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Prospects for a space-based gravitational-wave observatory

General relativity can be summarized by John Wheeler’s nowfamous dictum: “Spacetime tells matter how to move, matter tells spacetime how to curve.”1 Within this framework, gravitational waves (GWs) are the messengers that tell distant observers about changes in local mass distributions. They will soon be discovered by ground-based observatories such as Advanced LIGO2 (Laser Interferometer Gravitational-wave Observatory) and Advanced Virgo3 in the 10Hz to 10kHz frequency range, and by pulsar timing arrays4 in the nHz to Hz range. However, the signal-rich mHz range can only be explored by proposed space-based GW observatories (SGOs) like the well-studied Laser Interferometer Space Antenna (LISA). LISA was a highly ranked joint NASA/ESA (European Space Agency) mission that was cancelled in 2011 due to budgetary constraints. Since then, eLISA5 in Europe and SGO-Mid6 in the US have replaced LISA as notional mission designs for future SGOs. These LISA-like missions would trace the history of massive black holes out to large redshifts (>20), take a census of compact galactic binaries, and probe general relativity in the ultra-relativistic regime (see Figure 1).7 The basic principle for measuring GWs dates back to Felix Pirani, who proved that GWs change distances between freely falling test masses (TMs).8 This simple idea is the foundation of all modern interferometric GW detectors. The amplitude, h, of a typical GW from a binary system can be estimated from the four length scales involved: the Schwarzschild radii (R1 and R2) of the two orbiting objects, the distance D between the objects, and the distance r to the observer, yielding h D R1R2=Dr . Expected strain amplitudes range from 10 14 in the nHz range to 10 23 in the kHz range. A LISA-like mission would place three spacecraft in a triangular constellation into a heliocentric orbit (see Figure 1). Free-falling TMs inside each spacecraft would define the endpoints of the interferometer arms, which would each be a Figure 1. Left: Artist’s view (not to scale) of LISA (Laser Interferometer Space Antenna) in its heliocentric orbit, trailing Earth (blue orbit) by 20. Right: The LISA strain (h) sensitivity curve and depictions of three typical LISA sources drawn in relevant regions of the graph: massive black-hole mergers (top sequence), compact galactic binaries (middle), and extreme-mass-ratio inspirals (bottom). AU: Astronomical unit. (Image credit: JPL/NASA.)