1,827 research outputs found
Death of Stellar Baryonic Dark Matter
The nature of the dark matter in the haloes of galaxies is one of the
outstanding questions in astrophysics. All stellar candidates, until recently
thought to be likely baryonic contributions to the Halo of our Galaxy, are
shown to be ruled out. Faint stars and brown dwarfs are found to constitute
only a few percent of the mass of the Galaxy. Stellar remnants, including white
dwarfs and neutron stars, are shown to be very constrained as well. High energy
gamma-rays observed in HEGRA data place the strongest constraints, , where is the Hubble constant in units of 100 km
s Mpc. Hence one is left with several unanswered questions: 1)
What are MACHOs seen in microlensing surveys? 2) What is the dark matter in our
Galaxy? Indeed a nonbaryonic component in the Halo seems to be required.Comment: 6 pages ps fil
Effects of visuospatial tasks on desensitization to emotive memories
Objectives: Vivid and intrusive memories of extreme trauma can disrupt a stepwise approach to
imaginal exposure. Concurrent tasks that load the visuo-spatial sketchpad (VSSP) of working memory
reduce memory vividness during imaginal exposure. Such tasks may help maintain a progressive
exposure protocol while minimizing distress during treatment. The current study tested whether relief
of distress from a competing VSSP load during emotive imagery is at the cost of impaired
desensitization.
Design: This study examined repeated exposure to emotive memories using 18 unselected
undergraduates, using a within-subjects design.
Method: Participants recalled three positive and three negative self-related memories, and rated the
vividness and emotiveness of the image. Participants then received all three conditions (Eye
Movements; Visual Noise; Control) in a counterbalanced order. One positive and one negative
recollection were used for each condition. They then rated the vividness of the image and their
emotional response before proceeding to the next trial. There were 8 trials for each image. At a noninterference
session one week later, participants recalled each image, rating its vividness and their
emotional response.
Results: Consistent with previous research, vividness and distress during imaging were lower during
eye movements than in exposure alone, with passive visual interference giving intermediate results.
A small reduction in emotional responses a week later was of similar size for the three conditions
Massive Compact Halo Objects Viewed from a Cosmological Perspective: Contribution to the Baryonic Mass Density of the Universe
[Abridged] We estimate the contribution of Massive Compact Halo Objects
(Machos) and their stellar progenitors to the mass density of the Universe. If
the Machos that have been detected reside in the Halo of our Galaxy, then a
simple extrapolation of the Galactic population (out to 50 kpc) of Machos to
cosmic scales gives a cosmic density \rho_{Macho} = (1-5) \times 10^9 h \msun
\Mpc^{-3}, which in terms of the critical density corresponds to
. Such a mass density is comparable to
the baryon density implied by Big Bang Nucleosynthesis. If we take the central
values of the estimates, then Machos dominate the baryonic content of the
Universe today, with . However, the
cumulative uncertainties in the density determinations only require that
, where the fraction of
galaxies that contain Machos , and is the Hubble constant
in units of 100 km s Mpc. Our best estimate for
is hard to reconcile with the current best estimates of the baryonic content of
the intergalactic medium indicated by measurements of the Lyman-
forest. We explore the addition constraints that arise if the Machos are white
dwarfs as suggested by the present microlensing data. We discuss the challenges
this scenario presents at both the local and cosmic scales, emphasizing in
particular the constraints on the required mass budget and nucleosynthesis
products (particularly carbon).Comment: 18 pages, LaTeX, uses AASTeX macros. In press, New Astronomy
(submitted Jan. 20, 1998
Chain Inflation in the Landscape: "Bubble Bubble Toil and Trouble"
In the model of Chain Inflation, a sequential chain of coupled scalar fields
drives inflation. We consider a multidimensional potential with a large number
of bowls, or local minima, separated by energy barriers: inflation takes place
as the system tunnels from the highest energy bowl to another bowl of lower
energy, and so on until it reaches the zero energy ground state. Such a
scenario can be motivated by the many vacua in the stringy landscape, and our
model can apply to other multidimensional potentials. The ''graceful exit''
problem of Old Inflation is resolved since reheating is easily achieved at each
stage. Coupling between the fields is crucial to the scenario. The model is
quite generic and succeeds for natural couplings and parameters. Chain
inflation succeeds for a wide variety of energy scales -- for potentials
ranging from 10MeV scale inflation to GeV scale inflation.Comment: 31 pages, 3 figures, one reference adde
Fluid Interpretation of Cardassian Expansion
A fluid interpretation of Cardassian expansion is developed. Here, the
Friedmann equation takes the form where contains
only matter and radiation (no vacuum). The function g(\rhom) returns to the
usual 8\pi\rhom/(3 m_{pl}^2) during the early history of the universe, but
takes a different form that drives an accelerated expansion after a redshift . One possible interpretation of this function (and of the right hand
side of Einstein's equations) is that it describes a fluid with total energy
density \rho_{tot} = {3 m_{pl}^2 \over 8 \pi} g(\rhom) = \rhom + \rho_K
containing not only matter density (mass times number density) but also
interaction terms . These interaction terms give rise to an effective
negative pressure which drives cosmological acceleration. These interactions
may be due to interacting dark matter, e.g. with a fifth force between
particles . Such interactions may be intrinsically four
dimensional or may result from higher dimensional physics. A fully relativistic
fluid model is developed here, with conservation of energy, momentum, and
particle number. A modified Poisson's equation is derived. A study of
fluctuations in the early universe is presented, although a fully relativistic
treatment of the perturbations including gauge choice is as yet incomplete.Comment: 25 pages, 1 figure. Replaced with published version. Title changed in
journa
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
Friedmann cosmology with decaying vacuum density
Among the several proposals to solve the incompatibility between the observed
small value of the cosmological constant and the huge value obtained by quantum
field theories, we can find the idea of a decaying vacuum energy density,
leading from high values at early times of universe evolution to the small
value observed nowadays. In this paper we consider a variation law for the
vacuum density recently proposed by Schutzhold on the basis of quantum field
estimations in the curved, expanding background, characterized by a vacuum
density proportional to the Hubble parameter. We show that, in the context of
an isotropic and homogeneous, spatially flat model, the corresponding solutions
retain the well established features of the standard cosmology, and, in
addition, are in accordance with the observed cosmological parameters. Our
scenario presents an initial phase dominated by radiation, followed by a dust
era long enough to permit structure formation, and by an epoch dominated by the
cosmological term, which tends asymptotically to a de Sitter universe. Taking
the matter density equals to half of the vacuum energy density, as suggested by
observation, we obtain a universe age given by Ht = 1.1, and a decelerating
parameter equals to -1/2.Comment: Accepted for publication in General Relativity and Gravitatio
- …