2,319 research outputs found
The Geometry of Warped Product Singularities
In this article the degenerate warped products of singular semi-Riemannian
manifolds are studied. They were used recently by the author to handle
singularities occurring in General Relativity, in black holes and at the
big-bang. One main result presented here is that a degenerate warped product of
semi-regular semi-Riemannian manifolds with the warping function satisfying a
certain condition is a semi-regular semi-Riemannian manifold. The connection
and the Riemann curvature of the warped product are expressed in terms of those
of the factor manifolds. Examples of singular semi-Riemannian manifolds which
are semi-regular are constructed as warped products. Applications include
cosmological models and black holes solutions with semi-regular singularities.
Such singularities are compatible with a certain reformulation of the Einstein
equation, which in addition holds at semi-regular singularities too.Comment: 14 page
Short-range nuclear effects on axion emissivities by nucleon-nucleon bremsstrahlung
The rates of axion emission by nucleon-nucleon (NN) bremsstrahlung are
reconsidered by taking into account the NN short range correlations. The
analytical formulas for the neutron-neutron (nn), proton-proton (pp) and
neutron-proton (np) processes with the inclusion of the full momentum
dependence of an one- and two- pion exchange nuclear potentials, in the
non-degenerate limit, are explicitly given. We find that the two-pion exchange
(short range) effects can give a significant contribution to the emission
rates, and are temperature dependent. Other short range nuclear effects like
effective nucleon mass, polarization effects and use of correlated wave
functions, are discused as well. The trend of all these nuclear effects is to
diminish the corresponding axion emission rates. Further, we estimate that the
values of the emission rates calculated with the inclusion of all these effects
can differ from the corresponding ones derived with constant nuclear matrix
elements by a factor of . This leads to an uncertainty factor of when extracting bounds of the axion parametersComment: 11 pages, 4 figure
The Friedmann-Lemaitre-Robertson-Walker Big Bang singularities are well behaved
We show that the Big Bang singularity of the
Friedmann-Lemaitre-Robertson-Walker model does not raise major problems to
General Relativity. We prove a theorem showing that the Einstein equation can
be written in a non-singular form, which allows the extension of the spacetime
before the Big Bang. The physical interpretation of the fields used is
discussed. These results follow from our research on singular semi-Riemannian
geometry and singular General Relativity.Comment: 10 pages, 5 figure
Pion mass effects on axion emission from neutron stars through NN bremsstrahlung processes
The rates of axion emission by nucleon-nucleon bremsstrahlung are calculated
with the inclusion of the full momentum contribution from a nuclear one pion
exchange (OPE) potential. The contributions of the neutron-neutron (nn),
proton-proton (pp) and neutron-proton (np) processes in both the nondegenerate
and degenerate limits are explicitly given. We find that the finite momentum
corrections to the emissivities are quantitatively significant for the
non-degenerate regime and temperature-dependent, and should affect the existing
axion mass bounds. The trend of these nuclear effects is to diminish the
emissivities
High-energy black hole production
Black hole production in high-energy collisions is briefly surveyed. Included
is a summary of recent developments and open problems relevant to collider
(LHC) production, as well as of some theoretical issues pointing towards
fundamental principles of quantum gravity.Comment: Talk at PASCOS 2007, to appear in the proceedings. v2: minor
improvements, added refs. v3: reference correction. 11 pages, late
Spontaneous breaking of discrete symmetries in QCD on a small volume
In a compact space with non-trivial cycles, for sufficiently small values of
the compact dimensions, charge conjugation (C), spatial reflection (P) and time
reversal (T) are spontaneously broken in QCD. The order parameter for the
symmetry breaking is the trace of the Wilson line wrapping around the compact
dimension, which acquires an imaginary part in the broken phase. We show that a
physical signature for the symmetry breaking is a persistent baryonic current
wrapping in the compact directions. The existence of such a current is derived
analytically at first order in perturbation theory and confirmed in the
non-perturbative regime by lattice simulations.Comment: 4 pages, 2 figures, based on the poster presented by B. Lucini at
Pascos 0
Shell-model calculations of two-neutrino double-beta decay rates of Ca with GXPF1A interaction
The two-neutrino double beta decay matrix elements and half-lives of
Ca, are calculated within a shell-model approach for transitions to the
ground state and to the first excited state of Ti. We use the full
model space and the GXPF1A interaction, which was recently proposed to
describe the spectroscopic properties of the nuclei in the nuclear mass region
A=47-66. Our results are =
and = . The result for the
decay to the Ti 0 ground state is in good agreement with experiment.
The half-life for the decay to the 2 state is two orders of magnitude
larger than obtained previously.Comment: 6 pages, 4 figure
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