the most extreme gravitational fields. Supermassive black holes inhabit the centers of galaxies, and they somehow—following the laws of gravity—generate tremendous outflows of energetic particles and radiation, twisting magnetic fields into concentrated pockets of magnetism. Scientists cannot help but strive to understand these extreme environments and to take advantage of them as laboratories to put theories of gravitation to their most demanding tests.

Gravitation is a unifying theme in nearly all of today’s most pressing astrophysics issues. Much of the precursor work of the past decade was motivated by the scientific imperative of understanding gravitation, and an intense period of technology development to build the necessary tools is reaching fruition and must now be exploited. Scientists now have ground-based laser interferometric detectors that are on a path to reaching the level of sensitivity at which the detection of gravitational waves is virtually assured. They have a plan and a design for a network of spacecraft that will measure long-wavelength gravitational waves where astrophysical sources are predicted to be the most abundant. They have developed high-precision techniques for pulsar observations that are promising probes of the gravitational waves associated with inflation and with supermassive black holes. Recognizing these developments, the Panel on Particle Astrophysics and Gravitation presents a program of gravitational-wave astrophysics that will bring the investments in technology to fruition. The panel recommends that the Laser Interferometer Space Antenna (LISA) be given a new start immediately; that ground-based-laser gravitational-wave detectors continue their ongoing program of operation, upgrade, and further operation; and that the detection of gravitational waves through the timing of milli second pulsars move forward. To complement the use of gravitational waves as a beacon for astrophysics and fundamental physics, the panel recommends that the theoretical foundations of gravity themselves be put to stringent test, when such tests can be carried out in a cost-effective manner. These tests of gravitation will be provided by LISA’s observations of strong-field astrophysical systems, by electro magnetic surveys to characterize dark energy (considered by other Astro2010 Program Prioritization Panels), by precise monitoring of the dynamics of the Earth-Moon system, and by controlled tests of gravity theories done in the nearly noise-free environment of space. The time has arrived to explore the still-unknown regions of the universe with the new tool of gravitation.

Understanding the nature of three-quarters of the universe is an important goal, but the other one-quarter, which is known to be some form of matter, must not be overlooked. Scientists have identified one-sixth of this matter: it is in the form of stars, galaxies, and gas that have been studied extensively for centuries. However, the nature of the other five-sixths is still a mystery. Evidence exists that the unknown part is not made up of familiar materials but rather must be a diffuse substance that interacts only weakly with ordinary matter. The leading candidates for this so-called dark matter are new families of particles predicted by some theo-

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