General relativity also predicts the existence of gravitational waves, which travel at the speed of light. Our understanding of gravity waves has improved recently to include solving the relevant equations when gravity is strong, thanks to theoretical breakthroughs in numerical relativity. Astrophysicists now have the ability, in principle, to calculate the complete waveforms that should be observed from most types of powerful sources. To date, the effects of gravitational radiation have been observed only indirectly using sensitive measurements of spinning magnetized neutron stars, or pulsars, when they have orbiting stellar companions. These measurements are consistent with the theory, but the goal of detecting gravitational waves directly has not yet been met.

The first of these ripples in space-time likely to be detected will probably arise from the death spiral of a binary neutron star. Sustained international investments over the last 20 years would culminate with the mid-decade completion of the advanced Laser Interferometer Gravitational Wave Observatory (LIGO), which should make regular detections of this and many other types of sources at relatively short wavelengths.

However, the ultimate goal is to measure the full gravitational waveform for direct comparison with theoretical expectations. To accomplish this, measurements are needed at longer wavelengths to test the theory by means of sustained observations of merging black holes. This is the primary purpose of LISA, from which the signals will be of such high quality that the full gravitational waveform can be measured. A key recent development has been the solution of the theoretical problem of calculating the signals that should be seen from merging black holes. The results will test current understanding of general relativity and provide accurate measurements of the spin and mass of the merging black holes. These are vital parameters for understanding the origins and growth of the most massive black holes in the universe. We should also witness the capture of stars by massive black holes with signals of such long duration and fidelity that the space-time of the black hole can be directly mapped.

In summary, this survey recommends supplementing the current ability to use the universe as a giant cosmic laboratory to study dark energy, inflation, and black holes. Success in this endeavor would provide critical constraints on the laws of physics and the behavior of the universe that would inform efforts to realize a unification of gravity and quantum mechanics through string theory or other approaches; see Box 7.3.

THE LARGER SCIENCE PROGRAM

The three primary science objectives played a large role in motivating the difficult prioritization choices the committee had to make. They represent goals against which progress and prospects for individual facilities can be assessed over



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