2,628 research outputs found
Remarks on the consistency of minimal deviations from General Relativity
We study the consequences of the modification of the phase space structure of
General Relativity imposed by breaking the full diffeomorphism invariance but
retaining the time foliation preserving diffeomorphisms. We examine the
different sectors in phase space that satisfy the new structure of constraints.
For some sectors we find an infinite tower of constraints. In spite of that, we
also show that these sectors allow for solutions, among them some well known
families of black hole and cosmologies which fulfill all the constraints. We
raise some physical concerns on the consequences of an absolute Galilean time,
on the thermodynamical pathologies of such models and on their unusual vacuum
structure.Comment: latex 28 pages, 1 figure. Added comments and a reference. Text
improved
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
Development of an internal restraint system for an integrated restraint-pressure suit system Report, 7 Jun. 1965 - 28 Jun. 1966
Internal restraint system, composed of liquid filled garment and separate auxiliary system, for integrated restraint pressure suit for acceleration protection and thermal transpor
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
Strong field effects on binary systems in Einstein-aether theory
"Einstein-aether" theory is a generally covariant theory of gravity
containing a dynamical preferred frame. This article continues an examination
of effects on the motion of binary pulsar systems in this theory, by
incorporating effects due to strong fields in the vicinity of neutron star
pulsars. These effects are included through an effective approach, by treating
the compact bodies as point particles with nonstandard, velocity dependent
interactions parametrized by dimensionless "sensitivities". Effective
post-Newtonian equations of motion for the bodies and the radiation damping
rate are determined. More work is needed to calculate values of the
sensitivities for a given fluid source, so precise constraints on the theory's
coupling constants cannot yet be stated. It is shown, however, that strong
field effects will be negligible given current observational uncertainties if
the dimensionless couplings are less than roughly 0.01 and two conditions that
match the PPN parameters to those of pure general relativity are imposed. In
this case, weak field results suffice and imply one further condition on the
couplings. Thus, there exists a one-parameter family of Einstein-aether
theories with "small-enough" couplings that passes all current observational
tests. No conclusion can yet be reached for large couplings.Comment: 23 pages, 1 figure; v2: fixed error in Eqn. (70) and resulting bounds
on c'
Constraints on Lorentz Invariance Violation using INTEGRAL/IBIS observations of GRB041219A
One of the experimental tests of Lorentz invariance violation is to measure
the helicity dependence of the propagation velocity of photons originating in
distant cosmological obejcts. Using a recent determination of the distance of
the Gamma-Ray Burst GRB 041219A, for which a high degree of polarization is
observed in the prompt emission, we are able to improve by 4 orders of
magnitude the existing constraint on Lorentz invariance violation, arising from
the phenomenon of vacuum birefringence.Comment: 5 pages, 3 figures, accepted for publication as a Rapid Communication
in Physical Review
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