standard model of particle physics is incomplete. Indeed, astrophysical research has provided much of the evidence for physics beyond the standard model.

Our ability to probe the fundamental properties of neutrinos by using astro-physical measurements will continue in the coming decade. The neutrino oscillation measurements of the past decade probed only the difference in the squares of the neutrino masses, not the absolute masses, and we currently have only upper limits on the actual masses. Neutrinos were produced in abundance in the big bang, and although they constitute only a minor component of the dark matter, they affected the clustering of matter on large scales in a way that depends on their mass. Thus, the determination of the masses of the neutrinos—fundamental input to theories of the very small—may come from observations of the very large. In the coming decade, precise measurements of the structure seen in the CMB combined with measurements of large-scale structure from the next generation of visible/infrared imaging and spectroscopic surveys plus X-ray observations of clusters of galaxies will allow us to measure the neutrino mass or push its upper limit downward by an order of magnitude, and thereby help constrain particle physics models governing the behavior of all mass.

The Nature of Compact Objects and Probes of Relativity

Astronomical observations have verified that general relativity provides an accurate description of gravity on solar system scales, but an unanswered question, and the most challenging test of general relativity, is whether it works in the strong gravity fields around black holes. Current studies using X-ray spectroscopy of gas disks around black holes are consistent with the predictions of general relativity and yield preliminary estimates of the black hole spin. Over the next decade the precision of these tests can be dramatically improved.

Also feasible within the decade is the detection of gravitational waves from mergers of million-solar-mass black holes or low-mass objects captured by more massive ones. Such events produce clean signals that can be used to map space-time with tremendous precision in regions where gravity is very strong. An important theoretical and computational breakthrough in this decade was the ability to compute the merger of two black holes, yielding highly accurate predictions of the gravitational wave emission patterns. Combined with detections of these waves, such computations provide stringent tests of the theory of relativity in regimes not accessible by any other means. Deviations from Einstein’s predictions would cause us to rethink one of the foundational pillars of all of physical science.

Gravitational wave detection would not only test general relativity but also measure the spins and masses of the merging black holes. Furthermore, the discovery and understanding of such merging systems would uniquely probe the conditions at the centers of galaxies and the cosmological history of galaxy formation

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