Gravitational Waves

This article reviews current efforts and plans for gravitational-wave detection, the gravitational-wave sources that might be detected, and the information that the detectors might extract from the observed waves. Special attention is paid to (i) the LIGO/VIRGO network of earth-based, kilometer-scale laser interferometers, which is now under construction and will operate in the high-frequency band ($1$ to $10^4$ Hz), and (ii) a proposed 5-million-kilometer-long Laser Interferometer Space Antenna (LISA), which would fly in heliocentric orbit and operate in the low-frequency band ($10^{-4}$ to $1$ Hz). LISA would extend the LIGO/VIRGO studies of stellar-mass ($M\sim2$ to $300 M_\odot$) black holes into the domain of the massive black holes ($M\sim1000$ to $10^8M_\odot$) that inhabit galactic nuclei and quasars.Comment: Latex; 25 pages, 14 figures. Figures are in eps files that are bundled together in a tarred, compressed, and uuencoded form; figures are inserted into text via a "special" command rather than psfig or epsf. Uses a style file "snow.sty" that is bundled with the figure

An Overview of Gravitational-Wave Sources

We review current best estimates of the strength and detectability of the gravitational waves from a variety of sources, for both ground-based and space-based detectors, and we describe the information carried by the waves.Comment: 40 pages, 5 figures, to appear in Proceedings of GR16 (Durban, South Africa, 2001

Human gravity-gradient noise in interferometric gravitational-wave detectors

Among all forms of routine human activity, the one which produces the strongest gravity-gradient noise in interferometric gravitational-wave detectors (e.g. LIGO) is the beginning and end of weight transfer from one foot to the other during walking. The beginning and end of weight transfer entail sharp changes (time scale τ∼20 msec) in the horizontal jerk (first time derivative of acceleration) of a person’s center of mass. These jerk pairs, occurring about twice per second, will produce gravity-gradient noise in LIGO in the frequency band 2.5 Hz≲f≲1/(2τ)≃25 Hz with the form sqrt[Sh(f)]∼0.6×10-23 Hz-1/2(f/10 Hz)-6[∑i(ri/10 m)-6]1/2. Here the sum is over all the walking people, ri is the distance of the i’th person from the nearest interferometer test mass, and we estimate this formula to be accurate to within a factor 3. To ensure that this noise is negligible in advanced LIGO interferometers, people should be prevented from coming nearer to the test masses than r≃10 m. A r≃10 m exclusion zone will also reduce to an acceptable level gravity gradient noise from the slamming of a door and the striking of a fist against a wall. The dominant gravity-gradient noise from automobiles and other vehicles is probably that from decelerating to rest. To keep this below the sensitivity of advanced LIGO interferometers will require keeping vehicles at least 30 m from all test masses

A New Family of Light Beams and Mirror Shapes for Future LIGO Interferometers

Advanced LIGO's present baseline design uses arm cavities with Gaussian light beams supported by spherical mirrors. Because Gaussian beams have large intensity gradients in regions of high intensity, they average poorly over fluctuating bumps and valleys on the mirror surfaces, caused by random thermal fluctuations (thermoelastic noise). Flat-topped light beams (mesa beams) are being considered as an alternative because they average over the thermoelastic fluctuations much more effectively. However, the proposed mesa beams are supported by nearly flat mirrors, which experience a very serious tilt instability. In this paper we propose an alternative configuration in which mesa-shaped beams are supported by nearly concentric spheres, which experience only a weak tilt instability. The tilt instability is analyzed for these mirrors in a companion paper by Savov and Vyatchanin. We also propose a one-parameter family of light beams and mirrors in which, as the parameter alpha varies continuously from 0 to pi, the beams and supporting mirrors get deformed continuously from the nearly flat-mirrored mesa configuration ("FM") at alpha=0, to the nearly concentric-mirrored mesa configuration ("CM") at alpha=pi. The FM and CM configurations at the endpoints are close to optically unstable, and as alpha moves away from 0 or pi, the optical stability improves.Comment: Submitted to Physical Review D on 21 September 2004; RevTeX, 6 pages, 4 Figure