28,149 research outputs found
Moduli Spaces and Formal Operads
Let overline{M}_{g,n} be the moduli space of stable algebraic curves of genus
g with n marked points. With the operations which relate the different moduli
spaces identifying marked points, the family (overline{M}_{g,n})_{g,n} is a
modular operad of projective smooth Deligne-Mumford stacks, overline{M}. In
this paper we prove that the modular operad of singular chains
C_*(overline{M};Q) is formal; so it is weakly equivalent to the modular operad
of its homology H_*(overline{M};Q). As a consequence, the "up to homotopy"
algebras of these two operads are the same. To obtain this result we prove a
formality theorem for operads analogous to Deligne-Griffiths-Morgan-Sullivan
formality theorem, the existence of minimal models of modular operads, and a
characterization of formality for operads which shows that formality is
independent of the ground field.Comment: 36 pages (v3: some typographical corrections
Integrable models, degenerate horizons and AdS_2 black holes
The near extremal Reissner-Nordstrom black holes in arbitrary dimensions ca
be modeled by the Jackiw-Teitelboim (JT) theory. The asymptotic Virasoro
symmetry of the corresponding JT model exactly reproduces, via Cardy's formula,
the deviation of the Bekenstein-Hawking entropy of the Reissner-Nordstrom black
holes from extremality. We also comment how can we extend this approach to
investigate the evaporation process.Comment: 4 pages, LaTeX file, uses espcrc2.sty. Talk given at the Third
Conference on Constrained Dynamics and Quantum Gravit
Tidal Torques and the Orientation of Nearby Disk Galaxies
We use numerical simulations to investigate the orientation of the angular
momentum axis of disk galaxies relative to their surrounding large scale
structure. We find that this is closely related to the spatial configuration at
turnaround of the material destined to form the galaxy, which is often part of
a coherent two-dimensional slab criss-crossed by filaments. The rotation axis
is found to align very well with the intermediate principal axis of the inertia
momentum tensor at this time. This orientation is approximately preserved
during the ensuing collapse, so that the rotation axis of the resulting disk
ends up lying on the plane traced by the protogalactic material at turnaround.
This suggests a tendency for disks to align themselves so that their rotation
axis is perpendicular to the minor axis of the structure defined by surrounding
matter. One example of this trend is provided by our own Galaxy, where the
Galactic plane is almost at right angles with the supergalactic plane (SGP)
drawn by nearby galaxies; indeed, the SGP latitude of the North Galactic Pole
is just 6 degrees. We have searched for a similar signature in catalogs of
nearby disk galaxies, and find a significant excess of edge-on spirals (for
which the orientation of the disk rotation axis may be determined
unambiguously) highly inclined relative to the SGP. This result supports the
view that disk galaxies acquire their angular momentum as a consequence of
early tidal torques acting during the expansion phase of the protogalactic
material.Comment: 5 pages, 2 figures, accepted for publication in ApJ
Unconventional cosmology on the (thick) brane
We consider the cosmology of a thick codimension 1 brane. We obtain the
matching conditions leading to the cosmological evolution equations and show
that when one includes matter with a pressure component along the extra
dimension in the brane energy-momentum tensor, the cosmology is of non-standard
type. In particular one can get acceleration when a dust of non-relativistic
matter particles is the only source for the (modified) Friedman equation. Our
equations would seem to violate the conservation of energy-momentum from a 4D
perspective, but in 5D the energy-momentum is conserved. One could write down
an effective conserved 4D energy-momentum tensor attaching a ``dark energy''
component to the energy-momentum tensor of matter that has pressure along the
extra dimension. This extra component could, on a cosmological scale, be
interpreted as matter-coupled quintessence. We comment on the effective 4D
description of this effect in terms of the time evolution of a scalar field
(the 5D radion) coupled to this kind of matter.Comment: 9 pages, v2. eq.(17) corrected, comments on effective theory change
Initiation and growth behavior of very-long microstructurally short fatigue cracks
The present paper describes a novel experimental technique recently presented that allows one tostudy interactions between the crack and microstructural barriers with an unprecedented level of ease and detail.The method consists in increasing the grain size of Al1050 Aluminium alloy until the centimetre scale byapplying a series of mechanical and heat treatments. Once the thermo-mechanical treatment is completed andthe desired microstructure obtained, a circular notch is machined on each specimen, and the samples aresubjected to push-pull fatigue loading. Several combinations of notch and microstructural sizes have beentested. This method provides an easy way to record and analyse the effect of the microstructure upon crackgrowth rate. It was observed that the space between successive crack-tip arrests correlates well with the materialgrain size. Another interesting observation is that in the majority of the cases studied the cracks did not initiateat the point of maximum stress concentration. This is surprising since the classical methods of notched fatiguelimit analysis clearly indicate the horizontal symmetry axis as the initiation and propagation direction for pushpullloading
Nuclear liquid-gas phase transition and supernovae evolution
It is shown that the large density fluctuations appearing at the onset of the
first order nuclear liquid-gas phase transition can play an important role in
the supernovae evolution. Due to these fluctuations, the neutrino gas may be
trapped inside a thin layer of matter near the proto-neutron star surface. The
resulting increase of pressure may induce strong particle ejection a few
hundred milliseconds after the bounce of the collapse, contributing to the
revival of the shock wave. The Hartree-Fock+RPA scheme, with a finite-range
nucleon-nucleon effective interaction, is employed to estimate the effects of
the neutrino trapping due to the strong density fluctuations, and to discuss
qualitatively the consequences of the suggested new scenario.Comment: version2 - precise that nuclear liquid-gas phase transition is 1st
order and the unique instable mode is isoscala
Dynamical friction and the evolution of satellites in virialized halos: the theory of linear response
The evolution of a small satellite inside a more massive truncated isothermal
spherical halo is studied using both the Theory of Linear Response for
dynamical friction and N-Body simulations. The analytical approach includes the
effects of the gravitational wake, of the tidal deformation and the shift of
the barycenter of the primary, so unifying the local versus global
interpretation of dynamical friction. Sizes, masses, orbital energies and
eccentricities are chosen as expected in hierarchical clustering models. We
find that in general the drag force in self-gravitating backgrounds is weaker
than in uniform media and that the orbital decay is not accompanied by a
significant circularization. We also show that the dynamical friction time
scale is weakly dependent on the initial circularity. We provide a fitting
formula for the decay time that includes the effect of mass and angular
momentum loss. Live satellites with dense cores can survive disruption up to an
Hubble time within the primary, notwithstanding the initial choice of orbital
parameters. Dwarf spheroidal satellites of the Milky Way, like Sagittarius A
and Fornax, have already suffered mass stripping and, with their present
masses, the sinking times exceed 10 Gyr even if they are on very eccentric
orbits.Comment: 27 pages including 9 figures. Accepted for publication in the
Astrophysical Journal. Part 2, issue November 10 1999, Volume 52
Dwarf Dark Matter Halos
We study properties of dark matter halos at high redshifts z=2-10 for a vast
range of masses with the emphasis on dwarf halos with masses 10^7-10^9 Msun/h.
We find that the density profiles of relaxed dwarf halos are well fitted by the
NFW profile and do not have cores. We compute the halo mass function and the
halo spin parameter distribution and find that the former is very well
reproduced by the Sheth & Tormen model while the latter is well fitted by a
lognormal distribution with lambda_0 = 0.042 and sigma_lambda = 0.63. We
estimate the distribution of concentrations for halos in mass range that covers
six orders of magnitude from 10^7 Msun/h to 10^13} Msun/h, and find that the
data are well reproduced by the model of Bullock et al. The extrapolation of
our results to z = 0 predicts that present-day isolated dwarf halos should have
a very large median concentration of ~ 35. We measure the subhalo circular
velocity functions for halos with masses that range from 4.6 x 10^9 Msun/h to
10^13 Msun/h and find that they are similar when normalized to the circular
velocity of the parent halo. Dwarf halos studied in this paper are many orders
of magnitude smaller than well-studied cluster- and Milky Way-sized halos. Yet,
in all respects the dwarfs are just down-scaled versions of the large halos.
They are cuspy and, as expected, more concentrated. They have the same spin
parameter distribution and follow the same mass function that was measured for
large halos.Comment: Accepted to be pusblished by ApJ, 12 pages, 8 figures, LaTeX
(documentclass preprint2). Differences with respect to the previous
submission are: (i) abstract was modified slightly to make it more
transparent to the reader, (ii) an extra figure has been added, and (3) some
minor modifications to the main text were also don
Explaining the entropy excess in clusters and groups of galaxies without additional heating
The X-ray luminosity and temperature of clusters and groups of galaxies do
not scale in a self-similar manner. This has often been interpreted as a sign
that the intracluster medium has been substantially heated by non-gravitational
sources. In this paper, we propose a simple model which, instead, uses the
properties of galaxy formation to explain the observations. Drawing on
available observations, we show that there is evidence that the efficiency of
galaxy formation was higher in groups than in clusters. If confirmed, this
would deplete the low-entropy gas in groups, increase their central entropy and
decrease their X-ray luminosity. A simple, empirical, hydrostatic model appears
to match both the luminosity-temperature relation of clusters and properties of
their internal structure as well.Comment: 5 pages, 4 figures, accepted in ApJL; added one reference, otherwise
unchange
- …