224 research outputs found
Superconducting Cosmic Strings and Primordial Nucleosynthesis
We show that the presence of superconducting cosmic strings in the early Universe may have dramatic consequences for primordial nucleosynthesis. Due to the enormous currents that they potentially can carry, very large magnetic fields can be produced in the vicinity of such strings. As they then move through the primordial plasma, charged particles are deflected away by the magnetic pressure surrounding the strings. We show that the predicted primordial abundances can differ radically from standard big-bang predictions, and may even be consistent with an Ωb=1 universe
Protecting Classical-Quantum Signals in Free Space Optical Channels
Due to turbulence and tracking errors, free-space optical channels involving
mobile transceivers are characterized by a signal's partial loss or complete
erasure. This work presents an error correction protocol capable of protecting
a signal passing through such channels by encoding it with an ancillary
entangled bipartite state. Beyond its ability to offer protection under
realistic channel conditions, novel to our protocol is its ability to encompass
both classical and quantum information on the encoded signal. We show how,
relative to non-encoded direct transmission, the protocol can improve the
fidelity of transmitted coherent states over a wide range of losses and erasure
probabilities. In addition, the use of ancillary non-Gaussian entangled
bipartite states in the signal encoding is considered, showing how this can
increase performance. Finally, we briefly discuss the application of our
protocol to the transmission of more complex input states, such as multi-mode
entangled states
Neutrino-Lasing in The Early Universe
Recently, Madsen has argued that relativistic decays of massive neutrinos
into lighter fermions and bosons may lead, via thermalization, to the formation
of a Bose condensate. If correct, this could generate mixed hot and cold dark
matter, with important consequences for structure formation.
From a detailed study of such decays, we arrive at substantially different
conclusions; for a wide range of masses and decay times, we find that
stimulated emission of bosons dominates the decay. This phenomenon can best be
described as a neutrino laser, pumped by the QCD phase transition. We discuss
the implications for structure formation and the dark-matter problem.Comment: 7 pages, 3 figures included as uuencoded file, CITA/93/
Brans-Dicke model constrained from Big Bang nucleosynthesis and magnitude redshift relations of Supernovae
The Brans-Dicke model with a variable cosmological term () has
been investigated with use of the coupling constant of .
Parameters inherent in this model are constrained from comparison between Big
Bang nucleosynthesis and the observed abundances. Furthermore, the magnitude
redshift () relations are studied for with and without another
constant cosmological term in a flat universe. Observational data of Type Ia
Supernovae are used in the redshift range of . It is found that our
model with energy density of the constant cosmological term with the value of
0.7 can explain the SNIa observations, though the model parameters are
insensitive to the relation.Comment: Submitted to A&A, 4 pages, 3 figure
Analysis of 26 Barium Stars II. Contributions of s-, r- and p-processes in the production of heavy elements
Barium stars show enhanced abundances of the slow neutron capture (s-process)
heavy elements, and for this reason they are suitable objects for the study of
s-process elements. The aim of this work is to quantify the contributions of
the s-, r- and p-processes for the total abundance of heavy elements from
abundances derived for a sample of 26 barium stars. The abundance ratios
between these processes and neutron exposures were studied. The abundances of
the sample stars were compared to those of normal stars thus identifying the
fraction relative to the s-process main component. The fittings of the sigmaN
curves (neutron capture cross section times abundance, plotted against atomic
mass number) for the sample stars suggest that the material from the companion
asymptotic giant branch star had approximately the solar isotopic composition
as concerns fractions of abundances relative to the s-process main component.
The abundance ratios of heavy elements, hs, ls and s and the computed neutron
exposure are similar to those of post-AGB stars. For some sample stars, an
exponential neutron exposure fits well the observed data, whereas for others, a
single neutron exposure provides a better fit. The comparison between barium
and AGB stars supports the hypothesis of binarity for the barium star
formation. Abundances of r-elements that are part of the s-process path in
barium stars are usually higher than those in normal stars,and for this reason,
barium stars seemed to be also enriched in r-elements, although in a lower
degree than s-elements. No dependence on luminosity classes was found in the
abundance ratios behaviour among the dwarfs and giants of the sample barium
stars.Comment: 30 pages including 24 figures, accepted to A&
Do primordial Lithium abundances imply there's no Dark Energy?
Explaining the well established observation that the expansion rate of the
universe is apparently accelerating is one of the defining scientific problems
of our age. Within the standard model of cosmology, the repulsive 'dark energy'
supposedly responsible has no explanation at a fundamental level, despite many
varied attempts. A further important dilemma in the standard model is the
Lithium problem, which is the substantial mismatch between the theoretical
prediction for 7-Li from Big Bang Nucleosynthesis and the value that we observe
today. This observation is one of the very few we have from along our past
worldline as opposed to our past lightcone. By releasing the untested
assumption that the universe is homogeneous on very large scales, both apparent
acceleration and the Lithium problem can be easily accounted for as different
aspects of cosmic inhomogeneity, without causing problems for other
cosmological phenomena such as the cosmic microwave background. We illustrate
this in the context of a void model.Comment: 14 pages, 4 figures. v2: minor rearrangements in the text, comments
and references expanded, results unchange
The Deuteron Confronts Big Bang Nucleosynthesis
Recent determinations of the deuterium abundance, H/H, in high redshift
Lyman limit hydrogen clouds challenge the usual picture of primordial
nucleosynthesis based on \lq\lq concordance\rq\rq\ of the calculated light
element (H, He, He, Li) nucleosynthesis yields with the
observationally-inferred abundances of these species. Concordance implies that
all light element yields can be made to agree with the observationally-inferred
abundances (within errors) for single global specifications of the
baryon-to-photon ratio, ; lepton number; neutron lifetime; and expansion
rate (or equivalently, effective number of light neutrino degrees of freedom
). Though one group studying Lyman limit systems obtains a high value
of H/H (), another group finds consistently low
values (). In the former case, concordance for is readily attained for the current observationally-inferred abundances of
He and Li. But if the latter case represents the primordial deuterium
abundance, then concordance for {\it any} is impossible unless the
primordial value of Li/H is considerably larger than the abundance of
lithium as measured in old, hot Pop II halo stars. Furthermore, concordance
with is possible for low H/H only if either (1) the primordial
He abundance has been significantly underestimated, or (2) new neutrino
sector physics is invoked. We argue that systematic underestimation of both the
Li and He primordial abundances is the likely resolution of this
problem, a conclusion which is strengthened by new results on He.Comment: To be published in Nucl. Phys. B (Proc. Suppl.), in the proceedings
of "Sources and Detection of Dark Matter in the Universe", held in Santa
Monica, Feb. 14-16 1996. 5 pages. Replaced version has a TeX command removed
that apparently caused some latex compilers to fai
Cosmic Strings in an Open Universe with Baryonic and Non-Baryonic Dark Matter
We study the effects of cosmic strings on structure formation in open
universes. We calculate the power spectrum of density perturbations for two
class of models: one in which all the dark matter is non baryonic (CDM) and one
in which it is all baryonic (BDM). Our results are compared to the 1 in 6 IRAS
QDOT power spectrum. The best candidates are then used to estimate , the
energy per unit length of the string network. Some comments are made on
mechanisms by which structures are formed in the two theories.Comment: uu-encoded compressed tar of postscript files, Imperial/TP/94-95/0
The Quark-Hadron Phase Transition, QCD Lattice Calculations and Inhomogeneous Big-Bang Nucleosynthesis
We review recent lattice QCD results for the surface tension at the finite
temperature quark-hadron phase transition and discuss their implications on the
possible scale of inhomogeneities. In the quenched approximation the average
distance between nucleating centers is smaller than the diffusion length of a
protron, so that inhomogeneities are washed out by the time nucleosynthesis
sets in. Consequently the baryon density fluctuations formed by a QCD phase
transition in the early universe cannot significantly affect standard big-bang
nucleosynthesis calculations and certainly cannot allow baryons to close the
universe. At present lattice results are inconclusive when dynamical fermions
are included.Comment: 8 pages, LaTe
Scales of the Extra Dimensions and their Gravitational Wave Backgrounds
Circumstances are described in which symmetry breaking during the formation
of our three-dimensional brane within a higher-dimensional space in the early
universe excites mesoscopic classical radion or brane-displacement degrees of
freedom and produces a detectable stochastic background of gravitational
radiation. The spectrum of the background is related to the unification energy
scale and the the sizes and numbers of large extra dimensions. It is shown that
properties of the background observable by gravitational-wave observatories at
frequencies Hz to Hz contain information about
unification on energy scales from 1 to TeV, gravity propagating
through extra-dimension sizes from 1 mm to mm, and the dynamical
history and stabilization of from one to seven extra dimensions.Comment: 6 pages, Latex, 1 figure, submitted to Phys. Re
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