239 research outputs found
Theoretical Aspects of the Equivalence Principle
We review several theoretical aspects of the Equivalence Principle (EP). We
emphasize the unsatisfactory fact that the EP maintains the absolute character
of the coupling constants of physics while General Relativity, and its
generalizations (Kaluza-Klein,..., String Theory), suggest that all absolute
structures should be replaced by dynamical entities. We discuss the
EP-violation phenomenology of dilaton-like models, which is likely to be
dominated by the linear superposition of two effects: a signal proportional to
the nuclear Coulomb energy, related to the variation of the fine-structure
constant, and a signal proportional to the surface nuclear binding energy,
related to the variation of the light quark masses. We recall the various
theoretical arguments (including a recently proposed anthropic argument)
suggesting that the EP be violated at a small, but not unmeasurably small
level. This motivates the need for improved tests of the EP. These tests are
probing new territories in physics that are related to deep, and mysterious,
issues in fundamental physics.Comment: 21 pages, no figures; submitted to a "focus issue" of Classical and
Quantum Gravity on Tests of the Weak Equivalence Principle, organized by
Clive Speake and Clifford Wil
Phenomenology of the Equivalence Principle with Light Scalars
Light scalar particles with couplings of sub-gravitational strength, which
can generically be called 'dilatons', can produce violations of the equivalence
principle. However, in order to understand experimental sensitivities one must
know the coupling of these scalars to atomic systems. We report here on a study
of the required couplings. We give a general Lagrangian with five independent
dilaton parameters and calculate the "dilaton charge" of atomic systems for
each of these. Two combinations are particularly important. One is due to the
variations in the nuclear binding energy, with a sensitivity scaling with the
atomic number as . The other is due to electromagnetism. We compare
limits on the dilaton parameters from existing experiments.Comment: 5 page
Is the Bursting Radio-source GCRT J1745-3009 a Double Neutron Star Binary ?
GCRT J1745-3009 is a peculiar transient radio-source in the direction of the
Galactic Center. It was observed to emit a series of ~ 1 Jy bursts at 0.33 GHz,
with typical duration ~ 10 min and at apparently regular intervals of ~ 77 min.
If the source is indeed at the distance of the Galactic Center as it seems
likely, we show that its observational properties are compatible with those
expected from a double neutron star binary, similar to the double pulsar system
J0737-3039. In the picture we propose the (coherent) radio emission comes from
the shock originating in the interaction of the wind of the more energetic
pulsar with the magnetosphere of the companion. The observed modulation of the
radio signal is the consequence of an eccentric orbit, along which the
separation between the two stars varies. This cyclically drives the shock
inside the light cylinder radius of the less energetic pulsar.Comment: 5 pages, 3 figures, accepted for publication in The Astrophysical
Journal Letters, comment on geodetic precession adde
Cosmological Evolution of Brane World Moduli
We study cosmological consequences of non-constant brane world moduli in five
dimensional brane world models with bulk scalars and two boundary branes. We
focus on the case where the brane tension is an exponential function of the
bulk scalar field, . In the limit , the model reduces to the two-brane model of Randall-Sundrum, whereas larger
values of allow for a less warped bulk geometry. Using the moduli
space approximation, we derive the four-dimensional low-energy effective action
from a supergravity-inspired five-dimensional theory. For arbitrary values of
, the resulting theory has the form of a bi-scalar-tensor theory. We
show that, in order to be consistent with local gravitational observations,
has to be small (less than ) and the separation of the branes
must be large. We study the cosmological evolution of the interbrane distance
and the bulk scalar field for different matter contents on each branes. Our
findings indicate that attractor solutions exist which drive the moduli fields
towards values consistent with observations. The efficiency of the attractor
mechanism crucially depends on the matter content on each branes. In the
five-dimensional description, the attractors correspond to the motion of the
negative tension brane towards a bulk singularity, which signals the eventual
breakdown of the four-dimensional description and the necessity of a better
understanding of the bulk singularity.Comment: 18 pages, 10 figures, typos and factor of 2 corrected, version to
appear in Physical Review
Models of quintessence coupled to the electromagnetic field and the cosmological evolution of alpha
We study the change of the effective fine structure constant in the
cosmological models of a scalar field with a non-vanishing coupling to the
electromagnetic field. Combining cosmological data and terrestrial observations
we place empirical constraints on the size of the possible coupling and explore
a large class of models that exhibit tracking behavior. The change of the fine
structure constant implied by the quasar absorption spectra together with the
requirement of tracking behavior impose a lower bound of the size of this
coupling. Furthermore, the transition to the quintessence regime implies a
narrow window for this coupling around in units of the inverse Planck
mass. We also propose a non-minimal coupling between electromagnetism and
quintessence which has the effect of leading only to changes of alpha
determined from atomic physics phenomena, but leaving no observable
consequences through nuclear physics effects. In doing so we are able to
reconcile the claimed cosmological evidence for a changing fine structure
constant with the tight constraints emerging from the Oklo natural nuclear
reactor.Comment: 13 pages, 10 figures, RevTex, new references adde
SL(2,C) Gravity with Complex Vierbein and Its Noncommutative Extension
We show that it is possible to formulate gravity with a complex vierbein
based on SL(2,C) gauge invariance. The proposed action is a four-form where the
metric is not introduced but results as a function of the complex vierbein.
This formulation is based on the first order formalism. The novel feature here
is that integration of the spin-connection gauge field gives rise to kinetic
terms for a massless graviton, a massive graviton with the Fierz-Pauli mass
term, and a scalar field. The resulting theory is equivalent to bigravity. We
then show that by extending the gauge group to GL(2,C} the formalism can be
easily generalized to apply to a noncommutative space with the star product. We
give the deformed action and derive the Seiberg-Witten map for the complex
vierbein and gauge fields.Comment: Minor corrections. The noncommutative action in section 3 is
simplified. Version to appear in Physical Review
Gravitational dipole radiations from binary systems
We investigate the possibility of generating sizeable dipole radiations in
relativistic theories of gravity. Optimal parameters to observe their effects
through the orbital period decay of binary star systems are discussed.
Constraints on gravitational couplings beyond general relativity are derived.Comment: One comment added, accepted for publication in Phys. Rev.
Gauge Coupling Variation in Brane Models
We consider the space-time variation of gauge couplings in brane-world models
induced by the coupling to a bulk scalar field. A variation is generated by the
running of the gauge couplings with energy and a conformal anomaly while going
from the Jordan to the Einstein frame. We indicate that the one-loop
corrections cancel implying that one obtains a variation of the fine structure
constant by either directly coupling the gauge fields to the bulk scalar field
or having bulk scalar field dependent Yukawa couplings. Taking into account the
cosmological dynamics of the bulk scalar field, we constrain the strength of
the gauge coupling dependence on the bulk scalar field and relate it to
modifications of gravity at low energy.Comment: 4 pages, 1 figur
Effective field theory analysis of the self-interacting chameleon
We analyse the phenomenology of a self-interacting scalar field in the
context of the chameleon scenario originally proposed by Khoury and Weltman. In
the absence of self-interactions, this type of scalar field can mediate long
range interactions and simultaneously evade constraints from violation of the
weak equivalence principle. By applying to such a scalar field the effective
field theory method proposed for Einstein gravity by Goldberger and Rothstein,
we give a thorough perturbative evaluation of the importance of non-derivative
self-interactions in determining the strength of the chameleon mediated force
in the case of orbital motion. The self-interactions are potentially dangerous
as they can change the long range behaviour of the field. Nevertheless, we show
that they do not lead to any dramatic phenomenological consequence with respect
to the linear case and solar system constraints are fulfilled.Comment: 15 pages, 2 figures. Final version accepted for publication on
General Relativity and Gravitatio
Measurement of Relativistic Orbital Decay in the PSR B1534+12 Binary System
We have made timing observations of binary pulsar PSR B1534+12 with radio
telescopes at Arecibo, Green Bank, and Jodrell Bank. By combining our new
observations with data collected up to seven years earlier, we obtain a
significantly improved solution for the astrometric, spin, and orbital
parameters of the system. For the first time in any binary pulsar system, no
fewer than five relativistic or "post-Keplerian" orbital parameters are
measurable with useful accuracies in a theory-independent way. We find the
orbital period of the system to be decreasing at a rate close to that expected
from gravitational radiation damping, according to general relativity, although
the precision of this test is limited to about 15% by the otherwise poorly
known distance to the pulsar. The remaining post-Keplerian parameters are all
consistent with one another and all but one of them have fractional accuracies
better than 1%. By assuming that general relativity is the correct theory of
gravity, at least to the accuracy demanded by this experiment, we find the
masses of the pulsar and companion star each to be 1.339+-0.003 Msun and the
system's distance to be d = 1.1+-0.2 kpc, marginally larger than the d ~ 0.7
kpc estimated from the dispersion measure. The increased distance reduces
estimates of the projected rate of coalescence of double neutron-star systems
in the universe, a quantity of considerable interest for experiments with
terrestrial gravitational wave detectors such as LIGO.Comment: 17 pages, 4 figures, submitted to the Ap
- …