3,356 research outputs found
Einstein-Gauss-Bonnet black holes in de Sitter spacetime and the quasilocal formalism
We propose to compute the action and global charges of the asymptotically de
Sitter solutions in Einstein-Gauss-Bonnet theory by using the counterterms
method in conjunction with the quasilocal formalism. The general expression of
the counterterms and the boundary stress tensor is presented for spacetimes of
dimension . We apply this tehnique for several different solutions in
Einstein-Gauss-Bonnet theory with a positive cosmological constant. Apart from
known solutions, we consider also vacuum rotating black holes with equal
magnitude angular momenta.Comment: 13 pages, 3 figures, typos corrected, text and one figure improve
On Yang-Mills instantons in a spherically symmetric background
We present arguments for the existence of self-dual Yang-Mills instantons for
several spherically symmetric backgrounds with Euclidean signature. The
time-independent Yang-Mills field has finite action and a vanishing energy
momentum tensor and does not disturb the geometry. We conjecture the existence
of similar solutions for any nonextremal SO(3)-spherically symmetric
background.Comment: 6 pages, 3 figures; v2: references adde
Skyrmions around Kerr black holes and spinning BHs with Skyrme hair
We study solutions of the Einstein-Skyrme model. Firstly we consider test
field Skyrmions on the Kerr background. These configurations -- hereafter
dubbed Skerrmions -- can be in equilibrium with a Kerr black hole (BH) by
virtue of a synchronisation condition. We consider two sectors for Skerrmions.
In the sector with non-zero baryon charge, Skerrmions are akin to the known
Skyrme solutions on the Schwarzschild background. These `topological'
configurations reduce to flat spacetime Skyrmions in a vanishing BH mass limit;
moreoever, they never become "small" perturbations on the Kerr background: the
non-linearities of the Skyrme model are crucial for all such Skerrmions. In the
non-topological sector, on the other hand, Skerrmions have no analogue on the
Schwarzschild background. Non-topological Skerrmions carry not baryon charge
and bifurcate from a subset of Kerr solutions defining an existence line.
Therein the appropriate truncation of the Skyrme model yield a linear scalar
field theory containing a complex plus a real field, both massive and
decoupled, and the Skerrmions reduce to the known stationary scalar clouds
around Kerr BHs. Moreover, non-topological Skerrmions trivialise in the
vanishing BH mass limit. We then discuss the backreaction of these Skerrmions,
that yield rotating BHs with synchronised Skyrme hair, which continously
connect to the Kerr solution (self-gravitating Skyrmions) in the
non-topological (topological) sector. In particular, the non-topological hairy
BHs provide a non-linear realisation, within the Skyrme model, of the
synchronous stationary scalar clouds around Kerr.Comment: 23 pages, 7 figures; to appear in JHE
Einstein-Yang-Mills-Chern-Simons solutions in D=2n+1 dimensions
We investigate finite energy solutions of the
Einstein--Yang-Mills--Chern-Simons system in odd spacetime dimensions, D=2n+1,
with n>1. Our configurations are static and spherically symmetric, approaching
at infinity a Minkowski spacetime background. In contrast with the Abelian
case, the contribution of the Chern-Simons term is nontrivial already in the
static, spherically symmetric limit. Both globally regular, particle-like
solutions and black holes are constructed numerically for several values of D.
These solutions carry a nonzero electric charge and have finite mass. For
globally regular solutions, the value of the electric charge is fixed by the
Chern-Simons coupling constant. The black holes can be thought as non-linear
superpositions of Reissner-Nordstrom and non-Abelian configurations. A
systematic discussion of the solutions is given for D=5, in which case the
Reissner-Nordstrom black hole becomes unstable and develops non-Abelian hair.
We show that some of these non-Abelian configurations are stable under linear,
spherically symmetric perturbations. A detailed discussion of an exact D=5
solution describing extremal black holes and solitons is also provided.Comment: 34 pages, 14 figures; v2: misprints corrected and references adde
Conformally coupled scalar solitons and black holes with negative cosmological constant
We present arguments for the existence of both globally regular and black hole solutions of the Einstein equations with a conformally coupled scalar field, in the presence of a negative cosmological constant, for space-time dimensions greater than or equal to four. These configurations approach asymptotically anti-de Sitter space-time and are indexed by the central value of the scalar field. We also study the stability of these solutions, and show that, at least for all the solutions studied numerically, they are linearly stable
Quantum States of Neutrons in Magnetic Thin Films
We have studied experimentally and theoretically the interaction of polarized
neutrons with magnetic thin films and magnetic multilayers. In particular, we
have analyzed the behavior of the critical edges for total external reflection
in both cases. For a single film we have observed experimentally and
theoretically a simple behavior: the critical edges remain fixed and the
intensity varies according to the angle between the polarization axis and the
magnetization vector inside the film. For the multilayer case we find that the
critical edges for spin up and spin down polarized neutrons move towards each
other as a function of the angle between the magnetization vectors in adjacent
ferromagnetic films. Although the results for multilayers and single thick
layers appear to be different, in fact the same spinor method explains both
results. An interpretation of the critical edges behavior for the multilyers as
a superposition of ferromagnetic and antifferomagnetic states is given.Comment: 6 pages, 5 figure
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