fall under the category of laboratory astrophysics.) Traditional areas include, for example, laboratory measurements of atomic cross sections and rates. The panel notes that the experimental techniques of particle astrophysics and gravitation are used in some measurements critical to the interpretation of experiments and observations under consideration by this panel and by other panels. Measurements of nuclear cross sections are needed to inform studies of nucleosynthesis and stellar evolution. Laboratory experiments in plasma and fluid physics are needed to inform quantitative modeling of astronomical systems in, for example, the areas of fluid and magnetohydrodynamic turbulence, high-energy particle acceleration in turbulent plasmas, magnetic reconnection, and collisionless shocks. When assessing their programs the funding agencies must include consideration of the indirect yet important benefit of such experiments.

RECOMMENDATIONS

The first detection of gravitational waves is likely to take place in the next decade with ground-based detectors. Substantial progress in testing general relativity and studying astrophysical sources requires exploiting the lower-frequency part of the spectrum, which can be done only from space. The potential scientific benefit is enormous, because quantitative strong-field tests of general relativity will be possible for the first time, and a qualitatively new window for studying astrophysical systems at a broad range of redshifts will be opened. A great deal of technical progress has been made in the past decade, and a successful LISA pathfinder will eliminate much of the remaining risk. The panel recommends that the LISA mission be given the highest priority for a new start in the next decade, given the extensive technology development that has already been completed, the expected short time until the LISA Pathfinder (LPF) mission launch, and the need to maintain momentum in the U.S. community and guarantee a smooth transition to a joint NASA-ESA mission. The panel recommends that NASA funding of LISA begin immediately, with continuation beyond LPF contingent on the success of that mission.

LISA will not be sensitive to the lowest frequencies of gravitational waves that are predicted. This portion of the spectrum probes the fundamental question, How did the universe begin? It might also provide signals from merging supermassive black holes. Pulsar timing is a promising technique for detecting very-low-frequency gravitational waves, and the panel recommends that NSF provide support for a coherent program in gravitational-wave detection through timing of millisecond pulsars.

Although much progress has been made in the past decade in testing general relativity in the weak-field limit and on scales of the solar system, little has been done to test strong-field general relativity and gravitation on large (cosmological) scales. The discovery that the universe is apparently accelerating may be a manifes-



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