815 research outputs found
Gravitation, C, P and T symmetries and the Second Law
The intimate links between gravitation and the second law are summarized and
two less known relations between gravity and thermodynamics are studied.
Firstly, the information cost required to operate a Maxwell's demon on a curved
spacetime is estimated using the Kolmogorov-Sinai entropy. More importantly,
the charge and time (C and T) reversal properties of the Kerr-Newman solution
in General relativity show that this solution, similarly to the Dirac equation,
appears to represent both a particle and its antiparticle and suggests a
definition of antimatter in general relativity. This definition leads to a
parameter free explanation of the cosmological constant term observed in the
supernovae data. The relation of this definition of antimatter with the coupled
systems through opposite time arrows studied by Schulman is also emphasized.Comment: 9 pages, two figures, invited contribution at the "Quantum Limits to
the Second Law" conference (San Diego, 2002
High resolution simulations of the reionization of an isolated Milky Way - M31 galaxy pair
We present the results of a set of numerical simulations aimed at studying
reionization at galactic scale. We use a high resolution simulation of the
formation of the Milky Way-M31 system to simulate the reionization of the local
group. The reionization calculation was performed with the post-processing
radiative transfer code ATON and the underlying cosmological simulation was
performed as part of the CLUES project. We vary the source models to bracket
the range of source properties used in the literature. We investigate the
structure and propagation of the galatic ionization fronts by a visual
examination of our reionization maps. Within the progenitors we find that
reionization is patchy, and proceeds locally inside out. The process becomes
patchier with decreasing source photon output. It is generally dominated by one
major HII region and 1-4 additional isolated smaller bubbles, which eventually
overlap. Higher emissivity results in faster and earlier local reionization. In
all models, the reionization of the Milky Way and M31 are similar in duration,
i.e. between 203 Myr and 22 Myr depending on the source model, placing their
zreion between 8.4 and 13.7. In all models except the most extreme, the MW and
M31 progenitors reionize internally, ignoring each other, despite being
relatively close to each other even during the epoch of reionization. Only in
the case of strong supernova feedback suppressing star formation in haloes less
massive than 10^9 M_sun, and using our highest emissivity, we find that the MW
is reionized by M31.Comment: Accepted for publication in ApJ. 14 pages, 4 figures, 1 tabl
320g Ionization-Heat Cryogenic Detector for Dark Matter Search in the EDELWEISS Experiment
The EDELWEISS experiment used in 2001 a 320g heat-and-ionization cryogenic Ge
detector operated in a low-background environment in the Laboratoire Souterrain
de Modane for direct WIMP detection. This detector presents an increase of more
than 4 times the mass of previous detectors. Calibrations of this detector are
used to determine its energy resolution and fiducial volume, and to optimize
the detector design for the 1kg phase of the EDELWEISS-I experiment. Analysis
of the calibrations and characteristics of a first series of 320g-detectors are
presented.Comment: 4 pages, 3 figure
Constraining the second half of reionization with the Ly β forest
We present an analysis of the evolution of the Lyman-series forest into the
epoch of reionization using cosmological radiative transfer simulations in a
scenario where reionization ends late. We explore models with different
midpoints of reionization and gas temperatures. We find that once the
simulations have been calibrated to match the mean flux of the observed
Lyman- forest at , they also naturally reproduce the
distribution of effective optical depths of the Lyman- forest in this
redshift range. We note that the tail of the largest optical depths that is
most challenging to match corresponds to the long absorption trough of ULAS
J0148+0600, which we have previously shown to be rare in our simulations. We
consider the evolution of the Lyman-series forest out to higher redshifts, and
show that future observations of the Lyman- forest at will
discriminate between different reionization histories. The evolution of the
Lyman- and Lyman- forests are less promising as a tool for
pushing studies of reionization to higher redshifts due to the stronger
saturation and foreground contamination, respectively
Regularity of prime ideals
We answer several natural questions which arise from a recent paper of McCullough and Peeva providing counterexamples to the Eisenbud\u2013Goto Regularity Conjecture. We give counterexamples using Rees algebras, and also construct counterexamples that do not rely on the Mayr\u2013Meyer construction. Furthermore, examples of prime ideals for which the difference between the maximal degree of a minimal generator and the maximal degree of a minimal first syzygy can be made arbitrarily large are given. Using a result of Ananyan-Hochster we show that there exists an upper bound on regularity of prime ideals in terms of the multiplicity alone
CPT symmetry and antimatter gravity in general relativity
The gravitational behavior of antimatter is still unknown. While we may be
confident that antimatter is self-attractive, the interaction between matter
and antimatter might be either attractive or repulsive. We investigate this
issue on theoretical grounds. Starting from the CPT invariance of physical
laws, we transform matter into antimatter in the equations of both
electrodynamics and gravitation. In the former case, the result is the
well-known change of sign of the electric charge. In the latter, we find that
the gravitational interaction between matter and antimatter is a mutual
repulsion, i.e. antigravity appears as a prediction of general relativity when
CPT is applied. This result supports cosmological models attempting to explain
the Universe accelerated expansion in terms of a matter-antimatter repulsive
interaction.Comment: 6 pages, to be published in EPL (http://epljournal.edpsciences.org/
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