and growth. Black holes are common in the centers of galaxies, and our estimates of their abundance, masses, and merger rate are poised for steady improvement in precision through a space-based interferometer that can reach back in time to “hear” the space-time echoes of mergers of supermassive black holes.

Observations with X-ray telescopes provide a complementary probe of the nature of space-time near the event horizon at the edge of a black hole. Such observations allow us to track the motions of material as it swirls “down the drain,” and thereby to measure the spin of the black hole. This is currently possible only for a handful of nearby black holes, but more powerful facilities in the future would enable us to extend these measurements to large samples. Since any black hole can be fully characterized by its mass and spin, this is fundamental information about how black holes work and how they were formed.

Yet another probe of black holes is the jets that are frequently created by massive spinning black holes in active galactic nuclei. Radio telescopes have shown that the emitting gas travels with speeds close to that of light. X-ray and now gamma-ray telescopes are able to trace the emission down to quite close to the black hole itself. Plasma and magnetohydrodynamic physics, which we understand best from solar and solar system studies, play important roles in many astrophysical contexts. It is proposed to combine the results from many types of telescopes operating simultaneously to understand how jets are made and how they shine. This will then lead to a better understanding of how gravity operates around a black hole. Black holes—either spinning massive holes in active galactic nuclei or newly formed stellar ones in gamma-ray bursts—are also suspected to be the source of the ultrahigh-energy cosmic rays that are detected when they hit Earth’s atmosphere. These can have energies as large as that of a well-hit baseball, but despite the great advances in understanding of their properties that have come from the Auger-South facility in Argentina, we still do not know for sure what they are, how they interact with matter, and how they are made.

Only slightly less remarkable than black holes are the neutron stars. It is with respect to neutron stars that the investments over the last decade in ground-based gravitational wave detectors are likely to pay off first, given that frequent detections of merging neutron stars in other galaxies are expected from Advanced LIGO. Formed as the catastrophic collapse of the core of a dying massive star, these amazing objects contain a mass larger than the Sun’s, squeezed into a region the size of a city. The centers of neutron stars contain the densest matter in the universe, even more tightly compressed than the matter inside the nucleus of a single atom. Some neutron stars also have the largest inferred magnetic field strengths in the universe, a thousand trillion times larger than that of Earth.

Studying the properties of neutron stars offers a unique window into the properties of nuclear matter. Measuring neutron star masses and radii yields direct information about the interior composition that can be compared with theoretical

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