791 research outputs found
Why we need to see the dark matter to understand the dark energy
The cosmological concordance model contains two separate constituents which
interact only gravitationally with themselves and everything else, the dark
matter and the dark energy. In the standard dark energy models, the dark matter
makes up some 20% of the total energy budget today, while the dark energy is
responsible for about 75%. Here we show that these numbers are only robust for
specific dark energy models and that in general we cannot measure the abundance
of the dark constituents separately without making strong assumptions.Comment: 4 pages, to be published in the Journal of Physics: Conference Series
as a contribution to the 2007 Europhysics Conference on High Energy Physic
Mass and Light in the Universe
We present a weak lensing and photometric study of six half by half degree
fields observed at the CFHT using the UH8K CCD mosaic camera. The fields were
observed for a total of 2 hours each in I and V, resulting in catalogs
containing ~ 20 000 galaxies per passband per field. We use V-I color and I
magnitude to select bright early type galaxies at redshifts 0.1 < z < 0.9. We
measure the gravitational shear from faint galaxies in the range 21 < m_I < 25
from a composite catalog and find a strong correlation with that predicted from
the early types if they trace the mass with mass-to-light ratio 300\pm75 h (in
solar units) for a flat (Omega_m0 = 0.3, Omega_l0 = 0.7) lambda cosmology and
400\pm100 h for Einstein-de Sitter. We make two-dimensional reconstructions of
the mass surface density. Cross-correlation of the measured mass surface
density with that predicted from the early type galaxy distribution shows a
strong peak at zero lag (significant at the 5.2-sigma level). We azimuthally
average the cross- and auto-correlation functions. We conclude that the
profiles are consistent with early type galaxies tracing mass on scales of > 45
arcsec (> 200 kpc at z = 0.5). We sub-divide our bright early type galaxies by
redshift and obtain similar conclusions. These mass-to-light ratios imply
\Omega_m0 = 0.10\pm0.02 (\Omega_m0 = 0.13\pm0.03 for Einstein-de Sitter) of
closure density.Comment: 27 pages, 19 figs (4 ps, 15 gif), 4 tables, accepted for publication
in Ap.J. (email Gillian for better resolution ps versions of gif greyscale
plots
Resolving the virial discrepancy in clusters of galaxies with modified Newtonian dynamics
A sample of 197 X-ray emitting clusters of galaxies is considered in the
context of Milgrom's modified Newtonian dynamics (MOND). It is shown that the
gas mass, extrapolated via an assumed model to a fixed radius of 3 Mpc,
is correlated with the gas temperature as predicted by MOND (). The observed temperatures are generally consistent with the inferred
mass of hot gas; no substantial quantity of additional unseen matter is
required in the context of MOND. However, modified dynamics cannot resolve the
strong lensing discrepancy in those clusters where this phenomenon occurs. The
prediction is that additional baryonic matter may be detected in the central
regions of rich clusters.Comment: Submitted to A&A, 4 pages, 3 figures, A&A macro
Bound on the Dark Matter Density in the Solar System from Planetary Motions
High precision planet orbital data extracted from direct observation,
spacecraft explorations and laser ranging techniques enable to put a strong
constraint on the maximal dark matter density of a spherical halo centered
around the Sun. The maximal density at Earth's location is of the order
and shows only a mild dependence on the slope of the halo
profile, taken between 0 and -2. This bound is somewhat better than that
obtained from the perihelion precession limits.Comment: 7 pages, 1 figur
Operator Relations for SU(3) Breaking Contributions to K and K* Distribution Amplitudes
We derive constraints on the asymmetry a1 of the momentum fractions carried
by quark and antiquark in K and K* mesons in leading twist. These constraints
follow from exact operator identities and relate a1 to SU(3) breaking
quark-antiquark-gluon matrix elements which we determine from QCD sum rules.
Comparing our results to determinations of a1 from QCD sum rules based on
correlation functions of quark currents, we find that, for a1^\parallel(K*) the
central values agree well and come with moderate errors, whereas for a1(K) and
a1^\perp(K*) the results from operator relations are consistent with those from
quark current sum rules, but come with larger uncertainties. The consistency of
results confirms that the QCD sum rule method is indeed suitable for the
calculation of a1. We conclude that the presently most accurate predictions for
a1 come from the direct determination from QCD sum rules based on correlation
functions of quark currents and are given by: a1(K) = 0.06\pm 0.03,
a1^\parallel(K*) = 0.03\pm 0.02, a1^\perp(K*) = 0.04\pm 0.03.Comment: 21 page
The Effect of Substructure on Mass Estimates of Galaxies
Large galaxies are thought to form hierarchically, from the accretion and
disruption of many smaller galaxies. Such a scenario should naturally lead to
galactic phase-space distributions containing some degree of substructure. We
examine the errors in mass estimates of galaxies and their dark halos made
using the projected phase-space distribution of a tracer population (such as a
globular cluster system or planetary nebulae) due to falsely assuming that the
tracers are distributed randomly. The level of this uncertainty is assessed by
applying a standard mass estimator to samples drawn from 11 random realizations
of galaxy halos containing levels of substructure consistent with current
models of structure formation. We find that substructure will distort our mass
estimates by up to ~20% - a negligible error compared to statistical and
measurement errors in current derivations of masses for our own and other
galaxies. However, this represents a fundamental limit to the accuracy of any
future mass estimates made under the assumption that the tracer population is
distributed randomly, regardless of the size of the sample or the accuracy of
the measurements.Comment: 9 pages, 8 figures, Astrophysical Journal, in pres
The Bright Side of Dark Matter
We show that it is not possible in the absence of dark matter to construct a
four-dimensional metric that explains galactic observations. In particular, by
working with an effective potential it is shown that a metric which is
constructed to fit flat rotation curves in spiral galaxies leads to the wrong
sign for the bending of light i.e. repulsion instead of attraction. Hence,
without dark matter the motion of particles on galactic scales cannot be
explained in terms of geodesic motion on a four- dimensional metric. This
reveals a new bright side to dark matter: it is indispensable if we wish to
retain the cherished equivalence principle.Comment: 7 pages, latex, no figures. Received an honorable mention in the 1999
Gravity research Foundation Essay Competition. Submitted to Phys. Rev. Let
Imaging the Cosmic Matter Distribution using Gravitational Lensing of Pregalactic HI
21-cm emission from neutral hydrogen during and before the epoch of cosmic
reionisation is gravitationally lensed by material at all lower redshifts.
Low-frequency radio observations of this emission can be used to reconstruct
the projected mass distribution of foreground material, both light and dark. We
compare the potential imaging capabilities of such 21-cm lensing with those of
future galaxy lensing surveys. We use the Millennium Simulation to simulate
large-area maps of the lensing convergence with the noise, resolution and
redshift-weighting achievable with a variety of idealised observation
programmes. We find that the signal-to-noise of 21-cm lens maps can far exceed
that of any map made using galaxy lensing. If the irreducible noise limit can
be reached with a sufficiently large radio telescope, the projected convergence
map provides a high-fidelity image of the true matter distribution, allowing
the dark matter halos of individual galaxies to be viewed directly, and giving
a wealth of statistical and morphological information about the relative
distributions of mass and light. For instrumental designs like that planned for
the Square Kilometer Array (SKA), high-fidelity mass imaging may be possible
near the resolution limit of the core array of the telescope.Comment: version accepted for publication in MNRAS (reduced-resolution
figures
Extracting the Gamma Ray Signal from Dark Matter Annihilation in the Galactic Center Region
The GLAST satellite mission will study the gamma ray sky with considerably
greater exposure than its predecessor EGRET. In addition, it will be capable of
measuring the arrival directions of gamma rays with much greater precision.
These features each significantly enhance GLAST's potential for identifying
gamma rays produced in the annihilations of dark matter particles. The combined
use of spectral and angular information, however, is essential if the full
sensitivity of GLAST to dark matter is to be exploited. In this paper, we
discuss the separation of dark matter annihilation products from astrophysical
backgrounds, focusing on the Galactic Center region, and perform a forecast for
such an analysis. We consider both point-like and diffuse astrophysical
backgrounds and model them using a point-spread-function for GLAST. While the
results of our study depend on the specific characteristics of the dark matter
signal and astrophysical backgrounds, we find that in many scenarios it is
possible to successfully identify dark matter annihilation radiation, even in
the presence of significant astrophysical backgrounds.Comment: 8 pages, 5 figures. Minor changes, note added. Matches published
versio
Gamma-ray signatures of annihilation to charged leptons in dark matter substructure
Due to their higher concentrations and small internal velocities, Milky Way
subhalos can be at least as important as the smooth halo in accounting for the
GeV positron excess via dark matter annihilation. After showing how this can be
achieved in various scenarios, including in Sommerfeld models, we demonstrate
that, in this case, the diffuse inverse-Compton emission resulting from
electrons and positrons produced in substructure leads to a nearly-isotropic
signal close to the level of the isotropic GeV gamma-ray background seen by
Fermi. Moreover, we show that HESS cosmic-ray electron measurements can be used
to constrain multi-TeV internal bremsstrahlung gamma rays arising from
annihilation to charged leptons.Comment: 8 pages, 4 figures; minor updates to match published versio
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