1,920 research outputs found
Nucleosynthesis Constraints on Scalar-Tensor Theories of Gravity
We study the cosmological evolution of massless single-field scalar-tensor
theories of gravitation from the time before the onset of annihilation
and nucleosynthesis up to the present. The cosmological evolution together with
the observational bounds on the abundances of the lightest elements (those
mostly produced in the early universe) place constraints on the coefficients of
the Taylor series expansion of , which specifies the coupling of the
scalar field to matter and is the only free function in the theory. In the case
when has a minimum (i.e., when the theory evolves towards general
relativity) these constraints translate into a stronger limit on the
Post-Newtonian parameters and than any other observational
test. Moreover, our bounds imply that, even at the epoch of annihilation and
nucleosynthesis, the evolution of the universe must be very close to that
predicted by general relativity if we do not want to over- or underproduce
He. Thus the amount of scalar field contribution to gravity is very small
even at such an early epoch.Comment: 15 pages, 2 figures, ReVTeX 3.1, submitted to Phys. Rev. D1
On the Energy-Momentum Tensor of the Scalar Field in Scalar--Tensor Theories of Gravity
We study the dynamical description of gravity, the appropriate definition of
the scalar field energy-momentum tensor, and the interrelation between them in
scalar-tensor theories of gravity. We show that the quantity which one would
naively identify as the energy-momentum tensor of the scalar field is not
appropriate because it is spoiled by a part of the dynamical description of
gravity. A new connection can be defined in terms of which the full dynamical
description of gravity is explicit, and the correct scalar field
energy-momentum tensor can be immediately identified. Certain inequalities must
be imposed on the two free functions (the coupling function and the potential)
that define a particular scalar-tensor theory, to ensure that the scalar field
energy density never becomes negative. The correct dynamical description leads
naturally to the Einstein frame formulation of scalar-tensor gravity which is
also studied in detail.Comment: Submitted to Phys. Rev D15, 10 pages. Uses ReVTeX macro
Phasing of gravitational waves from inspiralling eccentric binaries
We provide a method for analytically constructing high-accuracy templates for
the gravitational wave signals emitted by compact binaries moving in
inspiralling eccentric orbits. By contrast to the simpler problem of modeling
the gravitational wave signals emitted by inspiralling {\it circular} orbits,
which contain only two different time scales, namely those associated with the
orbital motion and the radiation reaction, the case of {\it inspiralling
eccentric} orbits involves {\it three different time scales}: orbital period,
periastron precession and radiation-reaction time scales. By using an improved
`method of variation of constants', we show how to combine these three time
scales, without making the usual approximation of treating the radiative time
scale as an adiabatic process. We explicitly implement our method at the 2.5PN
post-Newtonian accuracy. Our final results can be viewed as computing new
`post-adiabatic' short period contributions to the orbital phasing, or
equivalently, new short-period contributions to the gravitational wave
polarizations, , that should be explicitly added to the
`post-Newtonian' expansion for , if one treats radiative effects
on the orbital phasing of the latter in the usual adiabatic approximation. Our
results should be of importance both for the LIGO/VIRGO/GEO network of ground
based interferometric gravitational wave detectors (especially if Kozai
oscillations turn out to be significant in globular cluster triplets), and for
the future space-based interferometer LISA.Comment: 49 pages, 6 figures, high quality figures upon reques
Singularity Free (Homogeneous Isotropic) Universe in Graviton-Dilaton Models
We present a class of graviton-dilaton models in which a homogeneous
isotropic universe, such as our observed one, evolves with no singularity at
any time. Such models may stand on their own as interesting models for
singularity free cosmology, and may be studied further accordingly. They may
also arise from string theory. We discuss critically a few such possibilities.Comment: 11 pages. Latex file. Revised in response to referees' Comments.
Results remain same. To appear in Phys. Rev. Let
Complete model of a spherical gravitational wave detector with capacitive transducers. Calibration and sensitivity optimization
We report the results of a detailed numerical analysis of a real resonant
spherical gravitational wave antenna operating with six resonant two-mode
capacitive transducers read out by superconducting quantum interference devices
(SQUID) amplifiers. We derive a set of equations to describe the
electro-mechanical dynamics of the detector. The model takes into account the
effect of all the noise sources present in each transducer chain: the thermal
noise associated with the mechanical resonators, the thermal noise from the
superconducting impedance matching transformer, the back-action noise and the
additive current noise of the SQUID amplifier. Asymmetries in the detector
signal-to-noise ratio and bandwidth, coming from considering the transducers
not as point-like objects but as sensor with physically defined geometry and
dimension, are also investigated. We calculate the sensitivity for an
ultracryogenic, 30 ton, 2 meter in diameter, spherical detector with optimal
and non-optimal impedance matching of the electrical read-out scheme to the
mechanical modes. The results of the analysis is useful not only to optimize
existing smaller mass spherical detector like MiniGrail, in Leiden, but also as
a technological guideline for future massive detectors. Furthermore we
calculate the antenna patterns when the sphere operates with one, three and six
resonators. The sky coverage for two detectors based in The Netherlands and
Brasil and operating in coincidence is also estimated. Finally, we describe and
numerically verify a calibration and filtering procedure useful for diagnostic
and detection purposes in analogy with existing resonant bar detectors.Comment: 23 pages, 20 figures, codes of the simulations are available on
request by contacting the autho
Anatomy and origin of authochthonous late Pleistocene forced regression deposits, east Coromandel inner shelf, New Zealand: implications for the development and definition of the regressive systems tract
High-resolution seismic reflection data from the east Coromandel coast, New Zealand, provide details of the sequence stratigraphy beneath an autochthonous, wave dominated inner shelf margin during the late Quaternary (0-140 ka). Since c. 1 Ma, the shelf has experienced limited subsidence and fluvial sediment input, producing a depositional regime characterised by extensive reworking of coastal and shelf sediments during glacio-eustatic sea-level fluctuations. It appears that only one complete fifth-order (c. 100 000 yr) depositional sequence is preserved beneath the inner shelf, the late Pleistocene Waihi Sequence, suggesting any earlier Quaternary sequences were mainly cannibalised into successively younger sequences. The predominantly Holocene-age Whangamata Sequence is also evident in seismic data and modern coastal deposits, and represents an incomplete depositional sequence in its early stages of formation. A prominent aspect of the sequence stratigraphy off parts of the east Coromandel coast is the presence of forced regressive deposits (FRDs) within the regressive systems tract (RST) of the late Pleistocene Waihi Sequence. The FRDs are interpreted to represent regressive barrier-shoreface sands that were sourced from erosion and onshore reworking of underlying Pleistocene sediments during the period of slow falling sea level from isotope stages 5 to 2 (c. 112-18 ka). The RST is volumetrically the most significant depositional component of the Waihi Sequence; the regressive deposits form a 15-20 m thick, sharp-based, tabular seismic unit that downsteps and progrades continuously across the inner shelf. The sequence boundary for the Waihi Sequence is placed at the most prominent, regionally correlative, and chronostratigraphically significant surface, namely an erosional unconformity characterised in many areas by large incised valleys that was generated above the RST. This unconformity is interpreted as a surface of maximum subaerial erosion generated during the last glacial lowstand (c. 18 ka). Although the base of the RST is associated with a prominent regressive surface of erosion, this is not used as the sequence boundary as it is highly diachronous and difficult to identify and correlate where FRDs are not developed. The previous highstand deposits are limited to subaerial barrier deposits preserved behind several modern Holocene barriers along the coast, while the transgressive systems tract is preserved locally as incised-valley fill deposits beneath the regressive surface of erosion at the base of the RST. Many documented late Pleistocene RSTs have been actively sourced from fluvial systems feeding the shelf and building basinward-thickening, often stacked wedges of FRDs, for which the name allochthonous FRDs is suggested. The Waihi Sequence RST is unusual in that it appears to have been sourced predominantly from reworking of underlying shelf sediments, and thus represents an autochthonous FRD. Autochthonous FRDs are also present on the Forster-Tuncurry shelf in southeast Australia, and may be a common feature in other shelf settings with low subsidence and low sediment supply rates, provided shelf gradients are not too steep, and an underlying source of unconsolidated shelf sediments is available to source FRDs. The preservation potential of such autochthonous FRDs in ancient deposits is probably low given that they are likely to be cannibalised during subsequent sea-level falls
Contrasting carbonate depositional systems for Pliocene cool-water limestones cropping out in central Hawke's Bay, New Zealand
Pliocene limestone formations in central Hawke's Bay (eastern North Island, New Zealand) accumulated on and near the margins of a narrow forearc basin seaway within the convergent Australia/Pacific plate boundary zone. The active tectonic setting and varied paleogeographic features of the limestone units investigated, in association with probable glacioeustatic sea-level fluctuations, resulted in complex stratigraphic architectures and contrasting types of carbonate accumulation on either side of the seaway. Here, we recognise recurring patterns of sedimentary facies, and sequences and systems tracts bounded by key physical surfaces within the limestone sheets. The facies types range from Bioclastic (B) to Siliciclastic (S) end-members via Mixed (M) carbonate-siliciclastic deposits. Skeletal components are typical cool-water associations dominated by epifaunal calcitic bivalves, bryozoans, and especially barnacles. Siliciclastic contents vary from one formation to another, and highlight siliciclastic-rich limestone units in the western ranges versus siliciclastic-poor limestone units in the eastern coastal hills. Heterogeneities in facies types, stratal patterns, and also in diagenetic pathways between eastern and western limestone units are considered to originate in the coeval occurrence in different parts of the forearc basin of two main morphodynamic carbonate systems over time
Testing Theories of Gravity with a Spherical Gravitational Wave Detector
We consider the possibility of discriminating different theories of gravity
using a recently proposed gravitational wave detector of spherical shape. We
argue that the spin content of different theories can be extracted relating the
measurements of the excited spheroidal vibrational eigenmodes to the
Newman-Penrose parameters. The sphere toroidal modes cannot be excited by any
metric GW and can be thus used as a veto.Comment: latex file, 16 pages, 1 figur
Limits on Active-Sterile Neutrino Mixing and the Primordial Deuterium Abundance
Studies of limits on active-sterile neutrino mixing derived from big bang
nucleosynthesis considerations are extended to consider the dependance of these
constraints on the primordial deuterium abundance. This study is motivated by
recent measurements of D/H in quasar absorption systems, which at present yield
discordant results. Limits on active-sterile mixing are somewhat relaxed for
high D/H. For low D/H (), no active-sterile neutrino
mixing is allowed by currently popular upper limits on the primordial He
abundance . For such low primordial D/H values, the observational inference
of active-sterile neutrino mixing by upcoming solar neutrino experiments would
imply that has been systematically underestimated, unless there is new
physics not included in standard BBN.Comment: 10 pages + 2 figures, uses revtex macros, submitted to Phys. Rev. D.
Corrected figure captions and an added referenc
Quintessence, the Gravitational Constant, and Gravity
Dynamical vacuum energy or quintessence, a slowly varying and spatially
inhomogeneous component of the energy density with negative pressure, is
currently consistent with the observational data. One potential difficulty with
the idea of quintessence is that couplings to ordinary matter should be
strongly suppressed so as not to lead to observable time variations of the
constants of nature. We further explore the possibility of an explicit coupling
between the quintessence field and the curvature. Since such a scalar field
gives rise to another gravity force of long range (\simg H^{-1}_0), the solar
system experiments put a constraint on the non-minimal coupling: |\xi| \siml
10^{-2}.Comment: 9 pages, a version to be published in Phys.Rev.
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