73 research outputs found
Membrane Models as a Means of Propulsion in General Relativity: Super-Luminal Warp-Drive that Satisfies the Weak Energy Condition
Presented are toy-models for sub-luminal and super-luminal warp-drives in 3+1
dimensions. The models are constructed in a chimeric manner - as different bulk
space-times separated by thin membranes. The membranes contain
perfect-fluid-like stress-energy tensors. The Israel junction conditions relate
this stress-energy to a jump in extrinsic curvature across the brane, which in
turn manifests as apparent acceleration in the bulk space-times. The
acceleration on either side of the brane may be set individually by choice of
model parameters. The Weak Energy Condition (WEC) is shown to be satisfied
everywhere in both models. Although the branes in these toy models are not
compact, it is demonstrated that super-luminal warp-drive is possible that
satisfies the WEC. Additionally, the nature of these models provides framework
for speculation on a mechanism for transition from sub-luminal to super-luminal
warp. Neither quantum effects nor stability of the models is considered.Comment: 52 pages, 9 figure
Robustness of the Quintessence Scenario in Particle Cosmologies
We study the robustness of the quintessence tracking scenario in the context
of more general cosmological models that derive from high-energy physics. We
consider the effects of inclusion of multiple scalar fields, corrections to the
Hubble expansion law (such as those that arise in brane cosmological models),
and potentials that decay with expansion of the Universe. We find that in a
successful tracking quintessence model the average equation of state must
remain nearly constant. Overall, the conditions for successful tracking become
more complex in these more general settings. Tracking can become more fragile
in presence of multiple scalar fields, and more stable when temperature
dependent potentials are present. Interestingly though, most of the cases where
tracking is disrupted are those in which the cosmological model is itself
non-viable due to other constraints. In this sense tracking remains robust in
models that are cosmologically viable
Interacting Quintessence, Cosmic Acceleration and the Coincidence Problem
Faced by recent evidence for a flat universe dominated by dark energy,
cosmologists grapple with deep cosmic enigmas such as the cosmological constant
problem, extreme fine-tuning and the cosmic coincidence problem. The extent to
which we observe the dimming of distant supernovae suggests that the cosmic
acceleration is as least as severe as in cosmological constant models.
Extrapolating this to our cosmic future implies terrifying visions of either a
cold and empty universe or an explosive demise in a ``Big Rip.'' We construct a
class of dynamical scalar field models of dark energy and dark matter. Within
this class we can explain why supernovae imply a cosmic equation of state
, address fine tuning issues, protect the universe from premature
acceleration and predict a constant fraction of dark energy to dark matter in
the future (thus solving the coincidence problem), satisfy the dominant energy
condition, and ensure that gravitationally bound objects remain so forever
(avoid a Big Rip). This is achieved with a string theory inspired Lagrangian
containing standard kinetic terms, exponential potentials and couplings, and
parameters of order unity.Comment: 4 pages and 3 figures, submitted to PRL on March 25 200
Bayesian analysis of the low-resolution polarized 3-year WMAP sky maps
We apply a previously developed Gibbs sampling framework to the foreground
corrected 3-yr WMAP polarization data and compute the power spectrum and
residual foreground template amplitude posterior distributions. We first
analyze the co-added Q- and V-band data, and compare our results to the
likelihood code published by the WMAP team. We find good agreement, and thus
verify the numerics and data processing steps of both approaches. However, we
also analyze the Q- and V-bands separately, allowing for non-zero EB
cross-correlations and including two individual foreground template amplitudes
tracing synchrotron and dust emission. In these analyses, we find tentative
evidence of systematics: The foreground tracers correlate with each of the Q-
and V-band sky maps individually, although not with the co-added QV map; there
is a noticeable negative EB cross-correlation at l <~ 16 in the V-band map; and
finally, when relaxing the constraints on EB and BB, noticeable differences are
observed between the marginalized band powers in the Q- and V-bands. Further
studies of these features are imperative, given the importance of the low-l EE
spectrum on the optical depth of reionization tau and the spectral index of
scalar perturbations n_s.Comment: 5 pages, 4 figures, submitted to ApJ
Washington State Ferries: update on ferry vessel noise in the Salish Sea
Protecting the Salish Sea environment, the Southern Resident Killer Whales and other marine mammals is part of the mission of Washington State Ferries (WSF), and British Columbia Ferries (BCF). This presentation provides an update on underwater noise data analysis of WSF and BCF ferry vessels, plans for further data collection and potential noise mitigation strategies. WSF and BCF are members of the Port of Vancouver Enhancing Cetacean Habitat and Observation Program (ECHO)
A Markov Chain Monte Carlo Algorithm for analysis of low signal-to-noise CMB data
We present a new Monte Carlo Markov Chain algorithm for CMB analysis in the
low signal-to-noise regime. This method builds on and complements the
previously described CMB Gibbs sampler, and effectively solves the low
signal-to-noise inefficiency problem of the direct Gibbs sampler. The new
algorithm is a simple Metropolis-Hastings sampler with a general proposal rule
for the power spectrum, C_l, followed by a particular deterministic rescaling
operation of the sky signal. The acceptance probability for this joint move
depends on the sky map only through the difference of chi-squared between the
original and proposed sky sample, which is close to unity in the low
signal-to-noise regime. The algorithm is completed by alternating this move
with a standard Gibbs move. Together, these two proposals constitute a
computationally efficient algorithm for mapping out the full joint CMB
posterior, both in the high and low signal-to-noise regimes.Comment: Submitted to Ap
A re-analysis of the three-year WMAP temperature power spectrum and likelihood
We analyze the three-year WMAP temperature anisotropy data seeking to confirm
the power spectrum and likelihoods published by the WMAP team. We apply five
independent implementations of four algorithms to the power spectrum estimation
and two implementations to the parameter estimation. Our single most important
result is that we broadly confirm the WMAP power spectrum and analysis. Still,
we do find two small but potentially important discrepancies: On large angular
scales there is a small power excess in the WMAP spectrum (5-10% at l<~30)
primarily due to likelihood approximation issues between 13 <= l <~30. On small
angular scales there is a systematic difference between the V- and W-band
spectra (few percent at l>~300). Recently, the latter discrepancy was explained
by Huffenberger et al. (2006) in terms of over-subtraction of unresolved point
sources. As far as the low-l bias is concerned, most parameters are affected by
a few tenths of a sigma. The most important effect is seen in n_s. For the
combination of WMAP, Acbar and BOOMERanG, the significance of n_s =/ 1 drops
from ~2.7 sigma to ~2.3 sigma when correcting for this bias. We propose a few
simple improvements to the low-l WMAP likelihood code, and introduce two
important extensions to the Gibbs sampling method that allows for proper
sampling of the low signal-to-noise regime. Finally, we make the products from
the Gibbs sampling analysis publically available, thereby providing a fast and
simple route to the exact likelihood without the need of expensive matrix
inversions.Comment: 14 pages, 7 figures. Accepted for publication in ApJ. Numerical
results unchanged, but interpretation sharpened: Likelihood approximation
issues at l=13-30 far more important than potential foreground issues at l <=
12. Gibbs products (spectrum and sky samples, and "easy-to-use" likelihood
module) available from http://www.astro.uio.no/~hke/ under "Research
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The impact of local sources and long-range transport on aerosol properties over the northeast U.S. region during INTEX-NA
We use data collected aboard the NASA DC-8 aircraft during the summer 2004, Intercontinental Transport and Chemical Evolution Experiment over North America (INTEX-NA) field campaign to examine the origin, composition, physical and optical properties of aerosols within air masses sampled over and downwind of the northeastern U.S. We note that aerosol concentrations within the region exhibited steep vertical gradients and significant variability in both time and space. An examination of air mass chemical signatures and backward trajectories indicates that transport from four, significantly different source regions contributed to the variability: the subtropical Atlantic Ocean (AO); the U.S. west coast and eastern Pacific (WCP); the U.S. east coast and Midwestern states (EC); and northwest Canada and Alaska (CA). AO air masses were typically confined to below 2 km altitude, exhibited low pollutant contents, contained enhanced levels of sea salt, and were typically observed when the Bermuda High strengthened. The most common air mass present in the upper troposphere, WCP air often contained weak dust and aged pollution enhances from convective input occurring over the central part of the continent. CA air exhibited enhancements in anthropogenic pollution tracers below 2 km and contained some black-carbon rich haze layers between 3 and 5 km that could be traced to forest fires burning in western Canada and Alaska. EC air was prevalent at lower elevations throughout the study area and exhibited enhanced scattering along with elevated levels of sulfate aerosols and combustion tracers. There is an overall balance between the observed cations and anions for all cases, except EC air mass below 4 km
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