414 research outputs found
Hierarchy in the Phase Space and Dark Matter Astronomy
We develop a theoretical framework for describing the hierarchical structure
of the phase space of cold dark matter haloes, due to gravitationally bound
substructures. Because it includes the full hierarchy of the cold dark matter
initial conditions and is hence complementary to the halo model, the stable
clustering hypothesis is applied for the first time here to the small-scale
phase space structure. As an application, we show that the particle dark matter
annihilation signal could be up to two orders of magnitude larger than that of
the smooth halo within the Galactic virial radius. The local boost is inversely
proportional to the smooth halo density, and thus is O(1) within the solar
radius, which could translate into interesting signatures for dark matter
direct detection experiments: The temporal correlation of dark matter detection
can change by a factor of 2 in the span of 10 years, while there will be
significant correlations in the velocity space of dark matter particles. This
can introduce O(1) uncertainty in the direction of local dark matter wind,
which was believed to be a benchmark of directional dark matter searches or the
annual modulation signal.Comment: 5 pages, 4 figure
Saddle stresses for generic theories with a preferred acceleration scale
We show how scaling arguments may be used to generate templates for the tidal
stresses around saddles for a vast class of MONDian theories {\it detached from
their obligations as dark matter alternatives}. Such theories are to be seen
simply as alternative theories of gravity with a preferred acceleration scale,
and could be tested in the solar system by extending the LISA Pathfinder (LPF)
mission. The constraints thus obtained may then be combined, if one wishes,
with requirements arising from astrophysical and cosmological applications, but
a clear separation of the issues is achieved. The central technical content of
this paper is the derivation of a scaling prescription allowing complex
numerical work to be bypassed in the generation of templates. We find that LPF
could constrain very tightly the acceleration and the free parameter
present in these theories. As an application of our technique we also
produce predictions for the moon saddle (for which a similar scaling argument
is applicable) with the result that we recommend that it should be included in
orbit design.Comment: Analysis of the lunar saddle added to version to appear in Physical
Review
One Gravitational Potential or Two? Forecasts and Tests
The metric of a perturbed Robertson-Walker spacetime is characterized by
three functions: a scale-factor giving the expansion history and two potentials
which generalize the single potential of Newtonian gravity. The Newtonian
potential induces peculiar velocities and, from these, the growth of matter
fluctuations. Massless particles respond equally to the Newtonian potential and
to a curvature potential. The difference of the two potentials, called the
gravitational slip, is predicted to be very small in general relativity but can
be substantial in modified gravity theories. The two potentials can be
measured, and gravity tested on cosmological scales, by combining weak
gravitational lensing or the Integrated Sachs-Wolfe effect with galaxy peculiar
velocities or clustering.Comment: 15 pages, invited research article for Theo Murphy Meeting "Testing
general relativity with cosmology
The Omega Dependence of the Evolution of xi(r)
The evolution of the two-point correlation function, xi(r,z), and the
pairwise velocity dispersion, sigma(r,z), for both the matter and halo
population, in three different cosmological models:
(Omega_M,Omega_Lambda)=(1,0), (0.2,0) and (0.2,0.8) are described. If the
evolution of xi is parameterized by xi(r,z)=(1+z)^{-(3+eps)}xi(r,0), where
xi(r,0)=(r/r_0)^{-gamma}, then eps(mass) ranges from 1.04 +/- 0.09 for (1,0) to
0.18 +/- 0.12 for (0.2,0), as measured by the evolution of at 1 Mpc (from z ~ 5
to the present epoch). For halos, eps depends on their mean overdensity. Halos
with a mean overdensity of about 2000 were used to compute the halo two-point
correlation function tested with two different group finding algorithms: the
friends of friends and the spherical overdensity algorithm. It is certainly
believed that the rate of growth of this xihh will give a good estimate of the
evolution of the galaxy two-point correlation function, at least from z ~ 1 to
the present epoch. The values we get for eps(halos) range from 1.54 for (1,0)
to -0.36 for (0.2,0), as measured by the evolution of xi(halos) from z ~ 1.0 to
the present epoch. These values could be used to constrain the cosmological
scenario. The evolution of the pairwise velocity dispersion for the mass and
halo distribution is measured and compared with the evolution predicted by the
Cosmic Virial Theorem (CVT). According to the CVT, sigma(r,z)^2 ~ G Q rho(z)
r^2 xi(r,z) or sigma proportional to (1+z)^{-eps/2}. The values of eps measured
from our simulated velocities differ from those given by the evolution of xi
and the CVT, keeping gamma and Q constant: eps(CVT) = 1.78 +/- 0.13 for (1,0)
or 1.40 +/- 0.28 for (0.2,0).Comment: Accepted for publication in the ApJ. Also available at
http://manaslu.astro.utoronto.ca/~carlberg/cnoc/xiev/xi_evo.ps.g
Radial Alignment in Simulated Clusters
Observational evidence for the radial alignment of satellites with their dark
matter host has been accumulating steadily in the past few years. The effect is
seen over a wide range of scales, from massive clusters of galaxies down to
galaxy-sized systems, yet the underlying physical mechanism has still not been
established. To this end, we have carried out a detailed analysis of the shapes
and orientations of dark matter substructures in high-resolution N-body
cosmological simulations. We find a strong tendency for radial alignment of the
substructure with its host halo: the distribution of halo major axes is very
anisotropic, with the majority pointing towards the center of mass of the host.
The alignment peaks once the sub-halo has passed the virial radius of the host
for the first time, but is not subsequently diluted, even after the halos have
gone through as many as four pericentric passages. This evidence points to the
existence of a very rapid dynamical mechanism acting on these systems and we
argue that tidal torquing throughout their orbits is the most likely candidate.Comment: v2: 13 pages, 10 figures, ApJ in press. Revisions include a new
section (4.2) comparing our results with observations, and a few added
reference
The Mass Power Spectrum in Quintessence Cosmological Models
We present simple analytic approximations for the linear and fully evolved
nonlinear mass power spectrum for spatially flat cold dark matter (CDM)
cosmological models with quintessence (Q). Quintessence is a time evolving,
spatially inhomogeneous energy component with negative pressure and an equation
of state w_Q < 0. It clusters gravitationally on large length scales but
remains smooth like the cosmological constant on small length scales. We show
that the clustering scale is determined by the Compton wavelength of the
Q-field and derive a shape parameter, \Gamma_Q, to characterize the linear mass
power spectrum. The growth of linear perturbations as functions of redshift,
w_Q, and matter density \Omega_m is also quantified. Calibrating to N-body
simulations, we construct a simple extension of the formula by Ma (1998) that
closely approximates the nonlinear power spectrum for a range of plausible QCDM
models.Comment: 5 pages with 3 inserted postscript figures, AAS LaTeX v4.0
emulateapj.sty. Astrophysical Journal Letters, in pres
POTENT Reconstruction from Mark III Velocities
We present an improved POTENT method for reconstructing the velocity and mass
density fields from radial peculiar velocities, test it with mock catalogs, and
apply it to the Mark III Catalog. Method improvments: (a) inhomogeneous
Malmquist bias is reduced by grouping and corrected in forward or inverse
analyses of inferred distances, (b) the smoothing into a radial velocity field
is optimized to reduce window and sampling biases, (c) the density is derived
from the velocity using an improved nonlinear approximation, and (d) the
computational errors are made negligible. The method is tested and optimized
using mock catalogs based on an N-body simulation that mimics our cosmological
neighborhood, and the remaining errors are evaluated quantitatively. The Mark
III catalog, with ~3300 grouped galaxies, allows a reliable reconstruction with
fixed Gaussian smoothing of 10-12 Mpc/h out to ~60 Mpc/h. We present maps of
the 3D velocity and mass-density fields and the corresponding errors. The
typical systematic and random errors in the density fluctuations inside 40
Mpc/h are \pm 0.13 and \pm 0.18. The recovered mass distribution resembles in
its gross features the galaxy distribution in redshift surveys and the mass
distribution in a similar POTENT analysis of a complementary velocity catalog
(SFI), including the Great Attractor, Perseus-Pisces, and the void in between.
The reconstruction inside ~40 Mpc/h is not affected much by a revised
calibration of the distance indicators (VM2, tailored to match the velocities
from the IRAS 1.2Jy redshift survey). The bulk velocity within the sphere of
radius 50 Mpc/h about the Local Group is V_50=370 \pm 110 km/s (including
systematic errors), and is shown to be mostly generated by external mass
fluctuations. With the VM2 calibration, V_50 is reduced to 305 \pm 110 km/s.Comment: 60 pages, LaTeX, 3 tables and 27 figures incorporated (may print the
most crucial figures only, by commenting out one line in the LaTex source
Researching immunocontraceptive vaccines with mares (Equus caballus) as both a target and model for African elephant (Loxodonta africana) cows: a review
A sequence of studies is reviewed that reported the domestic horse (Equus caballus) mare as an appropriate and accessible research platform for recording clinical and laboratory data post-immunisation with anti- GnRH and -zona pellucida (ZP) immunocontraceptive vaccines. Experience with a native porcine ZP (pZP) vaccine in African elephant (Loxodonta africana) cows highlighted needs for improving vaccine formulations and more clearly defining associated ovarian effects and safety profiles. Initially, the efficacy, reversibility and safety of the GnRH vaccine Improvac® in mares was demonstrated using reproductive tract ultrasonography and concurrently measuring serum antibody titres and progesterone concentrations. Results informed the study design and minimally invasive monitoring of post-treatment ovarian steroid responses of this vaccine in free-ranging African elephant cows. A subsequent sequence of studies reported reversible contraceptive and immunological efficacy in pony mares immunised with pZP formulated with Freund’s adjuvants. By comparison, mares treated with a recombinant ZP3 and ZP4 (reZP) vaccine showed disappointing responses. Unexpectedly, most pZP-treated mares showed ovarian inactivity. In attempting to understand this response, results showed the involvement of cytotoxic (CD8+) T-cells negatively correlated to serum ovarian steroid and anti-Müllerian hormone (AMH) levels. Of concern was the prevalence of injection-site lesions ascribable to Freund’s adjuvants. Following this, mares treated with both pZP and a novel reZP vaccine formulated with non-Freund’s adjuvants showed comparable immunological responses and ovarian inactivity, notably without adverse treatment reactions. In addition, measuring AMH showed promise for monitoring ovarian function in anti-ZP-treated animals
On the Growth of Perturbations as a Test of Dark Energy
The strongest evidence for dark energy comes presently from geometric
techniques such as the supernova distance-redshift relation. By combining the
measured expansion history with the Friedmann equation one determines the
energy density and its time evolution, hence the equation of state of dark
energy. Because these methods rely on the Friedmann equation which has not been
independently tested it is desirable to find alternative methods that work for
both general relativity and other theories of gravity.
Assuming that sufficiently large patches of a perturbed Robertson-Walker
spacetime evolve like separate Robertson-Walker universes, that shear stress is
unimportant on large scales and that energy and momentum are locally conserved,
we derive several relations between long-wavelength metric and matter
perturbations. These relations include generalizations of the initial-value
constraints of general relativity. For a class of theories including general
relativity we reduce the long-wavelength metric, density, and velocity
potential perturbations to quadratures including curvature perturbations,
entropy perturbations, and the effects of nonzero background curvature. When
combined with the expansion history measured geometrically, the long-wavelength
solution provide a test that may distinguish modified gravity from other
explanations of dark energy.Comment: 25 pages, 1 figure, submitted to ApJ; references added; expanded
discussion of entropy perturbations, initial-value constraints and
alternative theories of gravit
- …