13 research outputs found
Brans-Dicke model constrained from Big Bang nucleosynthesis and magnitude redshift relations of Supernovae
The Brans-Dicke model with a variable cosmological term () has
been investigated with use of the coupling constant of .
Parameters inherent in this model are constrained from comparison between Big
Bang nucleosynthesis and the observed abundances. Furthermore, the magnitude
redshift () relations are studied for with and without another
constant cosmological term in a flat universe. Observational data of Type Ia
Supernovae are used in the redshift range of . It is found that our
model with energy density of the constant cosmological term with the value of
0.7 can explain the SNIa observations, though the model parameters are
insensitive to the relation.Comment: Submitted to A&A, 4 pages, 3 figure
Brans-Dicke Theory and primordial black holes in Early Matter-Dominated Era
We show that primordial black holes can be formed in the matter-dominated era
with gravity described by the Brans-Dicke theory. Considering an early
matter-dominated era between inflation and reheating, we found that the
primordial black holes formed during that era evaporate at a quicker than those
of early radiation-dominated era. Thus, in comparison with latter case, less
number of primordial black holes could exist today. Again the constraints on
primordial black hole formation tend towards the larger value than their
radiation-dominated era counterparts indicating a significant enhancement in
the formation of primordial black holes during the matter-dominaed era.Comment: 9 page
Orbital effects of a monochromatic plane gravitational wave with ultra-low frequency incident on a gravitationally bound two-body system
We analytically compute the long-term orbital variations of a test particle
orbiting a central body acted upon by an incident monochromatic plane
gravitational wave. We assume that the characteristic size of the perturbed
two-body system is much smaller than the wavelength of the wave. Moreover, we
also suppose that the wave's frequency is much smaller than the particle's
orbital one. We make neither a priori assumptions about the direction of the
wavevector nor on the orbital geometry of the planet. We find that, while the
semi-major axis is left unaffected, the eccentricity, the inclination, the
longitude of the ascending node, the longitude of pericenter and the mean
anomaly undergo non-vanishing long-term changes. They are not secular trends
because of the slow modulation introduced by the tidal matrix coefficients and
by the orbital elements themselves. They could be useful to indepenedently
constrain the ultra-low frequency waves which may have been indirectly detected
in the BICEP2 experiment. Our calculation holds, in general, for any
gravitationally bound two-body system whose characteristic frequency is much
larger than the frequency of the external wave. It is also valid for a generic
perturbation of tidal type with constant coefficients over timescales of the
order of the orbital period of the perturbed particle.Comment: LaTex2e, 24 pages, no figures, no tables. Changes suggested by the
referees include
Limits on a Stochastic Background of Gravitational Waves from Gravitational Lensing
We compute the effects of a stochastic background of gravitational waves on
multiply imaged systems or on weak lensing. There are two possible observable
effects, a static relative deflection of images or shear, and an induced time
dependent shift or proper motion. We evaluate the rms magnitude of these
effects for a COBE normalized, scale-invariant spectrum, which is an upper
limit on spectra produced by inflation. Previous work has shown that
large-scale structure may cause a relative deflection large enough to affect
observations, but we find that the corresponding effect of gravity waves is
smaller by and so cannot be observed. This results from the
oscillation in time as well as the redshifting of the amplitude of gravity
waves. We estimate the magnitude of the proper motion induced by deflection of
light due to large-scale structure, and find it to be arcsec per
year. This corresponds to km/s at cosmological distances, which is
quite small compared to typical peculiar velocities. The COBE normalized
gravity wave spectrum produces motions smaller still by . We
conclude that light deflection due to these cosmological perturbations cannot
produce observable proper motions of lensed images. On the other hand, there
are only a few known observational limits on a stochastic background of gravity
waves at shorter, astrophysical wavelengths. We calculate the expected
magnitudes of the effects of lensing by gravity waves of such wavelengths, and
find that they are too small to yield interesting limits on the energy density
of gravity waves.Comment: 14 pages, LaTex + 1 PS Figure, accepted version to be published in
Phys. Rev. D15, Dec. 1996. An incorrect assumption was removed, also various
other minor change
OSS (Outer Solar System): A fundamental and planetary physics mission to Neptune, Triton and the Kuiper Belt
The present OSS mission continues a long and bright tradition by associating
the communities of fundamental physics and planetary sciences in a single
mission with ambitious goals in both domains. OSS is an M-class mission to
explore the Neptune system almost half a century after flyby of the Voyager 2
spacecraft. Several discoveries were made by Voyager 2, including the Great
Dark Spot (which has now disappeared) and Triton's geysers. Voyager 2 revealed
the dynamics of Neptune's atmosphere and found four rings and evidence of ring
arcs above Neptune. Benefiting from a greatly improved instrumentation, it will
result in a striking advance in the study of the farthest planet of the Solar
System. Furthermore, OSS will provide a unique opportunity to visit a selected
Kuiper Belt object subsequent to the passage of the Neptunian system. It will
consolidate the hypothesis of the origin of Triton as a KBO captured by
Neptune, and improve our knowledge on the formation of the Solar system. The
probe will embark instruments allowing precise tracking of the probe during
cruise. It allows to perform the best controlled experiment for testing, in
deep space, the General Relativity, on which is based all the models of Solar
system formation. OSS is proposed as an international cooperation between ESA
and NASA, giving the capability for ESA to launch an M-class mission towards
the farthest planet of the Solar system, and to a Kuiper Belt object. The
proposed mission profile would allow to deliver a 500 kg class spacecraft. The
design of the probe is mainly constrained by the deep space gravity test in
order to minimise the perturbation of the accelerometer measurement.Comment: 43 pages, 10 figures, Accepted to Experimental Astronomy, Special
Issue Cosmic Vision. Revision according to reviewers comment
Varying constants, Gravitation and Cosmology
Fundamental constants are a cornerstone of our physical laws. Any constant
varying in space and/or time would reflect the existence of an almost massless
field that couples to matter. This will induce a violation of the universality
of free fall. It is thus of utmost importance for our understanding of gravity
and of the domain of validity of general relativity to test for their
constancy. We thus detail the relations between the constants, the tests of the
local position invariance and of the universality of free fall. We then review
the main experimental and observational constraints that have been obtained
from atomic clocks, the Oklo phenomenon, Solar system observations, meteorites
dating, quasar absorption spectra, stellar physics, pulsar timing, the cosmic
microwave background and big bang nucleosynthesis. At each step we describe the
basics of each system, its dependence with respect to the constants, the known
systematic effects and the most recent constraints that have been obtained. We
then describe the main theoretical frameworks in which the low-energy constants
may actually be varying and we focus on the unification mechanisms and the
relations between the variation of different constants. To finish, we discuss
the more speculative possibility of understanding their numerical values and
the apparent fine-tuning that they confront us with.Comment: 145 pages, 10 figures, Review for Living Reviews in Relativit
Soliton solutions and cosmological gravitational waves
We examine plane-symmetric cosmological solutions to Einstein's equations which can be generated by the "soliton" technique, using the homogeneous Bianchi solutions as seeds and arbitrary numbers of real or complex poles. In some circumstances, these solutions can be interpreted as "incipient" gravitational waves on the Bianchi background. At early times they look like nonlinear inhomogeneities propagating at nearly the speed of light ("gravisolitons"), while at late times they look like cosmological gravitational waves