5,058 research outputs found
Aperture synthesis for gravitational-wave data analysis: Deterministic Sources
Gravitational wave detectors now under construction are sensitive to the
phase of the incident gravitational waves. Correspondingly, the signals from
the different detectors can be combined, in the analysis, to simulate a single
detector of greater amplitude and directional sensitivity: in short, aperture
synthesis. Here we consider the problem of aperture synthesis in the special
case of a search for a source whose waveform is known in detail: \textit{e.g.,}
compact binary inspiral. We derive the likelihood function for joint output of
several detectors as a function of the parameters that describe the signal and
find the optimal matched filter for the detection of the known signal. Our
results allow for the presence of noise that is correlated between the several
detectors. While their derivation is specialized to the case of Gaussian noise
we show that the results obtained are, in fact, appropriate in a well-defined,
information-theoretic sense even when the noise is non-Gaussian in character.
The analysis described here stands in distinction to ``coincidence
analyses'', wherein the data from each of several detectors is studied in
isolation to produce a list of candidate events, which are then compared to
search for coincidences that might indicate common origin in a gravitational
wave signal. We compare these two analyses --- optimal filtering and
coincidence --- in a series of numerical examples, showing that the optimal
filtering analysis always yields a greater detection efficiency for given false
alarm rate, even when the detector noise is strongly non-Gaussian.Comment: 39 pages, 4 figures, submitted to Phys. Rev.
Second Order Phase Transitions : From Infinite to Finite Systems
We investigate the Equation of State (EOS) of classical systems having 300
and 512 particles confined in a box with periodic boundary conditions. We show
that such a system, independently on the number of particles investigated, has
a critical density of about 1/3 the ground state density and a critical
temperature of about . The mass distribution at the critical point
exhibits a power law with . Making use of the grand partition
function of Fisher's droplet model, we obtain an analytical EOS around the
critical point in good agreement with the one extracted from the numerical
simulations.Comment: RevTex file, 17 pages + 9 figures available upon request from
[email protected]
How Many Templates for GW Chirp Detection? The Minimal-Match Issue Revisited
In a recent paper dealing with maximum likelihood detection of gravitational
wave chirps from coalescing binaries with unknown parameters we introduced an
accurate representation of the no-signal cumulative distribution of the
supremum of the whole correlator bank. This result can be used to derive a
refined estimate of the number of templates yielding the best tradeoff between
detector's performance (in terms of lost signals among those potentially
detectable) and computational burden.Comment: submitted to Class. Quantum Grav. Typing error in eq. (4.8) fixed;
figure replaced in version
Testing Alternative Theories of Gravity using LISA
We investigate the possible bounds which could be placed on alternative
theories of gravity using gravitational wave detection from inspiralling
compact binaries with the proposed LISA space interferometer. Specifically, we
estimate lower bounds on the coupling parameter \omega of scalar-tensor
theories of the Brans-Dicke type and on the Compton wavelength of the graviton
\lambda_g in hypothetical massive graviton theories. In these theories,
modifications of the gravitational radiation damping formulae or of the
propagation of the waves translate into a change in the phase evolution of the
observed gravitational waveform. We obtain the bounds through the technique of
matched filtering, employing the LISA Sensitivity Curve Generator (SCG),
available online. For a neutron star inspiralling into a 10^3 M_sun black hole
in the Virgo Cluster, in a two-year integration, we find a lower bound \omega >
3 * 10^5. For lower-mass black holes, the bound could be as large as 2 * 10^6.
The bound is independent of LISA arm length, but is inversely proportional to
the LISA position noise error. Lower bounds on the graviton Compton wavelength
ranging from 10^15 km to 5 * 10^16 km can be obtained from one-year
observations of massive binary black hole inspirals at cosmological distances
(3 Gpc), for masses ranging from 10^4 to 10^7 M_sun. For the highest-mass
systems (10^7 M_sun), the bound is proportional to (LISA arm length)^{1/2} and
to (LISA acceleration noise)^{-1/2}. For the others, the bound is independent
of these parameters because of the dominance of white-dwarf confusion noise in
the relevant part of the frequency spectrum. These bounds improve and extend
earlier work which used analytic formulae for the noise curves.Comment: 16 pages, 9 figures, submitted to Classical & Quantum Gravit
Performance of Newtonian filters in detecting gravitational waves from coalescing binaries
Coalescing binary systems are one of the most promising sources of
gravitational waves. The technique of matched filtering used in the detection
of gravitational waves from coalescing binaries relies on the construction of
accurate templates. Until recently filters modelled on the quadrupole or the
Newtonian approximation were deemed sufficient. Recently it was shown that
post-Newtonian effects contribute to a secular growth in the phase difference
between the actual signal and its corresponding Newtonian template. In this
paper we investigate the possibility of compensating for the phase difference
caused by the post-Newtonian terms by allowing for a shift in the Newtonian
filter parameters. We find that Newtonian filters perform adequately for the
purpose of detecting the presence of the signal for both the initial and the
advanced LIGO detectors.Comment: Revtex 9 pages + 6 figures ( Can be obtained by "anonymous" ftp from
144.16.31.1 in dir /pub/rbs. Submitted to Physical Review D. IUCAA 1
Black Hole Spectroscopy: Testing General Relativity through Gravitational Wave Observations
Assuming that general relativity is the correct theory of gravity in the
strong field limit, can gravitational wave observations distinguish between
black hole and other compact object sources? Alternatively, can gravitational
wave observations provide a test of one of the fundamental predictions of
general relativity? Here we describe a definitive test of the hypothesis that
observations of damped, sinusoidal gravitational waves originated from a black
hole or, alternatively, that nature respects the general relativistic no-hair
theorem. For astrophysical black holes, which have a negligible charge-to-mass
ratio, the black hole quasi-normal mode spectrum is characterized entirely by
the black hole mass and angular momentum and is unique to black holes. In a
different theory of gravity, or if the observed radiation arises from a
different source (e.g., a neutron star, strange matter or boson star), the
spectrum will be inconsistent with that predicted for general relativistic
black holes. We give a statistical characterization of the consistency between
the noisy observation and the theoretical predictions of general relativity,
together with a numerical example.Comment: 19 pages, 7 figure
Finite-Range Gravity and Its Role in Gravitational Waves, Black Holes and Cosmology
Theoretical considerations of fundamental physics, as well as certain
cosmological observations, persistently point out to permissibility, and maybe
necessity, of macroscopic modifications of the Einstein general relativity. The
field-theoretical formulation of general relativity helped us to identify the
phenomenological seeds of such modifications. They take place in the form of
very specific mass-terms, which appear in addition to the field-theoretical
analog of the usual Hilbert-Einstein Lagrangian. We interpret the added terms
as masses of the spin-2 and spin-0 gravitons. The arising finite-range gravity
is a fully consistent theory, which smoothly approaches general relativity in
the massless limit, that is, when both masses tend to zero and the range of
gravity tends to infinity. We show that all local weak-field predictions of the
theory are in perfect agreement with the available experimental data. However,
some other conclusions of the non-linear massive theory are in a striking
contrast with those of general relativity. We show in detail how the
arbitrarily small mass-terms eliminate the black hole event horizon and replace
a permanent power-law expansion of a homogeneous isotropic universe with an
oscillatory behaviour. One variant of the theory allows the cosmological scale
factor to exhibit an `accelerated expansion'instead of slowing down to a
regular maximum of expansion. We show in detail why the traditional,
Fierz-Pauli, massive gravity is in conflict not only with the static-field
experiments but also with the available indirect gravitational-wave
observations. At the same time, we demonstrate the incorrectness of the widely
held belief that the non-Fierz-Pauli theories possess `negative energies' and
`instabilities'.Comment: 56 pages including 11 figures; significant modifications; in
particular, we demonstrate the incorrectness of the widely held belief that
the non-Fierz-Pauli theories should suffer from negative energies and
instabilities; to appear in Int. J. Mod. Phys.
Colliding black holes: how far can the close approximation go?
We study the head-on collision of two equal-mass momentarily stationary black
holes, using black hole perturbation theory up to second order. Compared to
first-order results, this significantly improves agreement with numerically
computed waveforms and energy. Much more important, second-order results
correctly indicate the range of validity of perturbation theory. This use of
second-order, to provide ``error bars,'' makes perturbation theory a viable
tool for providing benchmarks for numerical relativity in more generic
collisions and, in some range of collision parameters, for supplying waveform
templates for gravitational wave detection.Comment: 6 pages, RevTeX, 2 figures included with eps
Processing of gas in cosmological filaments around Virgo cluster
Galaxies have different morphology, gas content, and star formation rate
(SFR) in dense environments like galaxy clusters. The impact of environmental
density extends to several virial radii, and galaxies are pre-processed in
filaments and groups, before falling into the cluster. Our goal is to quantify
this pre-processing, in terms of gas content and SFR, as a function of density
in cosmic filaments. We have observed the two first CO transitions in 163
galaxies with the IRAM-30m telescope, and added 82 measurements from the
literature, for a sample of 245 galaxies in the filaments around Virgo. We
gathered HI-21cm measurements from the literature, and observed 69 galaxies
with the Nan\c{c}ay telescope, to complete our sample. We compare our filament
galaxies with comparable samples from the Virgo cluster and with the isolated
galaxies of the AMIGA sample. We find a clear progression from field, to
filament, and cluster galaxies for decreasing SFR, increasing fraction of
galaxies in the quenching phase, increasing proportion of early-type galaxies
and decreasing gas content. Galaxies in the quenching phase, defined as having
SFR below 1/3 of the main sequence rate, are between 0-20\% in the isolated
sample, while they are 20-60\% in the filaments and 30-80\% in the Virgo
cluster. Processes that lead to star formation quenching are already at play in
filaments. They depend mostly on the local galaxy density, while the distance
to filament spine is a secondary parameter. While the HI to stellar mass ratio
decreases with local density by ~1 dex in the filaments, and ~2 dex in the
Virgo cluster with respect to the field, the decrease is much less for the
H to stellar mass ratio. As the environmental density increases, the gas
depletion time decreases, since the gas content decreases faster than the SFR.
This suggests that gas depletion significantly precedes star formation
quenching.Comment: 24 pages, plus 98 pages of supplementary material, submitted to A&
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