834 research outputs found
Comment on ``Constraints on the strength of primordial B-fields from big bang nucleosynthesis reexamined''
Recently Cheng, Olinto, Schramm and Truran (COST) reexamined the constraints
from big bang nucleosynthesis (BBN) on the strength of primordial magnetic
fields. Their bottom line agreed with that of an earlier recent paper on the
subject (Kernan, Starkman and Vachaspati (KSV)), both in its final limit on the
magnetic field during BBN, and in its conclusion that for allowed values of the
magnetic field the dominant factor for BBN is the increased expansion rate at a
given temperature caused by the energy density of the magnetic field,
. However, their conclusion that weak interaction rates increased
with increasing B-field at these low field values contradicted the earlier
results of KSV. In this comment we point out that the Taylor series expansion
of the weak interaction rate about B=0 used in COST is not well-defined, while
the Euler-McLaurin expansion of KSV is well-behaved and reliable. Using the
Euler-McLaurin expansion we find that the weak interaction rates decrease
rather than increase with increasing B-field at small values of the B-field.Comment: 4 pages, Latex, submitted to Phys. Rev.
Big Bang Nucleosynthesis Constraints on Primordial Magnetic Fields
We reanalyze the effect of magnetic fields in BBN, incorporating several
features which were omitted in previous analyses. We find that the effects of
coherent magnetic fields on the weak interaction rates and the electron
thermodynamic functions (\rhoe, \Pe, and \drhoedt ) are unimportant in
comparison to the contribution of the magnetic field energy density in BBN. In
consequence the effect of including magnetic fields in BBN is well approximated
numerically by treating the additional energy density as effective neutrino
number. A conservative upper bound on the primordial magnetic field,
parameterized as , is (). This bound can be stronger than the conventional bound coming from
the Faraday rotation measures of distant quasars if the cosmological magnetic
field is generated by a causal mechanism.Comment: Latex, 20 pages, 3 uuencoded figures appende
Magnetic fields in the early universe in the string approach to MHD
There is a reformulation of magnetohydrodynamics in which the fundamental
dynamical quantities are the positions and velocities of the lines of magnetic
flux in the plasma, which turn out to obey equations of motion very much like
ideal strings. We use this approach to study the evolution of a primordial
magnetic field generated during the radiation-dominated era in the early
Universe. Causality dictates that the field lines form a tangled random
network, and the string-like equations of motion, plus the assumption of
perfect reconnection, inevitably lead to a self-similar solution for the
magnetic field power spectrum. We present the predicted form of the power
spectrum, and discuss insights gained from the string approximation, in
particular the implications for the existence or not of an inverse cascade.Comment: 12 pages, 2 figure
Neutrino propagation in a random magnetic field
The active-sterile neutrino conversion probability is calculated for neutrino
propagating in a medium in the presence of random magnetic field fluctuations.
Necessary condition for the probability to be positive definite is obtained.
Using this necessary condition we put constraint on the neutrino magnetic
moment from active-sterile electron neutrino conversion in the early universe
hot plasma and in supernova.Comment: 11 page
Primordial magnetic fields, anomalous isocurvature fluctuations and Big Bang nucleosynthesis
We show that the presence of primordial stochastic (hypercharge) magnetic
fields before the electroweak (EW) phase transition induces isocurvature
fluctuations (baryon number inhomogeneities). Depending on the details of the
magnetic field spectrum and on the particle physics parameters (such as the
strength of the EW phase transition and electron Yukawa couplings) these
fluctuations may survive until the Big Bang nucleosynthesis (BBN). Their
lenghtscale may exceed the neutron diffusion length at that time, while their
magnitude can be so large that sizable antimatter domains are present. This
provides the possibility of a new type of initial conditions for
non-homogeneous BBN or, from a more conservative point of view, stringent
bounds on primordial magnetic fields.Comment: 4 pages, Latex, 1 epsfi
Spin-Flavour Oscillations and Neutrinos from SN1987A
The neutrino signal from SN1987A is analysed with respect to spin-flavour
oscillations between electron antineutrinos, , and muon
neutrinos, , by means of a maximum likelihood analysis.
Following Jegerlehner et al. best fit values for the total energy released in
neutrinos, , and the temperature of the electron antineutrino,
, for a range of mixing parameters and progenitor models are
calculated. In particular the dependence of the inferred quantities on the
metallicity of the supernova is investigated and the uncertainties involved in
using the neutrino signal to determine the neutrino magnetic moment are pointed
out.Comment: 14 pages, RevTeX, 4 figures, to appear in Physical Review
APC/C and SCF cyclin F Constitute a Reciprocal Feedback Circuit Controlling S-Phase Entry
The anaphase promoting complex/cyclosome (APC/C) is an ubiquitin ligase and core component of the cell-cycle oscillator. During G1 phase, APC/C binds to its substrate receptor Cdh1 and APC/C(Cdh1) plays an important role in restricting S-phase entry and maintaining genome integrity. We describe a reciprocal feedback circuit between APC/C and a second ubiquitin ligase, the SCF (Skp1-Cul1-F box). We show that cyclin F, a cell-cycle-regulated substrate receptor (F-box protein) for the SCF, is targeted for degradation by APC/C. Furthermore, we establish that Cdh1 is itself a substrate of SCF(cyclin F). Cyclin F loss impairs Cdh1 degradation and delays S-phase entry, and this delay is reversed by simultaneous removal of Cdh1. These data indicate that the coordinated, temporal ordering of cyclin F and Cdh1 degradation, organized in a double-negative feedback loop, represents a fundamental aspect of cell-cycle control. This mutual antagonism could be a feature of other oscillating systems
The Age Of Globular Clusters In Light Of Hipparcos: Resolving the Age Problem?
We review five independent techniques which are used to set the distance
scale to globular clusters, including subdwarf main sequence fitting utilizing
the recent Hipparcos parallax catalogue. These data together all indicate that
globular clusters are farther away than previously believed, implying a
reduction in age estimates. This new distance scale estimate is combined with a
detailed numerical Monte Carlo study designed to assess the uncertainty
associated with the theoretical age-turnoff luminosity relationship in order to
estimate both the absolute age and uncertainty in age of the oldest globular
clusters. Our best estimate for the mean age of the oldest globular clusters is
now Gyr, with a one-sided, 95% confidence level lower limit of
9.5 Gyr. This represents a systematic shift of over 2 compared to our
earlier estimate, due completely to the new distance scale---which we emphasize
is not just due to the Hipparcos data. This now provides a lower limit on the
age of the universe which is consistent with either an open universe, or a
flat, matter dominated universe (the latter requiring H_0 \le 67 \kmsmpc).
Our new study also explicitly quantifies how remaining uncertainties in the
distance scale and stellar evolution models translate into uncertainties in the
derived globular cluster ages. Simple formulae are provided which can be used
to update our age estimate as improved determinations for various quantities
become available.Comment: 41 pages, including 10 eps figs, uses aaspp4.sty and flushrt.sty,
submitted to Ap.J., revised to incorporate FULL Hipparcos catalogue dat
Reconciling Present Neutrino Puzzles: Sterile Neutrinos as Mirror Neutrinos
We suggest that recent neutrino puzzles that are the solar and atmospheric
neutrino deficits as well as the possible neutrino oscillations reported by the
LSND experiment and the possibility of massive neutrinos providing the hot
component of the cosmological dark matter, can all be naturally explained by
assuming existence of a mirror world described by an ``electroweak'' gauge
symmetry , with the breaking scale larger by about factor
of 30 than the scale of the standard model. An interesting
aspect of this model is that the sterile neutrinos arise from the hidden mirror
sector of the theory and thus their lightness is more natural than in the usual
neutrino mass scenarios. The needed pattern of the neutrino mass matrix in this
model is obtained by assuming a conserved ZKM-type global lepton number , which is violated by Planck scale effects. One implication
of our proposal is that bulk of the dark matter in the universe is a warm dark
matter consisting of few KeV mass particles rather than the 100 GeV range
particles of the currently popular cold dark matter scenarios.Comment: 10 pages, Latex, no figure
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