1,225 research outputs found
Lensed Density Perturbations in Braneworlds: An Alternative to Perturbations from Inflation
We consider a scenario in which our observable universe is a 3-dimensional
surface (3-brane) living in extra dimensions with a warped geometry. We show
that ``lensed'' density perturbations from other branes serve as possible seeds
for structure formation on our observable brane (without inflation), and, in
addition, provide constraints on braneworld scenarios with warped bulk
geometry. Due to the warped bulk metric, any perturbation generated on one
brane (or in the bulk matter) appears to an observer on a second brane to have
a significantly different amplitude. We analyze lensed perturbations in the
Randall-Sundrum type scenarios and the ``shortcut metric'' scenarios. For
Lorentz violating metrics in the bulk, we find the attractive possibility that
large density fluctuations that are causally produced elsewhere can lead to
small density fluctuations on our brane on superhorizon (acausal) length
scales, as required by structure formation. Our most interesting result is that
the ``shortcut metrics'' in which geodesics traverse the extra dimensions
provide an alternative to inflation with two important features: a possible
solution to the horizon problem and a mechanism to generate perturbations
necessary for structure formation.Comment: 27 page
New Models for a Triaxial Milky Way Spheroid and Effect on the Microlensing Optical Depth to the Large Magellanic Cloud
We obtain models for a triaxial Milky Way spheroid based on data by Newberg
and Yanny. The best fits to the data occur for a spheroid center that is
shifted by 3kpc from the Galactic Center. We investigate effects of the
triaxiality on the microlensing optical depth to the Large Magellanic Cloud
(LMC). The optical depth can be used to ascertain the number of Massive Compact
Halo Objects (MACHOs); a larger spheroid contribution would imply fewer Halo
MACHOs. On the one hand, the triaxiality gives rise to more spheroid mass along
the line of sight between us and the LMC and thus a larger optical depth.
However, shifting the spheroid center leads to an effect that goes in the other
direction: the best fit to the spheroid center is_away_ from the line of sight
to the LMC. As a consequence, these two effects tend to cancel so that the
change in optical depth due to the Newberg/Yanny triaxial halo is at most 50%.
After subtracting the spheroid contribution in the four models we consider, the
MACHO contribution (central value) to the mass of the Galactic Halo varies from
\~(8-20)% if all excess lensing events observed by the MACHO collaboration are
assumed to be due to MACHOs. Here the maximum is due to the original MACHO
collaboration results and the minimum is consistent with 0% at the 1 sigma
error level in the data.Comment: 26 pages, 2 figures. v2: minor revisions. v3: expanded discussion of
the local spheroid density and minor revisions to match version published in
Journal of Cosmology and Astroparticle Physics (JCAP
The cosmological BCS mechanism and the Big Bang Singularity
We provide a novel mechanism that resolves the Big Bang Singularity present
in FRW space-times without the need for ghost fields. Building on the fact that
a four-fermion interaction arises in General Relativity when fermions are
covariantly coupled, we show that at early times the decrease in scale factor
enhances the correlation between pairs of fermions. This enhancement leads to a
BCS-like condensation of the fermions and opens a gap dynamically driving the
Hubble parameter to zero and results in a non-singular bounce, at least in
some special cases.Comment: replaced to match the journal versio
Calculation of Particle Production by Nambu Goldstone Bosons with Application to Inflation Reheating and Baryogenesis
A semiclassical calculation of particle production by a scalar field in a
potential is performed. We focus on the particular case of production of
fermions by a Nambu-Goldstone boson . We have derived a (non)local
equation of motion for the -field with the backreaction of the produced
particles taken into account. The equation is solved in some special cases,
namely for purely Nambu-Goldstone bosons and for the tilted potential . Enhanced production of bosons due to
parametric resonance is investigated; we argue that the resonance probably
disappears when the expansion of the universe is included. Application of our
work on particle production to reheating and an idea for baryogenesis in
inflation are mentioned.Comment: Submitted to Physical Review {\rm D}: October 4, 1994 21 page, UM-AC
94-3
Heisenberg-picture approach to the evolution of the scalar fields in an expanding universe
We present the Heisenberg-picture approach to the quantum evolution of the
scalar fields in an expanding FRW universe which incorporates relatively simply
the initial quantum conditions such as the vacuum state, the thermal
equilibrium state, and the coherent state. We calculate the Wightman function,
two-point function, and correlation function of a massive scalar field. We find
the quantum evolution of fluctuations of a self-interacting field
perturbatively and discuss the renormalization of field equations.Comment: 15 pages, RevTeX, no figure
Model-Independent Comparison of Direct vs. Indirect Detection of Supersymmetric Dark Matter
We compare the rate for elastic scattering of neutralinos from various nuclei
with the flux of upward muons induced by energetic neutrinos from neutralino
annihilation in the Sun and Earth. We consider both scalar and axial-vector
interactions of neutralinos with nuclei. We find that the event rate in a kg of
germanium is roughly equivalent to that in a - to -m muon
detector for a neutralino with primarily scalar coupling to nuclei. For an
axially coupled neutralino, the event rate in a 50-gram hydrogen detector is
roughly the same as that in a 10- to 500-m muon detector. Expected
experimental backgrounds favor forthcoming elastic-scattering detectors for
scalar couplings while the neutrino detectors have the advantage for
axial-vector couplings.Comment: 10 pages, self-unpacking uuencoded PostScript fil
Fatal affairs - conjugational transfer of a dinoflagellate-killing plasmid between marine Rhodobacterales
The roseobacter group of marine bacteria is characterized by a mosaic distribution of ecologically important phenotypes. These are often encoded on mobile extrachromosomal replicons. So far, conjugation had only been experimentally proven between the two model organisms Phaeobacter inhibens and Dinoroseobacter shibae. Here, we show that two large natural RepABC-type plasmids from D. shibae can be transferred into representatives of all known major Rhodobacterales lineages. Complete genome sequencing of the newly established Phaeobacter inhibens transconjugants confirmed their genomic integrity. The conjugated plasmids were stably maintained as single copy number replicons in the genuine as well as the new host. Co-cultivation of Phaeobacter inhibens and the transconjugants with the dinoflagellate Prorocentrum minimum demonstrated that Phaeobacter inhibens is a probiotic strain that improves the yield and stability of the dinoflagellate culture. The transconjugant carrying the 191 kb plasmid, but not the 126 kb sister plasmid, killed the dinoflagellate in co-culture
Dark Stars and Boosted Dark Matter Annihilation Rates
Dark Stars (DS) may constitute the first phase of stellar evolution, powered
by dark matter (DM) annihilation. We will investigate here the properties of DS
assuming the DM particle has the required properties to explain the excess
positron and elec- tron signals in the cosmic rays detected by the PAMELA and
FERMI satellites. Any possible DM interpretation of these signals requires
exotic DM candidates, with an- nihilation cross sections a few orders of
magnitude higher than the canonical value required for correct thermal relic
abundance for Weakly Interacting Dark Matter can- didates; additionally in most
models the annihilation must be preferentially to lep- tons. Secondly, we study
the dependence of DS properties on the concentration pa- rameter of the initial
DM density profile of the halos where the first stars are formed. We restrict
our study to the DM in the star due to simple (vs. extended) adiabatic
contraction and minimal (vs. extended) capture; this simple study is sufficient
to illustrate dependence on the cross section and concentration parameter. Our
basic results are that the final stellar properties, once the star enters the
main sequence, are always roughly the same, regardless of the value of boosted
annihilation or concentration parameter in the range between c=2 and c=5:
stellar mass ~ 1000M\odot, luminosity ~ 10^7 L\odot, lifetime ~ 10^6 yrs (for
the minimal DM models considered here; additional DM would lead to more massive
dark stars). However, the lifetime, final mass, and final luminosity of the DS
show some dependence on boost factor and concentration parameter as discussed
in the paper.Comment: 37 pages, 11 figure
Dark Matter Capture in the First Stars: a Power Source and Limit on Stellar Mass
The annihilation of weakly interacting massive particles can provide an
important heat source for the first (Pop. III) stars, potentially leading to a
new phase of stellar evolution known as a "Dark Star". When dark matter (DM)
capture via scattering off of baryons is included, the luminosity from DM
annihilation may dominate over the luminosity due to fusion, depending on the
DM density and scattering cross-section. The influx of DM due to capture may
thus prolong the lifetime of the Dark Stars. Comparison of DM luminosity with
the Eddington luminosity for the star may constrain the stellar mass of zero
metallicity stars; in this case DM will uniquely determine the mass of the
first stars. Alternatively, if sufficiently massive Pop. III stars are found,
they might be used to bound dark matter properties.Comment: 19 pages, 4 figures, 3 Tables updated captions and graphs, corrected
grammer, and added citations revised for submission to JCA
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