100 research outputs found
On Instability of Certain Bi-Metric and Massive-Gravity Theories
Stability about cosmological background solutions to the bi-metric
Hassan-Rosen theory is studied. The results of this analysis are presented, and
it is shown that a large class of cosmological backgrounds is classically
unstable. This sets serious doubts on the physical viability of the
Hassan-Rosen theory - and in turn also of the de Rham-Gadabaze-Tolley model, to
which the mentioned theory is parent. A way to overcome this instability by
means of curvature-type deformations is discussed.Comment: 4 pages, 2 figures; v2: minor changes to match PRD versio
Long-range correlated random field and random anisotropy O(N) models: A functional renormalization group study
We study the long-distance behavior of the O(N) model in the presence of
random fields and random anisotropies correlated as ~1/x^{d-sigma} for large
separation x using the functional renormalization group. We compute the fixed
points and analyze their regions of stability within a double epsilon=d-4 and
sigma expansion. We find that the long-range disorder correlator remains
analytic but generates short-range disorder whose correlator develops the usual
cusp. This allows us to obtain the phase diagrams in (d,sigma,N) parameter
space and compute the critical exponents to first order in epsilon and sigma.
We show that the standard renormalization group methods with a finite number of
couplings used in previous studies of systems with long-range correlated random
fields fail to capture all critical properties. We argue that our results may
be relevant to the behavior of He-3A in aerogel.Comment: 8 pages, 3 figures, revtex
Large-Scale Suppression from Stochastic Inflation
We show non-perturbatively that the power spectrum of a self-interacting
scalar field in de Sitter space-time is strongly suppressed on large scales.
The cut-off scale depends on the strength of the self-coupling, the number of
e-folds of quasi-de Sitter evolution, and its expansion rate. As a consequence,
the two-point correlation function of field fluctuations is free from infra-red
divergencies.Comment: 4 pages, 1 figure; v2 minor changes to match published PRL versio
Astrophysical Bose-Einstein condensates and superradiance
We investigate gravitational analogue models to describe slowly rotating objects (e.g., dark-matter halos, or boson stars) in terms of Bose-Einstein condensates, trapped in their own gravitational potentials. We begin with a modified Gross-Pitaevskii equation, and show that the resulting background equations of motion are stable, as long as the rotational component is treated as a small perturbation. The dynamics of the fluctuations of the velocity potential are effectively governed by the Klein-Gordon equation of a "Eulerian metric," where we derive the latter by the use of a relativistic Lagrangian extrapolation. Superradiant scattering on such objects is studied. We derive conditions for its occurence and estimate its strength. Our investigations might give an observational handle to phenomenologically constrain Bose-Einstein condensates
Primordial Black Holes as Dark Matter: Recent Developments
Although the dark matter is usually assumed to be made up of some form of elementary particle, primordial black holes (PBHs) could also provide some of it. However, various constraints restrict the possible mass windows to 1016–1017 g, 1020–1024 g, and 10–103M⊙. The last possibility is contentious but of special interest in view of the recent detection of black hole mergers by LIGO/Virgo. PBHs might have important consequences and resolve various cosmological conundra even if they account for only a small fraction of the dark matter density. In particular, those larger than 103M⊙ could generate cosmological structures through the seed or Poisson effect, thereby alleviating some problems associated with the standard cold dark matter scenario, and sufficiently large PBHs might provide seeds for the supermassive black holes in galactic nuclei. More exotically, the Planck-mass relics of PBH evaporations or stupendously large black holes bigger than 1012M⊙ could provide an interesting dark component
Stochastic Inflation and Replica Field Theory
We adopt methods from statistical field theory to stochastic inflation. For
the example of a free test field in de Sitter and power-law inflation, the
power spectrum of long-wavelength fluctuations is computed. We study its
dependence on the shape of the filter that separates long from short wavelength
modes. While for filters with infinite support the phenomenon of dimensional
reductions is found on large super-horizon scales, filters with compact support
return a scale-invariant power spectrum in the infra-red. Features of the power
spectrum, induced by the filter, decay within a few e-foldings. Thus the
late-time power spectrum is independent of the filter details.Comment: 15 pages, 14 figure
Island of Stability for Consistent Deformations of Einstein's Gravity
We construct explicitly deformations of Einstein's theory of gravity that are
consistent and phenomenologically viable since they respect, in particular,
cosmological backgrounds. We show that these deformations have unique
symmetries in accordance with unitarity requirements, and give rise to a
curvature induced self-stabilizing mechanism. As a consequence, any nonlinear
completed deformation must incorporate self-stabilization on generic spacetimes
already at lowest order in perturbation theory. Furthermore, our findings
include the possibility of consistent and phenomenologically viable
deformations of general relativity that are solely operative on curved
spacetime geometries, reducing to Einstein's theory on the Minkowski
background.Comment: 4 pages, 3 figures, v2: discussion of phenomenology and applications
added, presentation optimize
a replica field-theoretical study
In this thesis we apply methods from statistical physics to stochastic inflation. Those methods, the replica field theory and the Gaussian variational methods, have to our knowledge never been applied before in this context, and allow us to compute the power spectrum of a scalar test field in the most general set-up. It provides a framework to perform calculations in regions of arbitrarily large quantum fluctuations and may also serve as a starting point to address the issue of back reaction. We first give an introduction to cosmological inflation, cosmological perturbation theory and cosmic microwave background anisotropies. Then we explain the idea of stochastic inflation, including some detailed derivations, and give an overview over major progress in this field. This is followed by an introduction to replica field theory, presented in a way directly applicable to stochastic inflation. Our work continues with a detailed calculation of the power spectrum of a scalar test field in a Friedmann Universe. We show the effect of the quantum fluctuations on the spectrum and derive explicit expressions showing its dependence on time and other important parameters. The effect of self-interactions and possible effects on the cosmic microwave background are discussed. We conclude with a summary of our results and give an outlook. One part of our major results has been published in Phys. Rev. D 78, 103501 (2008), where for the first time we present a replica field-theoretical approach to stochastic inflation in which we find a manifestation of the phenomena of so-called dimensional reduction. It implies under certain conditions inevitable infra-red divergencies of correlation functions on large-scales. These conditions are examined in detail in Phys. Rev. D 79, 44009 (2009), where we find that generically for a wide class of circumstances the divergencies are pushed exponentially fast well beyond observable scales ...thesi
Virus infections in tumors pave the way for tumor-directed DC-vaccines
Effective treatment of solid cancers by tumor-directed DC-vaccines still remains a challenge in clinical oncology. For therapeutic success, knock-down of tumor-specific tolerance appears mandatory before a potent tumor-specific cytotoxic T-cell response can be triggered by DC-vaccinations. Evidence is emerging that lytic virus infection in tumors can provide valuable help
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