4,312 research outputs found
Why the Universe Started from a Low Entropy State
We show that the inclusion of backreaction of massive long wavelengths
imposes dynamical constraints on the allowed phase space of initial conditions
for inflation, which results in a superselection rule for the initial
conditions. Only high energy inflation is stable against collapse due to the
gravitational instability of massive perturbations. We present arguments to the
effect that the initial conditions problem {\it cannot} be meaningfully
addressed by thermostatistics as far as the gravitational degrees of freedom
are concerned. Rather, the choice of the initial conditions for the universe in
the phase space and the emergence of an arrow of time have to be treated as a
dynamic selection.Comment: 12 pages, 2 figs. Final version; agrees with accepted version in
Phys. Rev.
The Size Distribution of Trans-Neptunian Bodies
[Condensed] We search 0.02 deg^2 for trans-Neptunian objects (TNOs) with
m<=29.2 (diameter ~15 km) using the ACS on HST. Three new objects are
discovered, roughly 25 times fewer than expected from extrapolation of the
differential sky density Sigma(m) of brighter objects. The ACS and other recent
TNO surveys show departures from a power law size distribution. Division of the
TNO sample into ``classical Kuiper belt'' (CKB) and ``Excited'' samples reveals
that Sigma(m) differs for the two populations at 96% confidence. A double power
law adequately fits all data. Implications include: The total mass of the CKB
is ~0.010 M_Earth, only a few times Pluto's mass, and is predominately in the
form of ~100 km bodies. The mass of Excited objects is perhaps a few times
larger. The Excited class has a shallower bright-end size distribution; the
largest objects, including Pluto, comprise tens of percent of the total mass
whereas the largest CKBOs are only ~2% of its mass. The predicted mass of the
largest Excited body is close to the Pluto mass; the largest CKBO is ~60 times
less massive. The deficit of small TNOs occurs for sizes subject to disruption
by present-day collisions, suggesting extensive depletion by collisions. Both
accretion and erosion appearing to have proceeded to more advanced stages in
the Excited class than the CKB. The absence of distant TNOs implies that any
distant (60 AU) population must have less than the CKB mass in the form of
objects 40 km or larger. The CKB population is sparser than theoretical
estimates of the required precursor population for short period comets, but the
Excited population could be a viable precursor population.Comment: Revised version accepted to the Astronomical Journal. Numerical
results are very slightly revised. Implications for the origins of
short-period comets are substantially revised, and tedious material on
statistical tests has been collected into a new Appendi
Accelerating housing production in London: main findings
Over the last year LSE London organised a series of seminars, round tables and site visits around this the question of how to accelerate new housing development in the capital. Together with practitioners we have analysed barriers to increasing the pace of development in London and explored possible solutions. Participants at our events have included architects, developers and academics, as well as representatives of central government, boroughs, the GLA and housing advocacy groups. More detailed reports can be found at lselondonhousing.org/
Rising to the challenge: London's housing crisis
The capital’s most pressing problem is how to accelerate the production of new housing, which has been stubbornly unresponsive to market and demographic pressures. Over the past two years LSE London has been conducting a wide-ranging academic and practical inquiry into the issues and the possible solutions. Over the course of the project we held discussions in council chambers, architects’ studios and City offices, donned hard hats to tour construction sites and pored over financial models. This document presents our findings and recommendations. In the boxes five key players on the London development scene speak frankly (and anonymously) about the challenges they face
The Transit Light Curve Project. XII. Six Transits of the Exoplanet XO-2b
We present photometry of six transits of the exoplanet XO-2b. By combining
the light-curve analysis with theoretical isochrones to determine the stellar
properties, we find the planetary radius to be 0.996 +0.031/-0.018 rjup and the
planetary mass to be 0.565 +/- 0.054 mjup. These results are consistent with
those reported previously, and are also consistent with theoretical models for
gas giant planets. The mid-transit times are accurate to within 1 min and are
consistent with a constant period. However, the period we derive differs by 2.5
sigma from the previously published period. More data are needed to tell
whether the period is actually variable (as it would be in the presence of an
additional body) or if the timing errors have been underestimated.Comment: Accepted for publication in AJ. 20 pages, 3 tables, 4 figure
The potential for circular dichroism as an additional facile and sensitive method of monitoring low-molecular-weight heparins and heparinoids
The ultraviolet circular dichroism (CD) spectra of commercial low-molecular-weight heparins, heparinoids and other anticoagulant preparations have been recorded between 180 and 260 nm. Principal component analysis of the spectra allowed their differentiation into a number of groups related to the means of their production reflecting the structural changes introduced by each process. The findings suggest that CD provides a complementary technique for the rapid analysis of heparin preparations
Prospective relationships between body weight and physical activity: an observational analysis from the NAVIGATOR study
Objectives: While bidirectional relationships exist between body weight and physical activity, direction of causality remains uncertain and previous studies have been limited by self-reported activity or weight and small sample size. We investigated the prospective relationships between weight and physical activity.
Design: Observational analysis of data from the Nateglinide And Valsartan in Impaired Glucose Tolerance Outcomes Research (NAVIGATOR) study, a double-blinded randomised clinical trial of nateglinide and valsartan, respectively.
Setting Multinational study of 9306 participants.
Participants: Participants with biochemically confirmed impaired glucose tolerance had annual measurements of both weight and step count using research grade pedometers, worn for 7 days consecutively. Along with randomisation to valsartan or placebo plus nateglinide or placebo, participants took part in a lifestyle modification programme.
Outcome measures: Longitudinal regression using weight as response value and physical activity as predictor value was conducted, adjusted for baseline covariates. Analysis was then repeated with physical activity as response value and weight as predictor value. Only participants with a response value preceded by at least three annual response values were included.
Results: Adequate data were available for 2811 (30%) of NAVIGATOR participants. Previous weight (χ2=16.8; p<0.0001), but not change in weight (χ2=0.1; p=0.71) was inversely associated with subsequent step count, indicating lower subsequent levels of physical activity in heavier individuals. Change in step count (χ2=5.9; p=0.02) but not previous step count (χ2=0.9; p=0.34) was inversely associated with subsequent weight. However, in the context of trajectories already established for weight (χ2 for previous weight measurements 747.3; p<0.0001) and physical activity (χ2 for previous step count 432.6; p<0.0001), these effects were of limited clinical importance.
Conclusions: While a prospective bidirectional relationship was observed between weight and physical activity, the magnitude of any effect was very small in the context of natural trajectories already established for these variables
The Short Rotation Period of Hi'iaka, Haumea's Largest Satellite
Hi'iaka is the larger outer satellite of the dwarf planet Haumea. Using
relative photometry from the Hubble Space Telescope and Magellan and a phase
dispersion minimization analysis, we have identified the rotation period of
Hi'iaka to be ~9.8 hrs (double-peaked). This is ~120 times faster than its
orbital period, creating new questions about the formation of this system and
possible tidal evolution. The rapid rotation suggests that Hi'iaka could have a
significant obliquity and spin precession that could be visible in light curves
within a few years. We then turn to an investigation of what we learn about the
(presently unclear) formation of the Haumea system and family based on this
unexpectedly rapid rotation rate. We explore the importance of the initial
semi-major axis and rotation period in tidal evolution theory and find they
strongly influence the time required to despin to synchronous rotation,
relevant to understanding a wide variety of satellite and binary systems. We
find that despinning tides do not necessarily lead to synchronous spin periods
for Hi'iaka, even if it formed near the Roche limit. Therefore the short
rotation period of Hi'iaka does not rule out significant tidal evolution.
Hi'iaka's spin period is also consistent with formation near its current
location and spin up due to Haumea-centric impactors.Comment: 21 pages with 6 figures, to be published in The Astronomical Journa
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Slurry-Based 3DP and Fine Ceramic Components
Slurry-based 3DPTM (S-3DPÔ) is a solid freeform fabrication technique developed at MIT for
production of fine ceramic components with complex geometries and fired densities in excess of
99% of theoretical density. Current research involves identification of the factors controlling
minimum feature size in S-3DPTM. The ink-jet printed binder droplet size is the primary factor
controlling the minimum feature dimension when deposited on the powder layers. For a given
droplet size, however, a balance between spreading of the binder solution on the surface of the S3DPTM powderbed and infiltration determine the feature size, while interactions between the
polymeric binder and the powder surface (polymer adsorption) control the minimum feature
cross-section. Droplet-on-demand printing of the binder solution has been introduced to improve
resolution, decreasing the minimum feature width from 300 mm to less than 150 mm.Mechanical Engineerin
The Short Rotation Period of Nereid
We determine the period, p = 11.52 \pm 0.14 h, and a light curve peak-to-peak
amplitude, a = 0.029 \pm 0.003 magnitudes, of the Neptunian irregular satellite
Nereid. If the light curve variation is due to albedo variations across the
surface, rather than solely to the shape of Nereid variations, the rotation
period would be a factor of two shorter. In either case, such a rotation period
and light curve amplitude, together with Nereid's orbital period, p=360.14
days, imply that Nereid is almost certainly in a regular rotation state, rather
than the chaotic rotation state suggested by Schaefer and Schaefer (1988,2000)
and Dobrovolskis (1995).
Assuming that Nereid is perfectly spherical, the albedo variation is 3%
across the observed surface. Assuming a uniform geometric albedo, the observed
cross sectional area varies by 3%. We caution that the lightcurve found in this
paper only sets limits on the combination of albedo and physical irregularity
and that we cannot determine the orientation of Nereid's spin axis from our
data.Comment: Accepted by ApJ Letters, 11 pages (incl. 1 figure
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