6,998 research outputs found
Particle Dark Energy
We explore the physics of a gas of particles interacting with a condensate
that spontaneously breaks Lorentz invariance. The equation of state of this gas
varies from 1/3 to less than -1 and can lead to the observed cosmic
acceleration. The particles are always stable. In our particular class of
models these particles are fermions with a chiral coupling to the condensate.
They may behave as relativistic matter at early times, produce a brief period
where they dominate the expansion with w<0 today, and behave as matter at late
time. There are no small parameters in our models, which generically lead to
dark energy clustering and, depending on the choice of parameters, smoothing of
small scale power.Comment: 8 pages, 5 figures; minor update with added refs; version appearing
in Phys. Rev.
Draft Genome Sequence of the Salt Water Bacterium Oceanospirillum linum ATCC 11336T
Oceanospirillum linum ATCC 11336T is an aerobic, bipolar-tufted gammaproteobacterium first isolated in the Long Island Sound in the 1950s. This announcement offers a genome sequence for O. linum ATCC 11336T, which has a predicted genome size of 3,782,189 bp (49.13% G+C content) containing 3,540 genes and 3,361 coding sequences
Genome Sequencing of a Marine Spirillum, Oceanospirillum multiglobuliferum ATCC 33336T, from Japan
Oceanospirillum multiglobuliferum ATCC 33336T is a motile gammaproteobacterium with bipolar tufted flagella, noted for its low salt tolerance compared to other marine spirilla. This strain was originally isolated from the putrid infusions of Crassostrea gigas near Hiroshima, Japan. This paper presents a draft genome sequence for O. multiglobuliferum ATCC 33336T
Searches for massive neutrinos with mechanical quantum sensors
The development of quantum optomechanics now allows mechanical sensors with
femtogram masses to be controlled and measured in the quantum regime. If the
mechanical element contains isotopes that undergo nuclear decay, measuring the
recoil of the sensor following the decay allows reconstruction of the total
momentum of all emitted particles, including any neutral particles that may
escape detection in traditional detectors. As an example, for weak nuclear
decays the momentum of the emitted neutrino can be reconstructed on an
event-by-event basis. We present the concept that a single nanometer-scale,
optically levitated sensor operated with sensitivity near the standard quantum
limit can search for heavy sterile neutrinos in the keV-MeV mass range with
sensitivity significantly beyond existing constraints. We also comment on the
possibility that mechanical sensors operated well into the quantum regime might
ultimately reach the sensitivities required to provide an absolute measurement
of the mass of the light neutrino states.Comment: 11 pages + refs, 7 figures. v2: published version (+ an appendix
containing a quantum model of the 3-body decay in a nanosphere
Diffraction tomography using power extinction measurements
We propose a new method for determining structures of semitransparent media from measurements of the extinguished power in scattering experiments. The method circumvents the problem of measuring the phase of the scattered field. We illustrate how this technique may be used to reconstruct both deterministic and random scatterers
A Keck HIRES Doppler Search for Planets Orbiting Metal-Poor Dwarfs. II. On the Frequency of Giant Planets in the Metal-Poor Regime
We present an analysis of three years of precision radial velocity
measurements of 160 metal-poor stars observed with HIRES on the Keck 1
telescope. We report on variability and long-term velocity trends for each star
in our sample. We identify several long-term, low-amplitude radial-velocity
variables worthy of follow-up with direct imaging techniques. We place lower
limits on the detectable companion mass as a function of orbital period. Our
survey would have detected, with a 99.5% confidence level, over 95% of all
companions on low-eccentricity orbits with velocity semi-amplitude K > 100 m/s,
or M_p*sin(i) > 3.0 M_JUP*(P/yr)^(1/3), for orbital periods P< 3 yr. None of
the stars in our sample exhibits radial-velocity variations compatible with the
presence of Jovian planets with periods shorter than the survey duration. The
resulting average frequency of gas giants orbiting metal-poor dwarfs with -2.0
< [Fe/H] < -0.6 is f_p<0.67% (at the 1-sigma confidence level). We examine the
implications of this null result in the context of the observed correlation
between the rate of occurrence of giant planets and the metallicity of their
main-sequence solar-type stellar hosts. By combining our dataset with the
Fischer & Valenti (2005) uniform sample, we confirm that the likelihood of a
star to harbor a planet more massive than Jupiter within 2 AU is a steeply
rising function of the host's metallicity. However, the data for stars with
-1.0 < [Fe/H] < 0.0 are compatible, in a statistical sense, with a constant
occurrence rate f_p~1%. Our results can usefully inform theoretical studies of
the process of giant planet formation across two orders of magnitude in
metallicity.Comment: 59 pages, 7 tables, 8 figures. Accepted for publication in the
Astrophysical Journa
Rational Design and Assembly of Synthetic Trimodular Polyketide Synthases
SummaryType I polyketide synthases (PKSs) consist of modules that add two-carbon units in polyketide backbones. Rearranging modules from different sources can yield novel enzymes that produce unnatural products, but the rules that govern module-module communication are still not well known. The construction and assay of hybrid bimodular units with synthetic PKS genes were recently reported. Here, we describe the rational design of trimodular PKSs by combining bimodular units. A cloning-expression system was developed to assemble and test 54 unnatural trimodular PKSs flanked by the loading module and the thioesterase from the erythromycin synthase. Remarkably, 96% of them produced the expected polyketide. The obtained results represent an important milestone toward the ultimate goal of making new bioactive polyketides by rational design. Additionally, these results show a path for the production of customized tetraketides by fermentation, which can be an important source of advanced intermediates to facilitate the synthesis of complex products
The Absolute Magnitudes of Red Horizontal Branch Stars in the ugriz System
Based on photometric data of the central parts of eight globular clusters and
one open cluster presented by An and his collaborators, we select red
horizontal branch (RHB) stars in the (g-r)0-g0 diagram and make a statistical
study of the distributions of their colors and absolute magnitudes in the SDSS
ugriz system. Meanwhile, absolute magnitudes in the Johnson VRI system are
calculated through the translation formulae between gri and VRI in the
literature. The calibrations of absolute magnitude as functions of metallicity
and age are established by linear regressions of the data.
It is found that metallicity coefficients in these calibrations decrease,
while age coefficients increase, from the blue filter to the red
filter. The calibration of Mi= 0.06[Fe/H]+0.040t+0.03 has the smallest scatter
of 0.04 mag, and thus i is the best filter in the system when RHB stars
are used for distance indicators. The comparison of the MI calibration from our
data with that from red clump stars indicates that the previous suggestion that
the filter is better than the V filter in distance determination may not be
true because of its significant dependence on age.Comment: 19 pages, 7 figures, accepted by Ap
Book Reviews
THE PROSPECTS FOR REVOLUTION: A STUDY OF RIOTS, CIVIL DISOBEDIENCE, AND INSURRECTION IN CONTEMPORARY AMERICA. By Ralph W. Conant. New York: Harper\u27s Magazine Press, 1971. Pp. xi, 290. 5.95 (Cloth), 7.95
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