1,910 research outputs found
Wormholes, naked singularities and universes of ghost radiation
Both the static and homogeneous metrics describing the spherically symmetric
gravitational field of a crossflow of incoming and outgoing null dust streams
are generalized for the case of the two-component ghost radiation. Static
solutions represent either naked singularities or the wormholes recently found
by Hayward. The critical value of the parameter separating the two
possibilities is given. The wormhole is allowed to have positive mass. The
homogeneous solutions are open universes.Comment: 5 pages, 10 figures, minor changes to match the published versio
Asymmetric Swiss-cheese brane-worlds
We study a brane-world cosmological scenario with local inhomogeneities
represented by black holes. The brane is asymmetrically embedded into the bulk.
The black strings/cigars penetrating the Friedmann brane generate a
Swiss-cheese type structure. This universe forever expands and decelerates, as
its general relativistic analogue. The evolution of the cosmological fluid
however can proceed along four branches, two allowed to have positive energy
density, one of them having the symmetric embedding limit. On this branch a
future pressure singularity can arise for either (a) a difference in the
cosmological constants of the cosmological and black hole brane regions (b) a
difference in the left and right bulk cosmological constants. While the
behaviour (a) can be avoided by a redefinition of the fluid variables, (b)
establishes a critical value of the asymmetry over which the pressure
singularity occurs. We introduce the pressure singularity censorship which
bounds the degree of asymmetry in the bulk cosmological constant. We also show
as a model independent generic feature that the asymmetry source term due to
the bulk cosmological constant increases in the early universe. In order to
obey the nucleosynthesis constraints, the brane tension should be constrained
therefore both from below and from above. With the maximal degree of asymmetry
obeying the pressure singularity censorship, the higher limit is 10 times the
lower limit. The degree of asymmetry allowed by present cosmological
observations is however much less, pushing the upper limit to infinity.Comment: v2: considerably expanded, 19 pages, 8 figures, many new references.
Pressure singularity censorship introduced, strict limits on the possible
degree of asymmetry derived. v3: model independent analysis shows that the
asymmetry bounds the brane tension from above. Limits on the maximal tension
set. Version published in JCA
On the validity of the 5-dimensional Birkhoff theorem: The tale of an exceptional case
The 5-dimensional (5d) Birkhoff theorem gives the class of 5d vacuum
space-times containing spatial hypersurfaces with cosmological symmetries. This
theorem is violated by the 5d vacuum Gergely-Maartens (GM) space-time, which is
not a representant of the above class, but contains the static Einstein brane
as embedded hypersurface. We prove that the 5d Birkhoff theorem is still
satisfied in a weaker sense: the GM space-time is related to the degenerated
horizon metric of certain black-hole space-times of the allowed class. This
result resembles the connection between the Bertotti-Robinson space-time and
the horizon region of the extremal Reissner-Nordstrom space-time in general
relativity.Comment: 13 pages; v2: title amended, to be published in Classical and Quantum
Gravit
Constraining Ho\v{r}ava-Lifshitz gravity by weak and strong gravitational lensing
We discuss gravitational lensing in the Kehagias-Sfetsos space-time emerging
in the framework of Ho\v{r}ava-Lifshitz gravity. In weak lensing we show that
there are three regimes, depending on the value of , where is the Ho\v{r}ava-Lifshitz parameter and
characterizes the lensing geometry. When is close to zero,
light deflection typically produces two images, as in Schwarzschild lensing.
For very large the space-time approaches flatness, therefore
there is only one undeflected image. In the intermediate range of only the upper focused image is produced due to the existence of a
maximal deflection angle , a feature inexistent in the
Schwarzschild weak lensing. We also discuss the location of Einstein rings, and
determine the range of the Ho\v{r}ava-Lifshitz parameter compatible with
present day lensing observations. Finally, we analyze in the strong lensing
regime the first two relativistic Einstein rings and determine the constraints
on the parameter range to be imposed by forthcoming experiments.Comment: 11 pages, 6 figures, introductory part and reference list changed on
referee request, results expanded. Published versio
Active Galactic Nuclei with Starbursts: Sources for Ultra High Energy Cosmic Rays
Ultra high energy cosmic ray events presently show a spectrum, which we
interpret here as galactic cosmic rays due to a starburst in the radio galaxy
Cen A pushed up in energy by the shock of a relativistic jet. The knee feature
and the particles with energy immediately higher in galactic cosmic rays then
turn into the bulk of ultra high energy cosmic rays. This entails that all
ultra high energy cosmic rays are heavy nuclei. This picture is viable if the
majority of the observed ultra high energy events come from the radio galaxy
Cen A, and are scattered by intergalactic magnetic fields across most of the
sky.Comment: 4 pages, 1 figure, proceedings of "High-Energy Gamma-rays and
Neutrinos from Extra-Galactic Sources", Heidelber
Semi-transparent brane-worlds
We study the evolution of a closed Friedmann brane perturbed by the Hawking
radiation escaping a bulk black hole. The semi-transparent brane absorbes some
of the infalling radiation, the rest being transmitted across the brane to the
other bulk region. We characterize the cosmological evolution in terms of the
transmission rate . For small values of a critical-like
behaviour could be observed, when the acceleration due to radiation pressure
and the deceleration induced by the increasing self-gravity of the brane
roughly compensate each other, and cosmological evolution is approximately the
same as without radiation. Lighter (heavier) branes than those with the
critical energy density will recollapse slower (faster). This feature is
obstructed at high values of , where the overall effect of the
radiation is to speed-up the recollapse. We determine the maximal value of the
transmission rate for which the critical-like behaviour is observed. We also
study the effect of transmission on the evolution of different source terms of
the Friedmann equation. We conclude that among all semi-transparent branes the
slowest recollapse occurs for light branes with total absorption.Comment: 15 pages, 8 figure
Spin-spin effects in radiating compact binaries
The dynamics of a binary system with two spinning components on an eccentric
orbit is studied, with the inclusion of the spin-spin interaction terms
appearing at the second post-Newtonian order. A generalized true anomaly
parametrization properly describes the radial component of the motion. The
average over one radial period of the magnitude of the orbital angular momentum
is found to have no nonradiative secular change. All spin-spin terms
in the secular radiative loss of the energy and magnitude of orbital angular
momentum are given in terms of and other constants of the motion.
Among them, self-interaction spin effects are found, representing the second
post-Newtonian correction to the 3/2 post-Newtonian order Lense-Thirring
approximation.Comment: 12 pages, to appear in Phys. Rev.
Broken phase effective potential in the two-loop Phi-derivable approximation and nature of the phase transition in a scalar theory
We study the phase transition of a real scalar phi^4 theory in the two-loop
Phi-derivable approximation using the imaginary time formalism, extending our
previous (analytical) discussion of the Hartree approximation. We combine Fast
Fourier Transform algorithms and accelerated Matsubara sums in order to achieve
a high accuracy. Our results confirm and complete earlier ones obtained in the
real time formalism [1] but which were less accurate due to the integration in
Minkowski space and the discretization of the spectral density function. We
also provide a complete and explicit discussion of the renormalization of the
two-loop Phi-derivable approximation at finite temperature, both in the
symmetric and in the broken phase, which was already used in the real-time
approach, but never published. Our main result is that the two-loop
Phi-derivable approximation suffices to cure the problem of the Hartree
approximation regarding the order of the transition: the transition is of the
second order type, as expected on general grounds. The corresponding critical
exponents are, however, of the mean-field type. Using a "RG-improved" version
of the approximation, motivated by our renormalization procedure, we find that
the exponents are modified. In particular, the exponent delta, which relates
the field expectation value phi to an external field h, changes from 3 to 5,
getting then closer to its expected value 4.789, obtained from accurate
numerical estimates [2].Comment: 54 pages, 16 figure
Reverse engineering of linking preferences from network restructuring
We provide a method to deduce the preferences governing the restructuring
dynamics of a network from the observed rewiring of the edges. Our approach is
applicable for systems in which the preferences can be formulated in terms of a
single-vertex energy function with f(k) being the contribution of a node of
degree k to the total energy, and the dynamics obeys the detailed balance. The
method is first tested by Monte-Carlo simulations of restructuring graphs with
known energies, then it is used to study variations of real network systems
ranging from the co-authorship network of scientific publications to the asset
graphs of the New York Stock Exchange. The empirical energies obtained from the
restructuring can be described by a universal function f(k) -k ln(k), which is
consistent with and justifies the validity of the preferential attachment rule
proposed for growing networks.Comment: 7 pages, 6 figures, submitted to PR
Exploring the Kondo model in and out of equilibrium with alkaline-earth atoms
We propose a scheme to realize the Kondo model with tunable anisotropy using
alkaline-earth atoms in an optical lattice. The new feature of our setup is
Floquet engineering of interactions using time-dependent Zeeman shifts, that
can be realized either using state-dependent optical Stark shifts or magnetic
fields. The properties of the resulting Kondo model strongly depend on the
anisotropy of the ferromagnetic interactions. In particular, easy-plane
couplings give rise to Kondo singlet formation even though microscopic
interactions are all ferromagnetic. We discuss both equilibrium and dynamical
properties of the system that can be measured with ultracold atoms, including
the impurity spin susceptibility, the impurity spin relaxation rate, as well as
the equilibrium and dynamical spin correlations between the impurity and the
ferromagnetic bath atoms. We analyze the non-equilibrium time evolution of the
system using a variational non-Gaussian approach, which allows us to explore
coherent dynamics over both short and long timescales, as set by the bandwidth
and the Kondo singlet formation, respectively. In the quench-type experiments,
when the Kondo interaction is suddenly switched on, we find that real-time
dynamics shows crossovers reminiscent of poor man's renormalization group flow
used to describe equilibrium systems. For bare easy-plane ferromagnetic
couplings, this allows us to follow the formation of the Kondo screening cloud
as the dynamics crosses over from ferromagnetic to antiferromagnetic behavior.
On the other side of the phase diagram, our scheme makes it possible to measure
quantum corrections to the well-known Korringa law describing the temperature
dependence of the impurity spin relaxation rate. Theoretical results discussed
in our paper can be measured using currently available experimental techniques.Comment: 22 pages, 12 figure
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