2,543 research outputs found
Higgs condensation as an unwanted curvaton
During inflation in the early universe, the Higgs field continuously acquires
long-wave quantum fluctuations. They accumulate to yield a non-vanishing value
with an exponentially large correlation length. We study consequences of such
Higgs condensations to show that, in inflation models where the universe is
reheated through gravitational particle production at the transition to the
kination regime, they not only contribute to reheat the universe but also act
as a curvaton. Unfortunately, however, for parameters of the Standard Model
Higgs field, this curvaton produces density fluctuations too large, so the
inflation models followed by a long kination regime are ruled out.Comment: 13 pages; v2, layout adjusted, references adde
Floating and sinking: the imprint of massive scalars around rotating black holes
We study the coupling of massive scalar fields to matter in orbit around
rotating black holes. It is generally expected that orbiting bodies will lose
energy in gravitational waves, slowly inspiralling into the black hole.
Instead, we show that the coupling of the field to matter leads to a surprising
effect: because of superradiance, matter can hover into "floating orbits" for
which the net gravitational energy loss at infinity is entirely provided by the
black hole's rotational energy. Orbiting bodies remain floating until they
extract sufficient angular momentum from the black hole, or until perturbations
or nonlinear effects disrupt the orbit. For slowly rotating and nonrotating
black holes floating orbits are unlikely to exist, but resonances at orbital
frequencies corresponding to quasibound states of the scalar field can speed up
the inspiral, so that the orbiting body "sinks". These effects could be a
smoking gun of deviations from general relativity.Comment: 5 pages, two figures, RevTeX4.1. v2: Published in Physical Review
Letter
Superradiant instability of large radius doubly spinning black rings
We point out that 5D large radius doubly spinning black rings with rotation
along S^1 and S^2 are afflicted by a robust instability. It is triggered by
superradiant bound state modes. The Kaluza-Klein momentum of the mode along the
ring is responsible for the bound state. This kind of instability in black
strings and branes was first suggested by Marolf and Palmer and studied in
detail by Cardoso, Lemos and Yoshida. We find the frequency spectrum and
timescale of this instability in the black ring background, and show that it is
active for large radius rings with large rotation along S^2. We identify the
endpoint of the instability and argue that it provides a dynamical mechanism
that introduces an upper bound in the rotation of the black ring. To estimate
the upper bound, we use the recent black ring model of Hovdebo and Myers, with
a minor extension to accommodate an extra small angular momentum. This
dynamical bound can be smaller than the Kerr-like bound imposed by regularity
at the horizon. Recently, the existence of higher dimensional black rings is
being conjectured. They will be stable against this mechanism.Comment: 21 pages, 3 figures. Overall minor improvements in discussions added.
Matches published version in PR
Analytical treatment of 2D steady flames anchored in high-velocity streams
The problem of burning of high-velocity gas streams in channels is revisited.
Previous treatments of this issue are found to be incomplete. It is shown that
despite relative smallness of the transversal gas velocity, it plays crucial
role in determining flame structure. In particular, it is necessary in
formulating boundary conditions near the flame anchor, and for the proper
account of the flame propagation law. Using the on-shell description of steady
anchored flames, a consistent solution of the problem is given. Equations for
the flame front position and gas-velocity at the front are obtained. It is
demonstrated that they reduce to a second-order differential equation for the
front position. Numerical solutions of the derived equations are found.Comment: 15 pages, 6 figure
Quasinormal modes of a massless charged scalar field on a small Reissner-Nordstr\"om-anti-de Sitter black hole
We investigate quasinormal modes of a massless charged scalar field on a
small Reissner-Nordstr\"om-anti-de Sitter (RN-AdS) black hole both with
analytical and numerical approaches. In the analytical approach, by using the
small black hole approximation (r_+ << L), we obtain the quasinormal mode
frequencies in the limit of r_+/L -> 0, where r_+ and L stand for the black
hole event horizon radius and the AdS scale, respectively. We then show that
the small RN-AdS black hole is unstable if its quasinormal modes satisfy the
superradiance condition and that the instability condition of the RN-AdS black
hole in the limit of r_+/L -> 0 is given by Q>(3/eL)Q_c, where Q, Q_c, and e
are the charge of the black hole, the critical (maximum) charge of the black
hole, and the charge of the scalar field, respectively. In the numerical
approach, we calculate the quasinormal modes for the small RN-AdS black holes
with r_+ << L and confirm that the RN-AdS black hole is unstable if its
quasinormal modes satisfy the superradiance condition. Our numerical results
show that the RN-AdS black holes with r_+ =0.2L, 0.1L, and 0.01L become
unstable against scalar perturbations with eL=4 when the charge of the black
hole satisfies Q > 0.8Q_c, 0.78Q_c, and 0.76Q_c, respectively.Comment: 13 pages, 11 figure
BlackMax: A black-hole event generator with rotation, recoil, split branes and brane tension
We present a comprehensive black-hole event generator, BlackMax, which
simulates the experimental signatures of microscopic and Planckian black-hole
production and evolution at the LHC in the context of brane world models with
low-scale quantum gravity. The generator is based on phenomenologically
realistic models free of serious problems that plague low-scale gravity, thus
offering more realistic predictions for hadron-hadron colliders. The generator
includes all of the black-hole graybody factors known to date and incorporates
the effects of black-hole rotation, splitting between the fermions, non-zero
brane tension and black-hole recoil due to Hawking radiation (although not all
simultaneously).
The generator can be interfaced with Herwig and Pythia.Comment: 32 pages, 61 figures, webpage
http://www-pnp.physics.ox.ac.uk/~issever/BlackMax/blackmax.htm
Standing gravitational waves from domain walls
We construct a plane symmetric, standing gravitational wave for a domain wall
plus a massless scalar field. The scalar field can be associated with a fluid
which has the properties of `stiff' matter, i.e. matter in which the speed of
sound equals the speed of light. Although domain walls are observationally
ruled out in the present era the solution has interesting features which might
shed light on the character of exact non-linear wave solutions to Einstein's
equations. Additionally this solution may act as a template for higher
dimensional 'brane-world' model standing waves.Comment: 4 pages two-column format, no figures, added discussion of physical
meaning of solution, added refernces, to be published PR
Numerical Solutions of Inflating Higher Dimensional Global Defects
We find numerical solutions of Einstein equations and scalar field equation
for a global defect in higher dimensional spacetimes (). We examine in
detail the relation among the expansion rate and the symmetry-breaking
scale and the number of extra dimensions for these solutions. We
find that even if the extra dimensions do not have a cigar geometry, the
expansion rate grows as increases, which is opposite to what is
needed for the recently proposed mechanism for solving the cosmological
constant problem. We also find that the expansion rate decreases as
increases.Comment: 6 pages, 7 figures, ReVTeX4, Accepted for publication in PR
Perturbations and absorption cross-section of infinite-radius black rings
We study scalar field perturbations on the background of non-supersymmetric
black rings and of supersymmetric black rings. In the infinite-radius limit of
these geometries, we are able to separate the wave equation, and to study wave
phenomena in its vicinities. In this limit, we show that (i) both geometries
are stable against scalar field perturbations, (ii) the absorption
cross-section for scalar fields is equal to the area of the event horizon in
the supersymmetric case, and proportional to it in the non-supersymmetric
situation.Comment: ReVTeX4. 15 pages, 3 figures. References added. Published versio
How particle collisions increase the rate of accretion from the cosmological background onto primordial black holes in braneworld cosmology
It is shown that, contrary to the widespread opinion, particle collisions
considerably increase accretion rate from the cosmological background onto 5D
primordial black holes formed during the high-energy phase of the
Randall-Sundrum Type II braneworld scenario. Increase of accretion rate leads
to much tighter constraints on initial primordial black hole mass fraction
imposed by the critical density limit and measurements of high-energy diffuse
photon background and antiproton excess.Comment: 5 pages, 4 figure
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