75 research outputs found
Emergent Lorentz invariance with chiral fermions
We study renormalization group flows in strongly interacting field theories
with fermions that correspond to transitions between a theory without Lorentz
invariance at high energies down to a theory with approximate Lorentz symmetry
in the infrared. Holographic description of the strong coupling is used. The
emphasis is made on emergence of chiral fermions in the low-energy theory.Comment: 28 pages, 2 figure
On stability of electroweak vacuum during inflation
We study Coleman-De Luccia tunneling of the Standard Model Higgs field during
inflation in the case when the electroweak vacuum is metastable. We verify that
the tunneling rate is exponentially suppressed. The main contribution to the
suppression is the same as in flat space-time. We analytically estimate the
corrections due to the expansion of the universe and an effective mass term in
the Higgs potential that can be present at inflation.Comment: 9 pages, 2 figures, dependence of the grav. corrections to the bounce
action on non-minimal coupling updated, corresponding references adde
Models of non-relativistic quantum gravity: the good, the bad and the healthy
Horava's proposal for non-relativistic quantum gravity introduces a preferred
time foliation of space-time which violates the local Lorentz invariance. The
foliation is encoded in a dynamical scalar field which we call `khronon'. The
dynamics of the khronon field is sensitive to the symmetries and other details
of the particular implementations of the proposal. In this paper we examine
several consistency issues present in three non-relativistic gravity theories:
Horava's projectable theory, the healthy non-projectable extension, and a new
extension related to ghost condensation. We find that the only model which is
free from instabilities and strong coupling is the non-projectable one. We
elaborate on the phenomenology of the latter model including a discussion of
the couplings of the khronon to matter. In particular, we obtain the parameters
of the post-Newtonian expansion in this model and show that they are compatible
with current observations.Comment: 50 pages, JHEP styl
Semiclassical S-matrix for black holes
We propose a semiclassical method to calculate S-matrix elements for
two-stage gravitational transitions involving matter collapse into a black hole
and evaporation of the latter. The method consistently incorporates
back-reaction of the collapsing and emitted quanta on the metric. We illustrate
the method in several toy models describing spherical self-gravitating shells
in asymptotically flat and AdS space-times. We find that electrically neutral
shells reflect via the above collapse-evaporation process with probability
exp(-B), where B is the Bekenstein-Hawking entropy of the intermediate black
hole. This is consistent with interpretation of exp(B) as the number of black
hole states. The same expression for the probability is obtained in the case of
charged shells if one takes into account instability of the Cauchy horizon of
the intermediate Reissner-Nordstrom black hole. Our semiclassical method opens
a new systematic approach to the gravitational S-matrix in the non-perturbative
regime.Comment: 41 pages, 13 figures; Introduction rewritten, references added;
journal versio
Constraints on violation of Lorentz invariance from atmospheric showers initiated by multi-TeV photons
We discuss the effect of hypothetical violation of Lorentz invariance at high
energies on the formation of atmospheric showers by very-high-energy gamma
rays. In the scenario where Lorentz invariance violation leads to a decrease of
the photon velocity with energy the formation of the showers is suppressed
compared to the Lorentz invariant case. Absence of such suppression in the
high-energy part of spectrum of the Crab nebula measured independently by HEGRA
and H.E.S.S. collaborations is used to set lower bounds on the energy scale of
Lorentz invariance violation. These bounds are competitive with the strongest
existing constraints obtained from timing of variable astrophysical sources and
the absorption of TeV photons on the extragalactic background light. They will
be further improved by the next generation of multi-TeV gamma-ray
observatories.Comment: 21 pages, 4 figures. References adde
Slowly moving black holes in khrono-metric model
We search for solutions describing slowly moving black holes in the
khrono-metric model, a modified gravity theory with preferred time (khronon)
which arises at low energies from the non-projectable Horava gravity. We work
in the decoupling limit when the back-reaction of the khronon on the metric is
small and can be treated perturbatively. For a given black hole velocity, we
find a family of solutions parameterized by the khronon propagation speed and
regular everywhere outside the universal horizon. On the universal horizon they
have a weak singularity manifesting itself in a non-analyticity of the khronon
field. Using the behavior of khronon at infinity we extract the leading black
hole sensitivity for which we obtain a simple analytic expression valid
throughout the phenomenologically allowed parameter space.Comment: 40 pages, 6 figure
Testing Lorentz invariance of dark matter with satellite galaxies
We develop the framework for testing Lorentz invariance in the dark matter
sector using galactic dynamics. We consider a Lorentz violating (LV) vector
field acting on the dark matter component of a satellite galaxy orbiting in a
host halo. We introduce a numerical model for the dynamics of satellites in a
galactic halo and for a galaxy in a rich cluster to explore observational
consequences of such an LV field. The orbital motion of a satellite excites a
time dependent LV force which greatly affects its internal dynamics. Our
analysis points out key observational signatures which serve as probes of LV
forces. These include modifications to the line of sight velocity dispersion,
mass profiles and shapes of satellites. With future data and a more detailed
modeling these signatures can be exploited to constrain a new region of the
parameter space describing the LV in the dark matter sector.Comment: 27 pages, 11 figures, 2 tables, 1 appendix. Minor corrections in
section 4.3.
Gravity Cutoff in Theories with Large Discrete Symmetries
We set an upper bound on the gravitational cutoff in theories with exact
quantum numbers of large N periodicity, such as Z_N discrete symmetries. The
bound stems from black hole physics. It is similar to the bound appearing in
theories with N particle species, though a priori, a large discrete symmetry
does not imply a large number of species. Thus, there emerges a potentially
wide class of new theories, that address the hierarchy problem by lowering the
gravitational cutoff due to existence of large Z_{10^32}-type symmetries.Comment: example and discussion adde
Completing Lorentz violating massive gravity at high energies
Theories with massive gravitons are interesting for a variety of physical
applications, ranging from cosmological phenomena to holographic modeling of
condensed matter systems. To date, they have been formulated as effective field
theories with a cutoff proportional to a positive power of the graviton mass
m_g and much smaller than that of the massless theory (M_P ~ 10^19 GeV in the
case of general relativity). In this paper we present an ultraviolet completion
for massive gravity valid up to a high energy scale independent of the graviton
mass. The construction is based on the existence of a preferred time foliation
combined with spontaneous condensation of vector fields. The perturbations of
these fields are massive and below their mass the theory reduces to a model of
Lorentz violating massive gravity. The latter theory possesses instantaneous
modes whose consistent quantization we discuss in detail. We briefly study some
modifications to gravitational phenomenology at low-energies. The homogeneous
cosmological solutions are the same as in the standard cosmology. The
gravitational potential of point sources agrees with the Newtonian one at
distances small with respect to m_g^(-1). Interestingly, it becomes repulsive
at larger distances.Comment: 33 pages, 3 figures; to appear in a special issue of JETP dedicated
to the 60th birthday of Valery Rubakov; minor changes with respect to v1,
references update
Time-Sliced Perturbation Theory for Large Scale Structure I: General Formalism
We present a new analytic approach to describe large scale structure
formation in the mildly non-linear regime. The central object of the method is
the time-dependent probability distribution function generating correlators of
the cosmological observables at a given moment of time. Expanding the
distribution function around the Gaussian weight we formulate a perturbative
technique to calculate non-linear corrections to cosmological correlators,
similar to the diagrammatic expansion in a three-dimensional Euclidean quantum
field theory, with time playing the role of an external parameter. For the
physically relevant case of cold dark matter in an Einstein--de Sitter
universe, the time evolution of the distribution function can be found exactly
and is encapsulated by a time-dependent coupling constant controlling the
perturbative expansion. We show that all building blocks of the expansion are
free from spurious infrared enhanced contributions that plague the standard
cosmological perturbation theory. This paves the way towards the systematic
resummation of infrared effects in large scale structure formation. We also
argue that the approach proposed here provides a natural framework to account
for the influence of short-scale dynamics on larger scales along the lines of
effective field theory.Comment: 29 pages, 2 figures, discussion on IR safety expanded, appendix C
added; version published in JCA
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