144 research outputs found
Comparison of LISA and Atom Interferometry for Gravitational Wave Astronomy in Space
One of the atom interferometer gravitational wave missions proposed by
Dimopoulos et al.1 in 2008 was called AGIS-Sat. 2. It had a suggested
gravitational wave sensitivity set by the atom state detection shot noise level
that started at 1 mHz, was comparable to LISA sensitivity from 1 to about 20
mHz, and had better sensitivity from 20 to 500 mHz. The separation between the
spacecraft was 1,000 km, with atom interferometers 200 m long and shades from
sunlight used at each end. A careful analysis of many error sources was
included, but requirements on the time-stability of both the laser wavefront
aberrations and the atom temperatures in the atom clouds were not investigated.
After including these considerations, the laser wavefront aberration stability
requirement to meet the quoted sensitivity level is about 1\times10-8
wavelengths, and is far tighter than for LISA. Also, the temperature
fluctuations between atom clouds have to be less than 1 pK. An alternate atom
interferometer GW mission in Earth orbit called AGIS-LEO with 30 km satellite
separation has been suggested recently. The reduction of wavefront aberration
noise by sending the laser beam through a high-finesse mode-scrubbing optical
cavity is discussed briefly, but the requirements on such a cavity are not
given. Unfortunately, such an Earth-orbiting mission seems to be considerably
more difficult to design than a non-geocentric mission and does not appear to
have comparably attractive scientific goals.Comment: Submitted to Proc. 46th Rencontres de Moriond: Gravitational Waves
and Experimental Gravity, March 20 - 27, 2011, La Thuile, Ital
Noise characterization for LISA
We consider the general problem of estimating the inflight LISA noise power
spectra and cross-spectra, which are needed for detecting and estimating the
gravitational wave signals present in the LISA data. For the LISA baseline
design and in the long wavelength limit, we bound the error on all spectrum
estimators that rely on the use of the fully symmetric Sagnac combination
(). This procedure avoids biases in the estimation that would otherwise
be introduced by the presence of a strong galactic background in the LISA data.
We specialize our discussion to the detection and study of the galactic white
dwarf-white dwarf binary stochastic signal.Comment: 9 figure
Effects of finite arm-length of LISA on analysis of gravitational waves from MBH binaries
Response of an interferometer becomes complicated for gravitational wave
shorter than the arm-length of the detector, as nature of wave appears
strongly. We have studied how parameter estimation for merging massive black
hole binaries are affected by this complicated effect in the case of LISA. It
is shown that three dimensional positions of some binaries might be determined
much better than the past estimations that use the long wave approximation. For
equal mass binaries this improvement is most prominent at \sim 10^5\sol.Comment: 10 pages, 3 figures, to appear in Phys.Rev.
Algebraic approach to time-delay data analysis for LISA
Cancellation of laser frequency noise in interferometers is crucial for
attaining the requisite sensitivity of the triangular 3-spacecraft LISA
configuration. Raw laser noise is several orders of magnitude above the other
noises and thus it is essential to bring it down to the level of other noises
such as shot, acceleration, etc. Since it is impossible to maintain equal
distances between spacecrafts, laser noise cancellation must be achieved by
appropriately combining the six beams with appropriate time-delays. It has been
shown in several recent papers that such combinations are possible. In this
paper, we present a rigorous and systematic formalism based on algebraic
geometrical methods involving computational commutative algebra, which
generates in principle {\it all} the data combinations cancelling the laser
frequency noise. The relevant data combinations form the first module of
syzygies, as it is called in the literature of algebraic geometry. The module
is over a polynomial ring in three variables, the three variables corresponding
to the three time-delays around the LISA triangle. Specifically, we list
several sets of generators for the module whose linear combinations with
polynomial coefficients generate the entire module. We find that this formalism
can also be extended in a straight forward way to cancel Doppler shifts due to
optical bench motions. The two modules are infact isomorphic.
We use our formalism to obtain the transfer functions for the six beams and
for the generators. We specifically investigate monochromatic gravitational
wave sources in the LISA band and carry out the maximisiation over linear
combinations of the generators of the signal-to-noise ratios with the frequency
and source direction angles as parameters.Comment: 27 Pages, 6 figure
Annual modulation of the Galactic binary confusion noise bakground and LISA data analysis
We study the anisotropies of the Galactic confusion noise background and its
effects on LISA data analysis. LISA has two data streams of the gravitational
waves signals relevant for low frequency regime. Due to the anisotropies of the
background, the matrix for their confusion noises has off-diagonal components
and depends strongly on the orientation of the detector plane. We find that the
sky-averaged confusion noise level could change by a factor of 2
in three months, and would be minimum when the orbital position of LISA is
either around the spring or autumn equinox.Comment: 13 pages, 6 figure
Testing Scalar-Tensor Gravity Using Space Gravitational-Wave Interferometers
We calculate the bounds which could be placed on scalar-tensor theories of
gravity of the Jordan, Fierz, Brans and Dicke type by measurements of
gravitational waveforms from neutron stars (NS) spiralling into massive black
holes (MBH) using LISA, the proposed space laser interferometric observatory.
Such observations may yield significantly more stringent bounds on the
Brans-Dicke coupling parameter \omega than are achievable from solar system or
binary pulsar measurements. For NS-MBH inspirals, dipole gravitational
radiation modifies the inspiral and generates an additional contribution to the
phase evolution of the emitted gravitational waveform. Bounds on \omega can
therefore be found by using the technique of matched filtering. We compute the
Fisher information matrix for a waveform accurate to second post-Newtonian
order, including the effect of dipole radiation, filtered using a currently
modeled noise curve for LISA, and determine the bounds on \omega for several
different NS-MBH canonical systems. For example, observations of a 1.4 solar
mass NS inspiralling to a 1000 solar mass MBH with a signal-to-noise ratio of
10 could yield a bound of \omega > 240,000, substantially greater than the
current experimental bound of \omega > 3000.Comment: 18 pages, 4 figures, 1 table; to be submitted to Phys. Rev.
On the topological classification of binary trees using the Horton-Strahler index
The Horton-Strahler (HS) index has been shown to
be relevant to a number of physical (such at diffusion limited aggregation)
geological (river networks), biological (pulmonary arteries, blood vessels,
various species of trees) and computational (use of registers) applications.
Here we revisit the enumeration problem of the HS index on the rooted,
unlabeled, plane binary set of trees, and enumerate the same index on the
ambilateral set of rooted, plane binary set of trees of leaves. The
ambilateral set is a set of trees whose elements cannot be obtained from each
other via an arbitrary number of reflections with respect to vertical axes
passing through any of the nodes on the tree. For the unlabeled set we give an
alternate derivation to the existing exact solution. Extending this technique
for the ambilateral set, which is described by an infinite series of non-linear
functional equations, we are able to give a double-exponentially converging
approximant to the generating functions in a neighborhood of their convergence
circle, and derive an explicit asymptotic form for the number of such trees.Comment: 14 pages, 7 embedded postscript figures, some minor changes and typos
correcte
Non-adiabatic quantum effects from a Standard Model time-dependent Higgs vev
We consider the time-dependence of the Higgs vacuum expectation value (vev)
given by the dynamics of the Standard Model and study the non-adiabatic
production of both bosons and fermions, which is intrinsically
non-perturbative. In the Hartree approximation, we analyse the general
expressions that describe the dissipative dynamics due to the back-reaction of
the produced particles. In particular, we solve numerically some relevant cases
for the Standard Model phenomenology in the regime of relatively small
oscillations of the Higgs vev.Comment: 35 pages, 15 figures and 2 table
Orbital Tests of Relativistic Gravity using Artificial Satellites
We reexamine non-Einsteinian effects observable in the orbital motion of
low-orbit artificial Earth satellites. The motivations for doing so are
twofold: (i) recent theoretical studies suggest that the correct theory of
gravity might contain a scalar contribution which has been reduced to a small
value by the effect of the cosmological expansion; (ii) presently developed
space technologies should soon give access to a new generation of satellites
endowed with drag-free systems and tracked in three dimensions at the
centimeter level. Our analysis suggests that such data could measure two
independent combinations of the Eddington parameters (beta - 1) and (gamma - 1)
at the 10^-4 level and probe the time variability of Newton's "constant" at the
d(ln G)/dt ~ 10^-13 yr^-1 level. These tests would provide well-needed
complements to the results of the Lunar Laser Ranging experiment, and of the
presently planned experiments aiming at measuring (gamma -1). In view of the
strong demands they make on the level of non- gravitational perturbations,
these tests might require a dedicated mission consisting of an optimized
passive drag-free satellite.Comment: 17 pages, IHES/P/94/22 and CPT-94/P.E.302
Correlations in Scale-Free Networks: Tomography and Percolation
We discuss three related models of scale-free networks with the same degree
distribution but different correlation properties. Starting from the
Barabasi-Albert construction based on growth and preferential attachment we
discuss two other networks emerging when randomizing it with respect to links
or nodes. We point out that the Barabasi-Albert model displays dissortative
behavior with respect to the nodes' degrees, while the node-randomized network
shows assortative mixing. These kinds of correlations are visualized by
discussig the shell structure of the networks around their arbitrary node. In
spite of different correlation behavior, all three constructions exhibit
similar percolation properties.Comment: 6 pages, 2 figures; added reference
- …