1,859 research outputs found
A Comment on "Brans-Dicke Cosmology with a scalar field potential"
We show that a recent letter claiming to present exact cosmological solutions
in Brans-Dicke theory actually uses a flawed set of equations as the starting
point for their analysis. The results presented in the letter are therefore not
valid.Comment: 2 pages, no figures. To appear in Europhysics Letter
General relativistic treatment of LISA optical links
LISA is a joint space mission of the NASA and the ESA for detecting low
frequency gravitational waves in the band Hz. In order to attain
the requisite sensitivity for LISA, the laser frequency noise must be
suppressed below the other secondary noises such as the optical path noise,
acceleration noise etc. This is achieved by combining time-delayed data for
which precise knowledge of time-delays is required. The gravitational field,
mainly that of the Sun and the motion of LISA affect the time-delays and the
optical links. Further, the effect of the gravitational field of the Earth on
the orbits of spacecraft is included. This leads to additional flexing over and
above that of the Sun. We have written a numerical code which computes the
optical links, that is, the time-delays with great accuracy
metres - more than what is required for time delay interferometry (TDI) - for
most of the orbit and with sufficient accuracy within metres for an
integrated time window of about six days, when one of the arms tends to be
tangent to the orbit. Our analysis of the optical links is fully general
relativistic and the numerical code takes into account effects such as the
Sagnac, Shapiro delay, etc.. We show that with the deemed parameters in the
design of LISA, there are symmetries inherent in the configuration of LISA and
in the physics, which may be used effectively to suppress the residual laser
noise in the modified first generation TDI. We demonstrate our results for some
important TDI variables
Temperature effects on the universal equation of state of solids
Recently it has been argued based on theoretical calculations and experimental data that there is a universal form for the equation of state of solids. This observation was restricted to the range of temperatures and pressures such that there are no phase transitions. The use of this universal relation to estimate pressure-volume relations (i.e., isotherms) required three input parameters at each fixed temperature. It is shown that for many solids the input data needed to predict high temperature thermodynamical properties can be dramatically reduced. In particular, only four numbers are needed: (1) the zero pressure (P=0) isothermal bulk modulus; (2)it P=0 pressure derivative; (3) the P=0 volume; and (4) the P=0 thermal expansion; all evaluated at a single (reference) temperature. Explicit predictions are made for the high temperature isotherms, the thermal expansion as a function of temperature, and the temperature variation of the isothermal bulk modulus and its pressure derivative. These predictions are tested using experimental data for three representative solids: gold, sodium chloride, and xenon. Good agreement between theory and experiment is found
Universality in the compressive behavior of solids
It was discovered that the isothermal equation of state for solids in compression is a simple, universal form. This single form accurately describes the pressure and bulk modulus as a function of volume for tonic, metallic, covalent, and rare gas solids
Radiation Pressure Induced Instabilities in Laser Interferometric Detectors of Gravitational Waves
The large scale interferometric gravitational wave detectors consist of
Fabry-Perot cavities operating at very high powers ranging from tens of kW to
MW for next generations. The high powers may result in several nonlinear
effects which would affect the performance of the detector. In this paper, we
investigate the effects of radiation pressure, which tend to displace the
mirrors from their resonant position resulting in the detuning of the cavity.
We observe a remarkable effect, namely, that the freely hanging mirrors gain
energy continuously and swing with increasing amplitude. It is found that the
`time delay', that is, the time taken for the field to adjust to its
instantaneous equilibrium value, when the mirrors are in motion, is responsible
for this effect. This effect is likely to be important in the optimal operation
of the full-scale interferometers such as VIRGO and LIGO.Comment: 27 pages, 11 figures, RevTex styl
Optimising the directional sensitivity of LISA
It was shown in a previous work that the data combinations canceling laser
frequency noise constitute a module - the module of syzygies. The cancellation
of laser frequency noise is crucial for obtaining the requisite sensitivity for
LISA. In this work we show how the sensitivity of LISA can be optimised for a
monochromatic source - a compact binary - whose direction is known, by using
appropriate data combinations in the module. A stationary source in the
barycentric frame appears to move in the LISA frame and our strategy consists
of "coherently tracking" the source by appropriately "switching" the data
combinations so that they remain optimal at all times. Assuming that the
polarisation of the source is not known, we average the signal over the
polarisations. We find that the best statistic is the `network' statistic, in
which case LISA can be construed of as two independent detectors. We compare
our results with the Michelson combination, which has been used for obtaining
the standard sensitivity curve for LISA, and with the observable obtained by
optimally switching the three Michelson combinations. We find that for sources
lying in the ecliptic plane the improvement in SNR increases from 34% at low
frequencies to nearly 90% at around 20 mHz. Finally we present the
signal-to-noise ratios for some known binaries in our galaxy. We also show
that, if at low frequencies SNRs of both polarisations can be measured, the
inclination angle of the plane of the orbit of the binary can be estimated.Comment: 16 pages, 8 figures, submitted to Phys Rev
A Quantum Analogue of the Algebra
We define a natural quantum analogue for the algebra, and which we
refer to as the algebra, by modding out the Heisenberg algebra
from the quantum affine algebra with level . We discuss
the representation theory of this algebra. In particular, we
exhibit its reduction to a group algebra, and to a tensor product of a group
algebra with a quantum Clifford algebra when , and , and thus, we
recover the explicit constructions of \uq-standard modules as achieved by
Frenkel-Jing and Bernard, respectively. Moreover, for arbitrary nonzero level
, we show that the explicit basis for the simplest -generalized
Verma module as constructed by Lepowsky and primc is also a basis for its
corresponding -module, i.e., it is invariant under the
q-deformation for generic q. We expect this algebra (associated
with \uq at level ), to play the role of a dynamical symmetry in the
off-critical statistical models.Comment: 32 pages, LATEX, minor change
Free-Field Representation of Group Element for Simple Quantum Group
A representation of the group element (also known as ``universal -matrix'') which satisfies , is given in the form where , and and
are the generators of quantum group associated respectively with
Cartan algebra and the {\it simple} roots. The ``free fields'' $\chi,\
\vec\phi,\ \psi\psi^{(s)}\psi^{(s')} =
q^{-\vec\alpha_{i(s)} \vec\alpha_{i(s')}} \psi^{(s')}\psi^{(s)}, &
\chi^{(s)}\chi^{(s')} = q^{-\vec\alpha_{i(s)}\vec\alpha_{i(s')}}
\chi^{(s')}\chi^{(s)}& {\rm for} \ s<s', \\ q^{\vec h\vec\phi}\psi^{(s)} =
q^{\vec h\vec\alpha_{i(s)}} \psi^{(s)}q^{\vec h\vec\phi}, & q^{\vec
h\vec\phi}\chi^{(s)} = q^{\vec h \vec\alpha_{i(s)}}\chi^{(s)}q^{\vec
h\vec\phi}, & \\ &\psi^{(s)} \chi^{(s')} = \chi^{(s')}\psi^{(s)} & {\rm for\
any}\ s,s'.d_Ggg \rightarrow g'\cdot g''{\cal
R}{\cal R} (g\otimes I)(I\otimes g) =
(I\otimes g)(g\otimes I){\cal R}$Comment: 68 page
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