature. Our approach uses the recently proposed
Laguerre reconstruction technique for the BAO feature and its linear point
, and substantially extends it to simultaneously model the
multipoles of the anisotropic galaxy 2-point correlation function
(2pcf). The approach is `model-agnostic': it assumes that the non-linear growth
of structure smears the BAO feature by an approximately Gaussian kernel with a
smearing scale , but does not assume any fiducial cosmology for
describing the shape of the feature itself. Using mock observations for two
realistic survey configurations assuming cold dark matter
(CDM), combined with Bayesian parameter inference, we show that the
linear point and smearing scale can be accurately
recovered by our method in both existing and upcoming surveys. The precision of
the recovery of is always better than , while can be recovered with uncertainty provided the linear galaxy
bias is separately constrained, e.g., using weak lensing observations. Our
method is also sensitive to the linear growth rate , albeit with larger
uncertainties and systematic errors, especially for upcoming surveys such as
DESI. We discuss how our model can be modified to improve the recovery of ,
such that the resulting constraints on can
potentially be used as a test of cosmological models including and beyond
CDM.Comment: 17 pages, 6 figures, submitted to MNRA
Bias deconstructed: Unravelling the scale dependence of halo bias using real space measurements
We explore the scale dependence of halo bias using real space
cross-correlation measurements in N-body simulations and in Pinocchio, an
algorithm based on Lagrangian Perturbation Theory. Recent work has shown how to
interpret such real space measurements in terms of k-dependent bias in Fourier
space, and how to remove the k-dependence to reconstruct the k-independent
peak-background split halo bias parameters. We compare our reconstruction of
the linear bias, which requires no free parameters, with previous estimates
from N-body simulations which were obtained directly in Fourier space at large
scales, and find very good agreement. Our reconstruction of the quadratic bias
is similarly parameter-free, although in this case there are no previous
Fourier space measurements to compare with. Our analysis of N-body simulations
explicitly tests the predictions of the excursion set peaks (ESP) formalism of
Paranjape et al. (2013) for the scale dependence of bias; we find that the ESP
predictions accurately describe our measurements. In addition, our measurements
in Pinocchio serve as a useful, successful consistency check between Pinocchio
and N-body simulations that is not accessible to traditional measurements.Comment: 13 pages, 9 figures; v3 -- Matches published versio
Halo abundances and counts-in-cells: The excursion set approach with correlated steps
The Excursion Set approach has been used to make predictions for a number of
interesting quantities in studies of nonlinear hierarchical clustering. These
include the halo mass function, halo merger rates, halo formation times and
masses, halo clustering, analogous quantities for voids, and the distribution
of dark matter counts in randomly placed cells. The approach assumes that all
these quantities can be mapped to problems involving the first crossing
distribution of a suitably chosen barrier by random walks. Most analytic
expressions for these distributions ignore the fact that, although different
k-modes in the initial Gaussian field are uncorrelated, this is not true in
real space: the values of the density field at a given spatial position, when
smoothed on different real-space scales, are correlated in a nontrivial way. As
a result, the problem is to estimate first crossing distribution by random
walks having correlated rather than uncorrelated steps. In 1990, Peacock &
Heavens presented a simple approximation for the first crossing distribution of
a single barrier of constant height by walks with correlated steps. We show
that their approximation can be thought of as a correction to the distribution
associated with what we call smooth completely correlated walks. We then use
this insight to extend their approach to treat moving barriers, as well as
walks that are constrained to pass through a certain point before crossing the
barrier. For the latter, we show that a simple rescaling, inspired by bivariate
Gaussian statistics, of the unconditional first crossing distribution,
accurately describes the conditional distribution, independently of the choice
of analytical prescription for the former. In all cases, comparison with
Monte-Carlo solutions of the problem shows reasonably good agreement.
(Abridged)Comment: 14 pages, 9 figures; v2 -- revised version with explicit
demonstration that the original conclusions hold for LCDM, expanded
discussion on stochasticity of barrier. Accepted in MNRA
Peaks theory and the excursion set approach
We describe a model of dark matter halo abundances and clustering which
combines the two most widely used approaches to this problem: that based on
peaks and the other based on excursion sets. Our approach can be thought of as
addressing the cloud-in-cloud problem for peaks and/or modifying the excursion
set approach so that it averages over a special subset, rather than all
possible walks. In this respect, it seeks to account for correlations between
steps in the walk as well as correlations between walks. We first show how the
excursion set and peaks models can be written in the same formalism, and then
use this correspondence to write our combined excursion set peaks model. We
then give simple expressions for the mass function and bias, showing that even
the linear halo bias factor is predicted to be k-dependent as a consequence of
the nonlocality associated with the peak constraint. At large masses, our model
has little or no need to rescale the variable delta_c from the value associated
with spherical collapse, and suggests a simple explanation for why the linear
halo bias factor appears to lie above that based on the peak-background split
at high masses when such a rescaling is assumed. Although we have concentrated
on peaks, our analysis is more generally applicable to other traditionally
single-scale analyses of large-scale structure.Comment: 10 pages, 4 figures; v2 -- minor changes, added discussion in sec2.2,
fixed a typo. Accepted in MNRA
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