9,263 research outputs found

### Elliptic fibrations associated with the Einstein spacetimes

Given a conformally nonflat Einstein spacetime we define a fibration $P$ over
it. The fibres of this fibration are elliptic curves (2-dimensional tori) or
their degenerate counterparts. Their topology depends on the algebraic type of
the Weyl tensor of the Einstein metric. The fibration $P$ is a double branched
cover of the bundle of null direction over the spacetime and is equipped with
six linearly independent 1-forms which satisfy certain relatively simple system
of equations.Comment: 15 pages, Late

### On the algebraic classification of spacetimes

We briefly overview the Petrov classification in four dimensions and its
generalization to higher dimensions.Comment: Submitted to Journal of Physics, conference series, proceedings of
4th meeting on constrained dynamics and quantum gravity, 12-16 September
2005, Sardinia, Ital

### Orbital and interlayer Skyrmions crystals in bilayer graphene

A graphene bilayer in a transverse magnetic field has a set of Landau levels
with energies $E=\pm \sqrt{N(N+1)}\hslash \omega_{c}^{\ast}$ where
$\omega_{c}^{\ast}$ is the effective cyclotron frequency and $% N=0,1,2,...$
All Landau levels but N=0 are four times degenerate counting spin and valley
degrees of freedom. The Landau level N=0 has an extra degeneracy due to the
fact that orbitals $n=0$ and $n=1$ both have zero kinetic energies. At integer
filling factors, Coulomb interactions produce a set of broken-symmetry states
with partial or full alignement in space of the valley and orbital pseudospins.
These quantum Hall pseudo-ferromagnetic states support topological charged
excitations in the form of orbital and valley Skyrmions. Away from integer
fillings, these topological excitations can condense to form a rich variety of
Skyrme crystals with interesting properties. We study in this paper different
crystal phases that occur when an electric field is applied between the layers.
We show that orbital Skyrmions, in analogy with spin Skyrmions, have a texture
of electrical dipoles that can be controlled by an in-plane electric field.
Moreover, the modulation of electronic density in the crystalline phases are
experimentally accessible through a measurement of their local density of
statesComment: 18 pages with 13 figure

### Influence of Primary Cosmic Radiation Mass Composition on the Estimation of Eas Energy

At the Yakutsk EAS array E_em is determined by using measurements of EAS
Cherenkov light flux and charged particle flux. It is known from calculations
that these characteristics depend on a sort of primary particle and, therefore,
the estimation of E_em depends on a primary particle mass. In the work the
dependence of the E_em/E_0 ratio on the energy is given and experimental data
are compared with calculations by the QGSJET model. The calculations have been
carried out for the primary proton and iron nucleus. The average calculated
meaning of the value of E_em/E_0 ratio (between the proton and iron nucleus)
within experimental errors is in agreement with experimental data that doesnt
contradict to the mixed mass composition of primary cosmic radiation.Comment: 19th European Cosmic Ray Symposium. Aug 30 - Sep 3 2004, Florence,
Italy. 3 pages, 1 figure. Subbmitted for publication in International Journal
of Modern Physics

### Fluctuations of Xmax and Primary Particle Mass Composition in the Range of Energy 5 10^{17} - 3 10^{19} ev by Yakutsk Data

The experimental distributions of \Xmax obtained with the Yakutsk EAS array
at fixed energies of $5 \times10^{17}$, $1\times10^{18}$ and $5\times10^{18}$
eV are analysed. A recent version of the QGSJET model is used as a tool of our
analysis. In the framework of this model, the most adequate mass composition of
primary particles satisfying the experimental data on \Xmax is selected.Comment: 19th European Cosmic Ray Symposium, Aug 30 - Sep 3 2004, Florence,
Italy. 3 pages, 1 figure. Submitted for publication in International Journal
of Modern Physics

### A comment on a paper by Carot et al

In a recent paper Carot et al. considered carefully the definition of
cylindrical symmetry as a specialisation of the case of axial symmetry. One of
their propositions states that if there is a second Killing vector, which
together with the one generating the axial symmetry, forms the basis of a
two-dimensional Lie algebra, then the two Killing vectors must commute, thus
generating an Abelian group. In this comment a similar result, valid under
considerably weaker assumptions, is recalled: any two-dimensional Lie
transformation group which contains a one-dimensional subgroup whose orbits are
circles, must be Abelian. The method used to prove this result is extended to
apply to three-dimensional Lie transformation groups. It is shown that the
existence of a one-dimensional subgroup with closed orbits restricts the
Bianchi type of the associated Lie algebra to be I (Abelian), II, III, VII_0,
VIII or IX. The relationship between the present approach and that of the
original paper is discussed.Comment: 6 pages, Te

### Black-Hole Spin Dependence in the Light Curves of Tidal Disruption Events

A star orbiting a supermassive black hole can be tidally disrupted if the
black hole's gravitational tidal field exceeds the star's self gravity at
pericenter. Some of this stellar tidal debris can become gravitationally bound
to the black hole, leading to a bright electromagnetic flare with bolometric
luminosity proportional to the rate at which material falls back to pericenter.
In the Newtonian limit, this flare will have a light curve that scales as
t^-5/3 if the tidal debris has a flat distribution in binding energy. We
investigate the time dependence of the black-hole mass accretion rate when
tidal disruption occurs close enough the black hole that relativistic effects
are significant. We find that for orbits with pericenters comparable to the
radius of the marginally bound circular orbit, relativistic effects can double
the peak accretion rate and halve the time it takes to reach this peak
accretion rate. The accretion rate depends on both the magnitude of the
black-hole spin and its orientation with respect to the stellar orbit; for
orbits with a given pericenter radius in Boyer-Lindquist coordinates, a maximal
black-hole spin anti-aligned with the orbital angular momentum leads to the
largest peak accretion rate.Comment: 16 pages, 15 figures, 1 table, PRD published versio

### Radiative Symmetry Breaking and Dynamical Origin of Cosmological Constant in $\phi^4$ Theory with Non-Linear Curvature Coupling

A scalar self-interacting theory non-linearly coupled with some power of the
curvature have a possibility to explain the current smallness of the
cosmological constant. Here one concentrate on a massless scalar field in the
four-dimensional Fridmann-Robertson-Walker (FRW) spacetime with flat spatial
part. One show the phase structure of radiative symmetry breaking and review a
dynamical resolution of the cosmological constant problem.Comment: 9 pages. To appear in the proceedings of 7th Workshop on Quantum
Field Theory Under the Influence of External Conditions (QFEXT 05),
Barcelona, Catalonia, Spain, 5-9 Sep 200

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