1,385 research outputs found
Large-scale structure and matter in the universe
This paper summarizes the physical mechanisms that encode the type and
quantity of cosmological matter in the properties of large-scale structure, and
reviews the application of such tests to current datasets. The key lengths of
the horizon size at matter-radiation equality and at last scattering determine
the total matter density and its ratio to the relativistic density; acoustic
oscillations can diagnose whether the matter is collisionless, and small-scale
structure or its absence can limit the mass of any dark-matter relic particle.
The most stringent constraints come from combining data on present-day galaxy
clustering with data on CMB anisotropies. Such an analysis breaks the
degeneracies inherent in either dataset alone, and proves that the universe is
very close to flat. The matter content is accurately consistent with pure Cold
Dark Matter, with about 25% of the critical density, and fluctuations that are
scalar-only, adiabatic and scale-invariant. It is demonstrated that these
conclusions cannot be evaded by adjusting either the equation of state of the
vacuum, or the total relativistic density.Comment: 17 Pages. Review paper from the January 2003 Royal Society Discussion
Meeting, "The search for dark matter and dark energy in the universe
Benefits of Artificially Generated Gravity Gradients for Interferometric Gravitational-Wave Detectors
We present an approach to experimentally evaluate gravity gradient noise, a
potentially limiting noise source in advanced interferometric gravitational
wave (GW) detectors. In addition, the method can be used to provide sub-percent
calibration in phase and amplitude of modern interferometric GW detectors.
Knowledge of calibration to such certainties shall enhance the scientific
output of the instruments in case of an eventual detection of GWs. The method
relies on a rotating symmetrical two-body mass, a Dynamic gravity Field
Generator (DFG). The placement of the DFG in the proximity of one of the
interferometer's suspended test masses generates a change in the local
gravitational field detectable with current interferometric GW detectors.Comment: 16 pages, 4 figure
Stacking Gravitational Wave Signals from Soft Gamma Repeater Bursts
Soft gamma repeaters (SGRs) have unique properties that make them intriguing
targets for gravitational wave (GW) searches. They are nearby, their burst
emission mechanism may involve neutron star crust fractures and excitation of
quasi-normal modes, and they burst repeatedly and sometimes spectacularly. A
recent LIGO search for transient GW from these sources placed upper limits on a
set of almost 200 individual SGR bursts. These limits were within the
theoretically predicted range of some models. We present a new search strategy
which builds upon the method used there by "stacking" potential GW signals from
multiple SGR bursts. We assume that variation in the time difference between
burst electromagnetic emission and burst GW emission is small relative to the
GW signal duration, and we time-align GW excess power time-frequency tilings
containing individual burst triggers to their corresponding electromagnetic
emissions. Using Monte Carlo simulations, we confirm that gains in GW energy
sensitivity of N^{1/2} are possible, where N is the number of stacked SGR
bursts. Estimated sensitivities for a mock search for gravitational waves from
the 2006 March 29 storm from SGR 1900+14 are also presented, for two GW
emission models, "fluence-weighted" and "flat" (unweighted).Comment: 17 pages, 16 figures, submitted to PR
Results from the First Science Run of the ZEPLIN-III Dark Matter Search Experiment
The ZEPLIN-III experiment in the Palmer Underground Laboratory at Boulby uses
a 12kg two-phase xenon time projection chamber to search for the weakly
interacting massive particles (WIMPs) that may account for the dark matter of
our Galaxy. The detector measures both scintillation and ionisation produced by
radiation interacting in the liquid to differentiate between the nuclear
recoils expected from WIMPs and the electron recoil background signals down to
~10keV nuclear recoil energy. An analysis of 847kg.days of data acquired
between February 27th 2008 and May 20th 2008 has excluded a WIMP-nucleon
elastic scattering spin-independent cross-section above 8.1x10(-8)pb at
55GeV/c2 with a 90% confidence limit. It has also demonstrated that the
two-phase xenon technique is capable of better discrimination between electron
and nuclear recoils at low-energy than previously achieved by other xenon-based
experiments.Comment: 12 pages, 17 figure
Quenching Factor for Low Energy Nuclear Recoils in a Plastic Scintillator
Plastic scintillators are widely used in industry, medicine and scientific
research, including nuclear and particle physics. Although one of their most
common applications is in neutron detection, experimental data on their
response to low-energy nuclear recoils are scarce. Here, the relative
scintillation efficiency for neutron-induced nuclear recoils in a
polystyrene-based plastic scintillator (UPS-923A) is presented, exploring
recoil energies between 125 keV and 850 keV. Monte Carlo simulations,
incorporating light collection efficiency and energy resolution effects, are
used to generate neutron scattering spectra which are matched to observed
distributions of scintillation signals to parameterise the energy-dependent
quenching factor. At energies above 300 keV the dependence is reasonably
described using the semi-empirical formulation of Birks and a kB factor of
(0.014+/-0.002) g/MeVcm^2 has been determined. Below that energy the measured
quenching factor falls more steeply than predicted by the Birks formalism.Comment: 8 pages, 9 figure
Correlated Gravitational Wave and Neutrino Signals from General-Relativistic Rapidly Rotating Iron Core Collapse
We present results from a new set of 3D general-relativistic hydrodynamic
simulations of rotating iron core collapse. We assume octant symmetry and focus
on axisymmetric collapse, bounce, the early postbounce evolution, and the
associated gravitational wave (GW) and neutrino signals. We employ a
finite-temperature nuclear equation of state, parameterized electron capture in
the collapse phase, and a multi-species neutrino leakage scheme after bounce.
The latter captures the important effects of deleptonization, neutrino cooling
and heating and enables approximate predictions for the neutrino luminosities
in the early evolution after core bounce. We consider 12-solar-mass and
40-solar-mass presupernova models and systematically study the effects of (i)
rotation, (ii) progenitor structure, and (iii) postbounce neutrino leakage on
dynamics, GW, and, neutrino signals. We demonstrate, that the GW signal of
rapidly rotating core collapse is practically independent of progenitor mass
and precollapse structure. Moreover, we show that the effects of neutrino
leakage on the GW signal are strong only in nonrotating or slowly rotating
models in which GW emission is not dominated by inner core dynamics. In rapidly
rotating cores, core bounce of the centrifugally-deformed inner core excites
the fundamental quadrupole pulsation mode of the nascent protoneutron star. The
ensuing global oscillations (f~700-800 Hz) lead to pronounced oscillations in
the GW signal and correlated strong variations in the rising luminosities of
antineutrino and heavy-lepton neutrinos. We find these features in cores that
collapse to protoneutron stars with spin periods <~ 2.5 ms and rotational
energies sufficient to drive hyper-energetic core-collapse supernova
explosions. Hence, joint GW + neutrino observations of a core collapse event
could deliver strong evidence for or against rapid core rotation. [abridged]Comment: 29 pages, 14 figures. Replaced with version matching published
versio
WIMP-nucleon cross-section results from the second science run of ZEPLIN-III
We report experimental upper limits on WIMP-nucleon elastic scattering cross
sections from the second science run of ZEPLIN-III at the Boulby Underground
Laboratory. A raw fiducial exposure of 1,344 kg.days was accrued over 319 days
of continuous operation between June 2010 and May 2011. A total of eight events
was observed in the signal acceptance region in the nuclear recoil energy range
7-29 keV, which is compatible with background expectations. This allows the
exclusion of the scalar cross-section above 4.8E-8 pb near 50 GeV/c^2 WIMP mass
with 90% confidence. Combined with data from the first run, this result
improves to 3.9E-8 pb. The corresponding WIMP-neutron spin-dependent
cross-section limit is 8.0E-3 pb. The ZEPLIN programme reaches thus its
conclusion at Boulby, having deployed and exploited successfully three liquid
xenon experiments of increasing reach
Accurate calibration of test mass displacement in the LIGO interferometers
We describe three fundamentally different methods we have applied to
calibrate the test mass displacement actuators to search for systematic errors
in the calibration of the LIGO gravitational-wave detectors. The actuation
frequencies tested range from 90 Hz to 1 kHz and the actuation amplitudes range
from 1e-6 m to 1e-18 m. For each of the four test mass actuators measured, the
weighted mean coefficient over all frequencies for each technique deviates from
the average actuation coefficient for all three techniques by less than 4%.
This result indicates that systematic errors in the calibration of the
responses of the LIGO detectors to differential length variations are within
the stated uncertainties.Comment: 10 pages, 6 figures, submitted on 31 October 2009 to Classical and
Quantum Gravity for the proceedings of 8th Edoardo Amaldi Conference on
Gravitational Wave
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