Session R10: Gravitational Wave Detection

Minimizing the Mechanical Loss in Fused Silica and Lowering the Thermal Noise in Advanced LIGO

We have measured the mechanical loss in fused silica from samples spanning a wide range of geometries and resonant frequencies in order to model the known variation of the loss with frequency and surface-to-volume ratio. Our model matches the data well and agrees with earlier work on the frequency dependence of the loss. This improved understanding of the mechanical loss has contributed significantly to the design of advanced interferometric gravitational wave detectors, which require ultra-low loss materials for their test mass mirrors. Fused silica has been chosen as the test mass material for Advanced LIGO. Noise estimates for Advanced LIGO show the mirror substrate thermal noise should be well below the quantum noise of the laser. However thermal noise in the mirror coatings is predicted to contribute significantly to the total noise budget in the central frequency region of 30-500 Hz. Work on reducing the mirror coating thermal noise is ongoing.   

Acceleration Noise Measurements for LISA

The close spacing between the proof mass and the housing in the LISA (Laser Interferometer Space Antenna) spacecraft has been a concern as there may be spurious feeble forces. Such forces may limit the performance of the gravity wave detector at frequencies below 3 mHz and must be studied experimentally. We are performing ultra sensitive torsion balance tests to investigate such effects. Our torsion pendulum and a nearby plate are designed to simulate the LISA proof mass with its adjacent housing surface. We study torque noise on the pendulum as a function of separation between the surfaces. In order to exceed the LISA requirement we are probing the acceleration noise at much closer separations, than those planned for LISA. We have taken data at separations as small as 0.15 mm.   

Observing Massive Black Hole Binary Coalescences with LISA

Massive black hole binary coalescences are among the most important astrophysical sources of gravitational waves to be observed by LISA. The ability to observe and characterize such sources with masses $\sim 10^5 M_\odot$ and larger at high redshifts is strongly dependent on the sensitivity of LISA in the low frequency (0.1 mHz and below) regime. We examine LISA's ability to observe these systems at redshifts up to $z \sim 10$ for various proposed values of the low frequency sensitivity, under current assumptions about the merger rates. The discussion will focus on the astrophysical information that can be gained by these observations.   

Ridgelines and Catastrophes: Analysis of LISA signals reveals how source parameter estimates sharpen non-linearly as observed signal duration increases

We have used a grid search technique to analyze simulated LISA gravitational wave signals to extract source parameters. We present parameter space graphs that illustrate the highly non-Gaussian nature of the probability distributions, and the high degree of correlation among various parameters. These correlations show up as ``ridgelines'' in the multidimensional parameter space. These graphs also illustrate the highly nonlinear rate with which the accuracy of the parameters extracted increases as a function of the duration of signal observed. The accuracy levels show plateaus followed by sudden jumps, which mathematicians call ``catastrophes.'' These extracted parameters can be used to perform the Ryan* test of the black hole uniqueness theorem. Results obtained to date support estimates that the Ryan test may be performed to an accuracy of better than 10{\%} if favorable cases of extreme-mass-ratio inspirals are observed for periods exceeding one year. Analysis of simulated LIGO cases suggests much less precise results for parameter extraction and much weaker limits on black hole non-uniqueness. \newline \newline * Ryan, F. D., \textit{Phys. Rev. D} \textbf{52}, 5707 (1995).