2,749 research outputs found
Low energy bounds on Poincare violation in causal set theory
In the causal set approach to quantum gravity, Poincar\'{e} symmetry is
modified by swerving in spacetime, induced by the random lattice discretization
of the space-time structure. The broken translational symmetry at short
distances is argued to lead to a residual diffusion in momentum space, whereby
a particle can acquire energy and momentum by drift along its mass shell and a
system in equilibrium can spontaneously heat up. We consider bounds on the rate
of momentum space diffusion coming from astrophysical molecular clouds, nuclear
stability and cosmological neutrino background. We find that the strongest
limits come from relic neutrinos, which we estimate to constrain the momentum
space diffusion constant by for neutrinos with
masses , improving the previously quoted bounds by
roughly 17 orders of magnitude.Comment: Additional discussion about behavior of alpha particles in nuclei
added. Version matches that accepted in PR
Causal sets and conservation laws in tests of Lorentz symmetry
Many of the most important astrophysical tests of Lorentz symmetry also
assume that energy-momentum of the observed particles is exactly conserved. In
the causal set approach to quantum gravity a particular kind of Lorentz
symmetry holds but energy-momentum conservation may be violated. We show that
incorrectly assuming exact conservation can give rise to a spurious signal of
Lorentz symmetry violation for a causal set. However, the size of this spurious
signal is much smaller than can be currently detected and hence astrophysical
Lorentz symmetry tests as currently performed are safe from causal set induced
violations of energy-momentum conservation.Comment: 8 pages, matches version published in PR
Relativistic kinematics beyond Special Relativity
In the context of departures from Special Relativity written as a momentum
power expansion in the inverse of an ultraviolet energy scale M, we derive the
constraints that the relativity principle imposes between coefficients of a
deformed composition law, dispersion relation, and transformation laws, at
first order in the power expansion. In particular, we find that, at that order,
the consistency of a modification of the energy-momentum composition law fixes
the modification in the dispersion relation. We therefore obtain the most
generic modification of Special Relativity that preserves the relativity
principle at leading order in 1/M.Comment: Version with minor corrections, to appear in Phys. Rev.
Mechanics of universal horizons
Modified gravity models such as Ho\v{r}ava-Lifshitz gravity or
Einstein-{\ae}ther theory violate local Lorentz invariance and therefore
destroy the notion of a universal light cone. Despite this, in the infrared
limit both models above possess static, spherically symmetric solutions with
"universal horizons" - hypersurfaces that are causal boundaries between an
interior region and asymptotic spatial infinity. In other words, there still
exist black hole solutions. We construct a Smarr formula (the relationship
between the total energy of the spacetime and the area of the horizon) for such
a horizon in Einstein-{\ae}ther theory. We further show that a slightly
modified first law of black hole mechanics still holds with the relevant area
now a cross-section of the universal horizon. We construct new analytic
solutions for certain Einstein-{\ae}ther Lagrangians and illustrate how our
results work in these exact cases. Our results suggest that holography may be
extended to these theories despite the very different causal structure as long
as the universal horizon remains the unique causal boundary when matter fields
are added.Comment: Minor clarifications. References update
Interferometers as Probes of Planckian Quantum Geometry
A theory of position of massive bodies is proposed that results in an
observable quantum behavior of geometry at the Planck scale, . Departures
from classical world lines in flat spacetime are described by Planckian
noncommuting operators for position in different directions, as defined by
interactions with null waves. The resulting evolution of position wavefunctions
in two dimensions displays a new kind of directionally-coherent quantum noise
of transverse position. The amplitude of the effect in physical units is
predicted with no parameters, by equating the number of degrees of freedom of
position wavefunctions on a 2D spacelike surface with the entropy density of a
black hole event horizon of the same area. In a region of size , the effect
resembles spatially and directionally coherent random transverse shear
deformations on timescale with typical amplitude . This quantum-geometrical "holographic noise" in position is not
describable as fluctuations of a quantized metric, or as any kind of
fluctuation, dispersion or propagation effect in quantum fields. In a Michelson
interferometer the effect appears as noise that resembles a random Planckian
walk of the beamsplitter for durations up to the light crossing time. Signal
spectra and correlation functions in interferometers are derived, and predicted
to be comparable with the sensitivities of current and planned experiments. It
is proposed that nearly co-located Michelson interferometers of laboratory
scale, cross-correlated at high frequency, can test the Planckian noise
prediction with current technology.Comment: 23 pages, 6 figures, Latex. To appear in Physical Review
Tests of Lorentz and CPT symmetry with hadrons and nuclei
We apply chiral-perturbation-theory techniques to the QCD sector of the Lorentz and CPT violating standard-model extension. We derive the effective Lagrangian in terms of pions and nucleons for a selected set of dimension-five operators involving quarks and gluons. This derivation is based on chiral-symmetry properties of the operators, as well as on their behaviour under C,P, and T transformations. We consider the power counting rules and apply the heavy-baryon approach to account for the large nucleon mass. Having obtained the relevant Lorentz-violating contributions to the pion-nucleon Lagrangian, we proceed to derive the particle and anti-particle Hamiltonian, from which we obtain the Lorentz-violating contribution to comagnetometer experiments. This allows us to place stringent limits on some of the parameters. For some other parameters we find that the best bounds will come from nucleon nucleon interactions, and we derive the relevant nucleon-nucleon potential. These considerations imply possible new opportunities for spin-precession experiments involving for example the deuteron.Portuguese Foundation for Science and Technology (FCT) [SFRH/BPD/101403/2014]; program POPH/FS
The Theory of a Quantum Noncanonical Field in Curved Spacetimes
Much attention has been recently devoted to the possibility that quantum
gravity effects could lead to departures from Special Relativity in the form of
a deformed Poincar\`e algebra. These proposals go generically under the name of
Doubly or Deformed Special Relativity (DSR). In this article we further explore
a recently proposed class of quantum field theories, involving noncanonically
commuting complex scalar fields, which have been shown to entail a DSR-like
symmetry. An open issue for such theories is whether the DSR-like symmetry has
to be taken as a physically relevant symmetry, or if in fact the "true"
symmetries of the theory are just rotations and translations while boost
invariance has to be considered broken. We analyze here this issue by extending
the known results to curved spacetime under both of the previous assumptions.
We show that if the symmetry of the free theory is taken to be a DSR-like
realization of the Poincar\'e symmetry, then it is not possible to render such
a symmetry a gauge symmetry of the curved physical spacetime. However, it is
possible to introduce an auxiliary spacetime which allows to describe the
theory as a standard quantum field theory in curved spacetime. Alternatively,
taking the point of view that the noncanonical commutation of the fields
actually implies a breakdown of boost invariance, the physical spacetime
manifold has to be foliated in surfaces of simultaneity and the field theory
can be coupled to gravity by making use of the ADM prescription.Comment: 9 pages, no figure
About Locality and the Relativity Principle Beyond Special Relativity
Locality of interactions is an essential ingredient of Special Relativity.
Recently, a new framework under the name of relative locality
\cite{AmelinoCamelia:2011bm} has been proposed as a way to consider Planckian
modifications of the relativistic dynamics of particles. We note in this paper
that the loss of absolute locality is a general feature of theories beyond
Special Relativity with an implementation of a relativity principle. We give an
explicit construction of such an implementation and compare it both with the
previously mentioned framework of relative locality and the so-called Doubly
Special Relativity theories.Comment: 10 pages, no figure
Study of time lags in HETE-2 Gamma-Ray Bursts with redshift: search for astrophysical effects and Quantum Gravity signature
The study of time lags between spikes in Gamma-Ray Bursts light curves in
different energy bands as a function of redshift may lead to the detection of
effects due to Quantum Gravity. We present an analysis of 15 Gamma-Ray Bursts
with measured redshift, detected by the HETE-2 mission between 2001 and 2006 in
order to measure time lags related to astrophysical effects and search for
Quantum Gravity signature in the framework of an extra-dimension string model.
The use of photon-tagged data allows us to consider various energy ranges.
Systematic effects due to selection and cuts are evaluated. No significant
Quantum Gravity effect is detected from the study of the maxima of the light
curves and a lower limit at 95% Confidence Level on the Quantum Gravity scale
parameter of 3.2x10**15 GeV is set.Comment: 4 pages, 5 figures. v3: Error corrected in Eq. 1. Results updated.
Proceedings of the 30th ICRC, Merida, Mexico (2007
Detecting Vanishing Dimensions Via Primordial Gravitational Wave Astronomy
Lower-dimensionality at higher energies has manifold theoretical advantages
as recently pointed out. Moreover, it appears that experimental evidence may
already exists for it - a statistically significant planar alignment of events
with energies higher than TeV has been observed in some earlier cosmic ray
experiments. We propose a robust and independent test for this new paradigm.
Since (2+1)-dimensional spacetimes have no gravitational degrees of freedom,
gravity waves cannot be produced in that epoch. This places a universal maximum
frequency at which primordial waves can propagate, marked by the transition
between dimensions. We show that this cut-off frequency may be accessible to
future gravitational wave detectors such as LISA.Comment: Somewhat expanded version with discussions that could not fit into
the PRL version; references adde
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