1,711 research outputs found
Exploring the Expansion History of the Universe
Exploring the recent expansion history of the universe promises insights into
the cosmological model, the nature of dark energy, and potentially clues to
high energy physics theories and gravitation. We examine the extent to which
precision distance-redshift observations can map out the history, including the
acceleration-deceleration transition, and the components and equations of state
of the energy density. We consider the ability to distinguish between various
dynamical scalar field models for the dark energy, as well as higher dimension
and alternate gravity theories. Finally, we present a new, advantageous
parametrization for the study of dark energy.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Letter
Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo
We re-examine a deep {\it Hubble Space Telescope} pencil-beam search for red
dwarfs, stars just massive enough to burn Hydrogen. The authors of this search
(Bahcall, Flynn, Gould \& Kirhakos 1994) found that red dwarfs make up less
than 6\% of the galactic halo. First, we extrapolate this result to include
brown dwarfs, stars not quite massive enough to burn hydrogen; we assume a
mass function. Then the total mass of red dwarfs and brown dwarfs
is 18\% of the halo. This result is consistent with microlensing results
assuming a popular halo model. However, using new stellar models and parallax
observations of low mass, low metallicity stars, we obtain much tighter bounds
on low mass stars. We find the halo red dwarf density to be of the halo,
while our best estimate of this value is 0.14-0.37\%. Thus our estimate of the
halo mass density of red dwarfs drops to 16-40 times less than the reported
result of Bahcall et al (1994). For a mass function, this suggests
a total density of red dwarfs and brown dwarfs of 0.25-0.67\% of the
halo, \ie , (0.9-2.5)\times 10^9\msun out to 50 kpc. Such a low result would
conflict with microlensing estimates by the \macho\ group (Alcock \etal
1995a,b).Comment: 13 pages, 2 figures. Figure one only available via fax or snail-mail
To be published in ApJL. fig. 2 now available in postscript. Some minor
changes in dealing with disk forground. Some cosmetic changes. Updated
reference
Dark Stars: Improved Models and First Pulsation Results
We use the stellar evolution code MESA to study dark stars. Dark stars (DSs),
which are powered by dark matter (DM) self-annihilation rather than by nuclear
fusion, may be the first stars to form in the Universe. We compute stellar
models for accreting DSs with masses up to 10^6 M_{sun}. The heating due to DM
annihilation is self-consistently included, assuming extended adiabatic
contraction of DM within the minihalos in which DSs form. We find remarkably
good overall agreement with previous models, which assumed polytropic
interiors. There are some differences in the details, with positive
implications for observability. We found that, in the mass range of 10^4 -10^5
M_{sun}, our DSs are hotter by a factor of 1.5 than those in Freese et
al.(2010), are smaller in radius by a factor of 0.6, denser by a factor of 3 -
4, and more luminous by a factor of 2. Our models also confirm previous
results, according to which supermassive DSs are very well approximated by
(n=3)-polytropes. We also perform a first study of dark star pulsations. Our DS
models have pulsation modes with timescales ranging from less than a day to
more than two years in their rest frames, at z ~ 15, depending on DM particle
mass and overtone number. Such pulsations may someday be used to identify
bright, cool objects uniquely as DSs; if properly calibrated, they might, in
principle, also supply novel standard candles for cosmological studies.Comment: 17 pages; 11 figures; revised version; accepted by Ap
The Paths of Quintessence
The structure of the dark energy equation of state phase plane holds
important information on the nature of the physics. We explain the bounds of
the freezing and thawing models of scalar field dark energy in terms of the
tension between the steepness of the potential vs. the Hubble drag.
Additionally, we extend the phase plane structure to modified gravity theories,
examine trajectories of models with certain properties, and categorize regions
in terms of scalar field hierarchical parameters, showing that dark energy is
generically not a slow roll phenomenon.Comment: 12 pages, 7 figures; matches PRD versio
On an easy transition from operator dynamics to generating functionals by Clifford algebras
Clifford geometric algebras of multivectors are treated in detail. These
algebras are build over a graded space and exhibit a grading or multivector
structure. The careful study of the endomorphisms of this space makes it clear,
that opposite Clifford algebras have to be used also. Based on this
mathematics, we give a fully Clifford algebraic account on generating
functionals, which is thereby geometric. The field operators are shown to be
Clifford and opposite Clifford maps. This picture relying on geometry does not
need positivity in principle. Furthermore, we propose a transition from
operator dynamics to corresponding generating functionals, which is based on
the algebraic techniques. As a calculational benefit, this transition is
considerable short compared to standard ones. The transition is not injective
(unique) and depends additionally on the choice of an ordering. We obtain a
direct and constructive connection between orderings and the explicit form of
the functional Hamiltonian. These orderings depend on the propagator of the
theory and thus on the ground state. This is invisible in path integral
formulations. The method is demonstrated within two examples, a non-linear
spinor field theory and spinor QED. Antisymmetrized and normal-ordered
functional equations are derived in both cases.Comment: 23p., 76kB, plain LaTeX, [email protected]
Effects of vertical vibration on hopper flows of granular material
The discharge of granular material from a hopper subject to vertical sinusoidal oscillations was investigated using experiments and discrete element computer simulations. With the hopper exit closed, side-wall convection cells are observed, oriented such that particles move up along the inclined walls of the hopper and down at the center line. The convection cells are a result of the granular bed dilation during free fall and the subsequent interaction with the hopper walls. The mass discharge rate for a vibrating hopper scaled by the discharge rate without vibration reaches a maximum value at a dimensionless velocity amplitude just greater than 1. Further increases in the velocity decrease the discharge rate. The decrease occurs due to a decrease in the bulk density of the discharging material when vibration is applied
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