36 research outputs found
Lorentz-violation and cosmological perturbations: a toy brane-world model
We study possible effects of Lorentz-violation on the generation of
cosmological perturbations at inflation by introducing a simple inflating
five-dimensional brane-world setup with violation of four-dimensional
Lorentz-invariance at an energy scale . We consider massless scalar field,
meant to mimic perturbations of inflaton and/or gravitational field, in this
background. At three-momenta below , there exists a zero mode localized on
the brane, whose behaviour coincides with that in four-dimensional theory. On
the contrary, at three-momenta above , the localized mode is absent and
physics is entirely five-dimensional. As three-momenta get redshifted, more
modes get localized on the brane, the phenomenon analogous to ``mode
generation''. We find that for , where is the inflationary Hubble
scale, the spectrum of perturbations coincides with that in four-dimensional
theory. For and time-dependent bulk parameters, the spectrum deviates,
possibly strongly, from the flat spectrum even for pure de Sitter inflation.Comment: 5 figures, iopart, minor changes, appendix adde
Real-Time Instantons and Suppression of Collision-Induced Tunneling
We consider tunneling processes in QFT induced by collisions of elementary
particles. We propose a semiclassical method for estimating the probability of
these processes in the limit of very high collision energy. As an illustration,
we evaluate the maximum probability of induced tunneling between different
vacua in a (1+1)-dimensional scalar model with boundary interaction.Comment: 5 pages, 1 figure, JETP Letters styl
ПРОГНОЗ ОБЛАСТЕЙ СЖАТИЯ И РАСТЯЖЕНИЯ В ГЕОЛОГИЧЕСКИХ СТРУКТУРАХ С ИСПОЛЬЗОВАНИЕМ ДАННЫХ ТОЛЬКО О СКОРОСТЯХ ПРОДОЛЬНЫХ ВОЛН В ГЕОЛОГИЧЕСКОЙ СРЕДЕ
The article presents accurate solutions for the problem for two elastic half‐spaces with an arbitrary curvilinear interface. Our study shows that dilatation solutions (Poisson integrals) are dependent on neither an overall compression modulus nor the Poisson ratio, and depend only on the velocity of longitudinal waves. These specific solutions can be supplemented by general solutions for an incompressible elastic medium, and the boundary conditions of the rigid contact for the sum of the solutions can thus be satisfied. Relatively simple calculations make it possible to determine the divergence of the displacement field and reduce the entire problem solving process to a study of Poisson equations with a known divergence. Furthermore, predictions of volumetric compression or extension are important for geological investigations, since the zones characterized by reduced pressure rates may act as fluid attractors.Приведены точные решения упругой задачи для двух полупространств, разделенных произвольной криволинейной поверхностью. Показано, что частные решения для дилатации (интегралы Пуассона) не зависят ни от модуля всестороннего сжатия, ни от коэффициента Пуассона, а зависят только от скорости продольных волн. Эти частные решения могут быть дополнены общими решениями для несжимаемой упругой среды, и тем самым будут выполнены граничные условия жесткого контакта для суммы означенных решений. Возникает возможность сравнительно простыми вычислениями определить дивергенцию поля перемещений и свести всю задачу к исследованию уравнений типа Пуассона при известной дивергенции. Кроме того, сам прогноз объемного сжатия или растяжения имеет важное геологическое значение, так как зоны пониженного давления могут быть аттракторами флюидов
Black Holes in Ho\v{r}ava Gravity with Higher Derivative Magnetic Terms
We consider Horava gravity coupled to Maxwell and higher derivative magnetic
terms. We construct static spherically symmetric black hole solutions in the
low-energy approximation. We calculate the horizon locations and temperatures
in the near-extremal limit, for asymptotically flat and (anti-)de Sitter
spaces. We also construct a detailed balanced version of the theory, for which
we find projectable and non-projectable, non-perturbative solutions.Comment: 17 pages. v2: Up to date with published version; some minor remarks
and more reference
The Generalized Second Law implies a Quantum Singularity Theorem
The generalized second law can be used to prove a singularity theorem, by
generalizing the notion of a trapped surface to quantum situations. Like
Penrose's original singularity theorem, it implies that spacetime is null
geodesically incomplete inside black holes, and to the past of spatially
infinite Friedmann--Robertson--Walker cosmologies. If space is finite instead,
the generalized second law requires that there only be a finite amount of
entropy producing processes in the past, unless there is a reversal of the
arrow of time. In asymptotically flat spacetime, the generalized second law
also rules out traversable wormholes, negative masses, and other forms of
faster-than-light travel between asymptotic regions, as well as closed timelike
curves. Furthermore it is impossible to form baby universes which eventually
become independent of the mother universe, or to restart inflation. Since the
semiclassical approximation is used only in regions with low curvature, it is
argued that the results may hold in full quantum gravity. An introductory
section describes the second law and its time-reverse, in ordinary and
generalized thermodynamics, using either the fine-grained or the coarse-grained
entropy. (The fine-grained version is used in all results except those relating
to the arrow of time.) A proof of the coarse-grained ordinary second law is
given.Comment: 46 pages, 8 figures. v2: discussion of global hyperbolicity revised
(4.1, 5.2), more comments on AdS. v3: major revisions including change of
title. v4: similar to published version, but with corrections to plan of
paper (1) and definition of global hyperbolicity (3.2). v5: fixed proof of
Thm. 1, changed wording of Thm. 3 & proof of Thm. 4, revised Sec. 5.2, new
footnote
Perturbative instabilities in Horava gravity
We investigate the scalar and tensor perturbations in Horava gravity, with
and without detailed balance, around a flat background. Once both types of
perturbations are taken into account, it is revealed that the theory is plagued
by ghost-like scalar instabilities in the range of parameters which would
render it power-counting renormalizable, that cannot be overcome by simple
tricks such as analytic continuation. Implementing a consistent flow between
the UV and IR limits seems thus more challenging than initially presumed,
regardless of whether the theory approaches General Relativity at low energies
or not. Even in the phenomenologically viable parameter space, the tensor
sector leads to additional potential problems, such as fine-tunings and
super-luminal propagation.Comment: 21 pages, version published at Class. Quant. Gra
Supergravity based inflation models: a review
In this review, we discuss inflation models based on supergravity. After
explaining the difficulties in realizing inflation in the context of
supergravity, we show how to evade such difficulties. Depending on types of
inflation, we give concrete examples, particularly paying attention to chaotic
inflation because the ongoing experiments like Planck might detect the tensor
perturbations in near future. We also discuss inflation models in Jordan frame
supergravity, motivated by Higgs inflation.Comment: 30 pages, invited review for Classical and Quantum Gravity, published
versio
Superluminal Travel Made Possible (in two dimensions)
We argue that superluminal signal propagation is possible in consistent
Poincare invariant quantum field theories in two space-time dimensions,
provided spatial parity is broken. This happens due to existence of the
``instantaneous'' causal structure, with one of the light cone variables being
a global time. In two dimensions this causal structure is invariant under the
Poincare group if one gives up the spatial parity. As a non-trivial example of
a consistent interacting quantum field theory with this causal structure we
discuss a non-linear SO(1,1) sigma-model, where SO(1,1) is the Lorentz
symmetry. We show that this theory is asymptotically free and argue that this
model is also well defined non-perturbatively, at least for some values of
parameters. It provides an example of a microscopic Poincare invariant quantum
field theory with local action, but non-local physical properties. Being
coupled to gravity this ``instantaneous'' theory mixes with the Liouville
field. If proves to be consistent, the resulting model can be used to construct
(non-critical) string theories with very unconventional properties by
introducing the instantaneous causal structure on the world-sheet.Comment: 33 pages, 13 figure
Supersymmetric Aether
It has been suggested by Groot Nibbelink and Pospelov that Lorentz invariance
can be an emergent symmetry of low-energy physics provided the theory enjoys a
non-relativistic version of supersymmetry. We construct a model that realizes
the latter symmetry dynamically: it breaks Lorentz invariance but leaves the
supersymmetry generators intact. The model is a supersymmetric extension of the
dynamical aether theory of Jacobson and Mattingly. It shows rich dynamics and
possesses a family of inequivalent vacua realizing different symmetry breaking
patterns. In particular, we find stable vacua that break spontaneously spatial
isotropy. Supersymmetry breaking terms give masses to fermionic and bosonic
partners of the aether field. We comment on the coupling of the model to
supergravity and on the implications for Horava gravity.Comment: 21 pages, no figure
Superluminal group velocity through near-maximal neutrino oscillations
Recently it was suggested that the observation of superluminal neutrinos by
the OPERA collaboration may be due to group velocity effects resulting from
close-to-maximal oscillation between neutrino mass eigenstates, in analogy to
known effects in optics. We show that superluminal propagation does occur
through this effect for a series of very narrow energy ranges, but this
phenomenum cannot explain the OPERA measurement.Comment: Reworking includes corrections due to finite width of the wave
packet, further references, and comments on other corrections, causality, the
new OPERA results, and why the size of neutrino wave packet provides
important constraint