57,109 research outputs found
What Do We Really Know About Cosmic Acceleration?
Essentially all of our knowledge of the acceleration history of the Universe
- including the acceleration itself - is predicated upon the validity of
general relativity. Without recourse to this assumption, we use SNeIa to
analyze the expansion history and find (i) very strong (5 sigma) evidence for a
period of acceleration, (ii) strong evidence that the acceleration has not been
constant, (iii) evidence for an earlier period of deceleration and (iv) only
weak evidence that the Universe has not been decelerating since z~0.3.Comment: 9 pages, 8 figure
Homotopy Quantum Field Theories and Related Ideas
In this short note we provide a review of some developments in the area of
homotopy quantum field theories, loosely based on a talk given by the second
author at the Xth Oporto Meeting on Geometry, Topology and Physics.Comment: 8 pages, 2 figures; correcte
Recycling controls membrane domains
We study the coarsening of strongly microphase separated membrane domains in
the presence of recycling of material. We study the dynamics of the domain size
distribution under both scale-free and size-dependent recycling. Closed form
solutions to the steady state distributions and its associated central moments
are obtained in both cases. Moreover, for the size-independent case, the~time
evolution of the moments is analytically calculated, which provide us with
exact results for their corresponding relaxation times. Since these moments and
relaxation times are measurable quantities, the biophysically significant free
parameters in our model may be determined by comparison with experimental data.Comment: 5 pages, 4 figure
Scalar Field as Dark Matter in the Universe
We investigate the hypothesis that the scalar field is the dark matter and
the dark energy in the Cosmos, wich comprises about 95% of the matter of the
Universe. We show that this hypothesis explains quite well the recent
observations on type Ia supernovae.Comment: 4 pages REVTeX, 1 eps figure. Minor changes. To appear in Classical
and Quantum Gravit
Radiation hydrodynamical models of the inner rim in protoplanetary disks
Many stars host planets orbiting within a few astronomical units (AU). The
occurrence rate and distributions of masses and orbits vary greatly with the
host stars mass. These close planets origins are a mystery that motivates
investigating protoplanetary disks central regions. A key factor governing the
conditions near the star is the silicate sublimation front, which largely
determines where the starlight is absorbed, and which is often called the inner
rim. We present the first radiation hydrodynamical modeling of the sublimation
front in the disks around the young intermediate-mass stars called Herbig Ae
stars. The models are axisymmetric, and include starlight heating, silicate
grains sublimating and condensing to equilibrium at the local, time-dependent
temperature and density, and accretion stresses parametrizing the results of
MHD magneto-rotational turbulence models. The results compare well with
radiation hydrostatic solutions, and prove to be dynamically stable. Passing
the model disks into Monte Carlo radiative transfer calculations, we show that
the models satisfy observational constraints on the inner rims location. A
small optically-thin halo of hot dust naturally arises between the inner rim
and the star. The inner rim has a substantial radial extent, corresponding to
several disk scale heights. While the fronts overall position varies with the
stellar luminosity, its radial extent depends on the mass accretion rate. A
pressure maximum develops near the location of thermal ionization at
temperatures about 1000 K. The pressure maximum is capable of halting solid
pebbles radial drift and concentrating them in a zone where temperatures are
sufficiently high for annealing to form crystalline silicates.Comment: accepted for Ap
The grand unified photon spectrum: A coherent view of the diffuse extragalactic background radiation
The spectrum of diffuse extragalactic background radiation (DEBRA) at wavelengths from 10(exp 5) to 10(exp -24) cm is presented in a coherent fashion. Each wavelength region, from the radio to ultra-high energy photons and cosmic rays, is treated both separately and as part of the grand unified photon spectrum (GUPS). A discussion of, and references to, the relevant literature for each wavelength region is included. This review should provide a useful tool for those interested in diffuse backgrounds, the epoch of galaxy formation, astrophysical/cosmological constraints to particle properties, exotic early Universe processes, and many other astrophysical and cosmological enterprises. As a worked example, researchers derive the cosmological constraints to an unstable-neutrino spies (with arbitrary branching ratio to a radiative decay mode) that follow from the GUPS
Effects of ordinary and superconducting cosmic strings on primordial nucleosynthesis
A precise calculation is done of the primordial nucleosynthesis constraint on the energy per length of ordinary and superconducting cosmic strings. A general formula is provided for the constraint on the string tension for ordinary strings. Using the current values for the various parameters that describe the evolution of loops, the constraint for ordinary strings is G mu less than 2.2 x 10 to the minus 5 power. Our constraint is weaker than previously quoted limits by a factor of approximately 5. For superconducting loops, with currents generated by primordial magnetic fields, the constraint can be less or more stringent than this limit, depending on the strength of the magnetic field. It is also found in this case that there is a negligible amount of entropy production if the electromagnetic radiation from strings thermalizes with the radiation background
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