nor have the applicable physical principles in these environments been elucidated. Observing the polarization of x rays from pulsars, magnetars, and perhaps gamma-ray bursts would allow just this.

How Were the Elements from Iron to Uranium Made?

The production of the light elements in the big bang and of the elements up to iron in supernovae is in quantitative agreement with observation. Beyond iron, the general conditions needed to make the elements seem clear, but the locale and means of production are unknown. Supernovae or neutron stars are thought to be likely sites for the origin of the heavy elements. By combining full three-dimensional calculations of supernova explosion in a terascale computation, experimental measurements of neutrino-oscillation physics, experimental data on the r-process and rp-process nuclei far from stability, and x-ray and gamma-ray observations of newly formed elements in supernovae, it may be possible to pin down the source of the heaviest elements.

How Do Cosmic Accelerators Work and What Are They Accelerating?

On both spectral and astrophysical grounds, it seems that ultrahigh-energy protons are formed in extremely powerful yet local sources. Perhaps these sources have already been identified with active galaxies or gamma-ray bursts. Alternatively, a completely new constituent of the universe could be involved, like a topological defect associated with the physics of grand unification. Only by observing many more of these particles, or perhaps the associated gamma rays, neutrinos, and gravitational waves, will scientists be able to distinguish these possibilities. To realize this opportunity, large cosmic-ray air shower detector arrays and observations of high-energy gamma rays and neutrinos will be needed, as described in Box 6.1.

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