Precise simulations for future gravitational wave detectors

Abstract

The emerging science of gravitational wave astronomy is optimistically named. Astronomy depends ultimately on observations, yet the only output of gravitational wave detectors has so far been noise generated within the instruments. There is good reason, based on experimental and theoretical progress, to believe that things are about to change. As an example of progress on the theoretical side, Kenta Kiuchi of Waseda University, Yuichiro Sekiguchi of the National Astronomical Observatory, Masaru Shibata of Kyoto University (all in Japan), and Keisuke Taniguchi of the University of Wisconsin, US, report in Physical Review Letters simulations of neutron star mergers that reveal new details of the gravitational waves they are expected to emit [1]. The effort to detect gravitational waves started humbly fifty years ago with Joe Weber’s bar detectors [2]. Today the field is a thriving example of Big Science, including large facilities [3] in the US (LIGO) and Italy (VIRGO), smaller installations in Germany (GEO 600) and Japan (TAMA, LCGT), and potential future detectors in Australia (AIGO) and India (INDIGO). LIGO, the best funded and so far the most sensitive of these instruments, is preparing a major upgrade called Advanced LIGO. In parallel with the development of ground-based detectors, there has been substantial design progress for detectors in space. The principal example is LISA [4], which received effusive endorsement from the National Academy of Sciences: “LISA is an extraordinarily original and technically bold mission concept. The first direct detection of low-frequency gravitational waves will be a momentous discovery, of the kind that wins Nobel Prizes” [5]. Space-based detectors will not likely be making that low-frequency (< 0.1 Hz) discovery for another ten years at least—not for lack of inherent sensitivity or progress in technology development, but rather because rapid deployment is not a characteristic of billion-dollar space research missions. Meantime, the effort to improve ground-based detectors, which operate at higher frequency (10 to 10, 000 Hz), is proceeding apace. What is the motivation compelling some of us to spend entire careers building telescopes that have yet to see the gravitational equivalent of first light? The thrall of zero is one conceivable answer. That is, the absence of signals at predicted levels places real constraints on astrophysical phenomena, and ultimately could test Einstein’s theory of general relativity. But most of us would trade a thick stack of publications with “search for” in the title for a single thin “discovery of.” It’s not about zero. Rather, the case that LIGO and VIRGO are almost good enough to see signals stands up to scrutiny. This is not simple optimism: the detection prediction is derived from a synthesis of electromagnetic astronomy and astrophysical models of sources that seem inevitable. For many years the favored source of gravitational waves for ground-based detectors has been the inspiral of a compact binary system consisting of one neutron star plus a companion that is either another neutron star or a black hole (Fig. 1, top). The orbital motion generates gravitational radiation at a frequency that chirps as the orbit decays and speeds up. The chirp waveform can be calculated accurately from a handful of parameters such as the masses and spins of the two stars and the inclination angle of the orbital plane. This waveform parameterization allows for coherent integration of the last minute or so of the life of the binary system, when the gravitational wave signal is strongest. In the 1987 proposal to the National Science Foundation that first described the LIGO concept [6], Kip Thorne had this to say about detection prospects in general: “The most certain of the sources is coalescence of neutron-star binaries: Estimates based on pulsar statistics in our own galaxy suggest that to see 3 such events