4,575 research outputs found
Non-linear bigravity and cosmic acceleration
We explore the cosmological solutions of classes of non-linear bigravity
theories. These theories are defined by effective four-dimensional Lagrangians
describing the coupled dynamics of two metric tensors, and containing, in the
linearized limit, both a massless graviton and an ultralight one. We focus on
two paradigmatic cases: the case where the coupling between the two metrics is
given by a Pauli-Fierz-type mass potential, and the case where this coupling
derives from five-dimensional brane constructions. We find that cosmological
evolutions in bigravity theories can be described in terms of the dynamics of
two ``relativistic particles'', moving in a curved Lorenzian space, and
connected by some type of nonlinear ``spring''. Classes of bigravity
cosmological evolutions exhibit a ``locking'' mechanism under which the two
metrics ultimately stabilize in a bi-de-Sitter configuration, with relative
(constant) expansion rates. In the absence of matter, we find that a generic
feature of bigravity cosmologies is to exhibit a period of cosmic acceleration.
This leads us to propose bigravity as a source of a new type of dark energy
(``tensor quintessence''), exhibiting specific anisotropic features. Bigravity
could also have been the source of primordial inflation.Comment: 55 pages, 4 figures, references and comments added, final version
published in Phys. Rev.
Two-Stream Instability of Counter-Rotating Galaxies
The present study of the two-stream instability in stellar disks with
counter-rotating components of stars and/or gas is stimulated by recently
discovered counter-rotating spiral and S0 galaxies. Strong linear two-stream
instability of tightly-wrapped spiral waves is found for one and two-armed
waves with the pattern angular speed of the unstable waves always intermediate
between the angular speed of the co-rotating matter () and that of the
counter-rotating matter (). The instability arises from the
interaction of positive and negative energy modes in the co- and
counter-rotating components. The unstable waves are in general convective -
they move in radius and radial wavenumber space - with the result that
amplification of the advected wave is more important than the local growth
rate. For a galaxy of co-rotating stars and counter-rotating stars of
mass-fraction , or of counter-rotating gas of mass-fraction
, the largest amplification is usually for the one-armed
leading waves (with respect to the co-rotating stars). For the case of both
counter-rotating stars and gas, the largest amplifications are for , also for one-armed leading waves. The two-armed trailing
waves usually have smaller amplifications. The growth rates and amplifications
all decrease as the velocity spreads of the stars and/or gas increase. It is
suggested that the spiral waves can provide an effective viscosity for the gas
causing its accretion.Comment: 14 pages, submitted to ApJ. One table and 17 figures can be obtained
by sending address to R. Lovelace at [email protected]
Why national health research systems matter
Some of the most outstanding problems in Computer Science (e.g. access to heterogeneous information sources, use of different e-commerce standards, ontology translation, etc.) are often approached through the identification of ontology mappings. A manual mapping generation slows down, or even makes unfeasible, the solution of particular cases of the aforementioned problems via ontology mappings. Some algorithms and formal models for partial tasks of automatic generation of mappings have been proposed. However, an integrated system to solve this problem is still missing. In this paper, we present AMON, a platform for automatic ontology mapping generation. First of all, we show the general structure. Then, we describe the current version of the system, including the ontology in which it is based, the similarity measures that it uses, the access to external sources, etc
Nature of 45 degree vortex lattice reorientation in tetragonal superconductors
The transformation of the vortex lattice in a tetragonal superconductor which
consists of its 45 degree reorientation relative to the crystal axes is studied
using the nonlocal London model. It is shown that the reorientation occurs as
two successive second order (continuous) phase transitions. The transition
magnetic fields are calculated for a range of parameters relevant for
borocarbide superconductors in which the reorientation has been observed
Two dimensional QCD with matter in adjoint representation: What does it teach us?
We analyse the highly excited states in
with adjoint matter by using such general methods as dispersion relations,
duality and unitarity. We find the Hagedorn-like spectrum where parameters and can be expressed in
terms of asymptotics of the following matrix elements f_{n_{\{k\}}} \sim \la
0|Tr(\bar{\Psi}\Psi)^{k}|n_{k}\ra. We argue that the asymptotical values
do not depend on (after appropriate normalization). Thus,
we obtain and in case of
Majorana fermions in the adjoint representation. The Hagedorn temperature is
the limiting temperature in this case. We also argue that the chiral condensate
\la 0|Tr(\bar{\Psi}\Psi) |0\ra is not zero in the model. Contrary to the 't
Hooft model, this condensate does not break down any continuous symmetries and
can not be considered as an order parameter. Thus, no Goldstone boson appears
as a consequence of the condensation. We also discuss a few apparently
different but actually tightly related problems: master field, condensate,
wee-partons and constituent quark model in the light cone framework.Comment: uuencoded Z-compressed file for figs at the end. Revised version to
appear in Nuclear Physics B. More detail disscusion about the condensate and
discrete chiral symmetry breaking phenomenon in the mode
Infrared afterglow of GRB041219 as a result of reradiation on dust in a circumstellar cloud
Observations of gamma ray bursts (GRB) afterglows in different spectral bands
provide a most valuable information about their nature, as well as about
properties of surrounding medium. Powerful infrared afterglow was observed from
the strong GRB041219. Here we explain the observed IR afterglow in the model of
a dust reradiation of the main GRB signal in the envelope surrounding the GRB
source. In this model we do not expect appearance of the prompt optical
emission which should be absorbed in the dust envelope. We estimate the
collimation angle of the gamma ray emission, and obtain restrictions on the
redshift (distance to GRB source), by fitting the model parameters to the
observational data.Comment: 6 pages, 2 figures, Submited to Astrofizik
Nonlinear paramagnetic magnetization in the mixed state of CeCoIn_5
Torque and magnetization measurements in magnetic fields up to 14 T were
performed on CeCoIn single crystals. The amplitude of the paramagnetic
torque shows an dependence in the mixed state and an
dependence in the normal state. In addition, the mixed-state magnetizations for
both and axes show anomalous behavior after the
subtraction of the corresponding paramagnetic contributions as linear
extrapolations of the normal-state magnetization. These experimental results
point towards a nonlinear paramagnetic magnetization in the mixed state of
CeCoIn, which is a result of the fact that both orbital and Pauli limiting
effects dominate in the mixed state.Comment: 2 pages, 2 figures, conferenc
Two-step flux penetration in layered antiferromagnetic superconductor
A layered antiferromagnetic superconductor in the mixed state may posses
magnetic domains created along the Josephson vortices. This may happen when an
external magnetic field is strong enough to flip over magnetic moments, lying
in the phase core of the Josephson vortex, from their ground state
configuration. The formation of the domain structure of the vortices modifies
the surface energy barrier of the superconductor. During this process the
entrance of the flux is stopped and a newly created state exhibits perfect
shielding. Such behavior should be visible as a plateau on the dependence of
flux density as a function of the external magnetic field. The end of the
plateau determines the critical field, which we call the second critical field
for flux penetration.Comment: 8 pages, 3 figure
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