1,693 research outputs found
The graceful exit in pre-big bang string cosmology
We re-examine the graceful exit problem in the pre-big bang scenario of
string cosmology, by considering the most general time-dependent classical
correction to the Lagrangian with up to four derivatives. By including possible
forms for quantum loop corrections we examine the allowed region of parameter
space for the coupling constants which enable our solutions to link smoothly
the two asymptotic low-energy branches of the pre-big bang scenario, and
observe that these solutions can satisfy recently proposed entropic bounds on
viable singularity free cosmologies.Comment: 14 pages, 6 figures, JHEP class. Added new section on the classical
correction and reference
Cosmological perturbations and the transition from contraction to expansion
We investigate both analytically and numerically the evolution of scalar
perturbations generated in models which exhibit a smooth transition from a
contracting to an expanding Friedmann universe. We find that the resulting
spectral index in the late radiation dominated universe depends on which of the
or \ variables passes regularly through the transition. The
results can be parameterized through the exponent defining the rate of
contraction of the universe. For we find that there are no stable
cases where both variables are regular during the transition. In particular,
for , we find that the resulting spectral index is close to scale
invariant if is regular, whereas it has a steep blue behavior if
is regular. We also show that as long as , perturbations arising
from the Bardeen potential remain small during contraction in the sense that
there exists a gauge in which all the metric and matter perturbation variables
are small.Comment: 30 pages, 16 figures. Version to appear in Phys. Rev. D. Slight
modifications, but no change in the conclusio
Revision of Earth-sized Kepler Planet Candidate Properties with High Resolution Imaging by Hubble Space Telescope
We present the results of our Hubble Space Telescope program and describe how
our analysis methods were used to re-evaluate the habitability of some of the
most interesting Kepler planet candidates. Our program observed 22 Kepler
Object of Interest (KOI) host stars, several of which were found to be multiple
star systems unresolved by Kepler. We use our high-resolution imaging to
spatially resolve the stellar multiplicity of Kepler-296, KOI-2626, and
KOI-3049, and develop a conversion to the Kepler photometry (Kp) from the F555W
and F775W filters on WFC3/UVIS. The binary system Kepler-296 (5 planets) has a
projected separation of 0.217" (80AU); KOI-2626 (1 planet candidate) is a
triple star system with a projected separation of 0.201" (70AU) between the
primary and secondary components and 0.161" (55AU) between the primary and
tertiary; and the binary system KOI-3049 (1 planet candidate) has a projected
separation of 0.464" (225AU). We use our measured photometry to fit the
separated stellar components to the latest Victoria-Regina Stellar Models with
synthetic photometry to conclude that the systems are coeval. The components of
the three systems range from mid-K dwarf to mid-M dwarf spectral types. We
solved for the planetary properties of each system analytically and via an MCMC
algorithm using our independent stellar parameters. The planets range from
~1.6R_Earth to ~4.2R_Earth, mostly Super Earths and mini-Neptunes. As a result
of the stellar multiplicity, some planets previously in the Habitable Zone are,
in fact, not, and other planets may be habitable depending on their assumed
stellar host.Comment: 16 pages, 10 figures, ApJ, 804, 9
Fourier Transforms of Lorentz Invariant Functions
Fourier transforms of Lorentz invariant functions in Minkowski space, with
support on both the timelike and the spacelike domains are performed by means
of direct integration. The cases of 1+1 and 1+2 dimensions are worked out in
detail, and the results for 1+n dimensions are given.Comment: 15 pages, 1 figur
Hubble Space Telescope High Resolution Imaging of Kepler Small and Cool Exoplanet Host Stars
High resolution imaging is an important tool for follow-up study of exoplanet
candidates found via transit detection with the Kepler Mission. We discuss here
HST imaging with the WFC3 of 23 stars that host particularly interesting Kepler
planet candidates based on their small size and cool equilibrium temperature
estimates. Results include detections, exclusion of background stars that could
be a source of false positives for the transits, and detection of
physically-associated companions in a number of cases providing dilution
measures necessary for planet parameter refinement. For six KOIs, we find that
there is ambiguity in which star hosts the transiting planet(s), with
potentially strong implications for planetary characteristics. Our sample is
evenly distributed in G, K, and M spectral types. Albeit with a small sample
size, we find that physically-associated binaries are more common than expected
at each spectral type, reaching a factor of 10 frequency excess at M. We
document the program detection sensitivities, detections, and deliverables to
the Kepler follow-up program archive.Comment: Accepted for the Astronomical Journal; 13 pages with 9 figure
On the "Causality Argument" in Bouncing Cosmologies
We exhibit a situation in which cosmological perturbations of astrophysical
relevance propagating through a bounce are affected in a scale-dependent way.
Involving only the evolution of a scalar field in a closed universe described
by general relativity, the model is consistent with causality. Such a specific
counter-example leads to the conclusion that imposing causality is not
sufficient to determine the spectrum of perturbations after a bounce provided
it is known before. We discuss consequences of this result for string motivated
scenarios.Comment: 4 pages, 1 figure, ReVTeX, to appear in Phys. Rev. Let
Therapeutic Vascular Compliance Change May Cause Significant Variation in Coronary Perfusion: A Numerical Study
In some pathological conditions like aortic stiffening and calcific aortic stenosis (CAS), the microstructure of the aortic root and the aortic valve leaflets are altered in response to stress resulting in changes in tissue thickness, stiffness, or both. This aortic stiffening and CAS are thought to affect coronary blood flow. The goal of the present paper was to include the flow in the coronary ostia in the previous fluid structure interaction model we have developed and to analyze the effect of diseased tissues (aortic root stiffening and CAS) on coronary perfusion. Results revealed a significant impact on the coronary perfusion due to a moderate increase in the aortic wall stiffness and CAS (increase of the aortic valve leaflets thickness). A marked drop of coronary peak velocity occurred when the values of leaflet thickness and aortic wall stiffness were above a certain threshold, corresponding to a threefold of their normal value. Consequently, mild and prophylactic treatments such as smoking cessation, exercise, or diet, which have been proven to increase the aortic compliance, may significantly improve the coronary perfusion
Operations on integral lifts of K(n)
This very rough sketch is a sequel to arXiv:1808.08587; it presents evidence
that operations on lifts of the functors K(n) to cohomology theories with
values in modules over valuation rings of local number fields, indexed by
Lubin-Tate groups of such fields, are extensions of the groups of automorphisms
of the indexing group laws, by the exterior algebras on the normal bundle to
the orbits of the group laws in the space of lifts.Comment: \S 2.0 hopefully less cryptic. To appear in the proceedings of the
2015 Nagoya conference honoring T Ohkawa. Comments very welcome
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