measured by the Chandra X-Ray Observatory, and the rotation speed of hydrogen gas disks surrounding galaxies measured by ground-based radio telescopes (Figure 2.12). With improved observations, astronomers have determined precisely how much dark matter there is, and learned that it interacts only with itself and very feebly with familiar matter only through gravity. These normal-matter constituents are small islands in a vast sea of dark matter of some unknown form.

An important clue to the nature of dark matter comes from indirect but powerful arguments based on the formation of the elements and the formation of galaxies. It has been found that only one-sixth of the total matter is in normal “baryonic” form and that the remainder is probably some exotic new elementary particle generated in copious quantities in the big bang but not yet detected by Earth-based particle accelerator experiments. If so, elucidating the nature and properties of the dark matter particle (or particles) will open an entirely new window to our understanding of the fundamental properties of matter.

The hunt for dark matter is the joint domain of elementary particle physics, astrophysics, and astronomy. Circumstances in all arenas are ripe for the detection of dark matter in the coming decade. Some of the most promising candidate dark matter particles predicted by theorists have properties that imply they will be produced anew in experiments at the Large Hadron Collider (LHC), while relic copies from the early universe will be detected at high energy from their self-interactions or decay in space, producing gamma rays and other high-energy particles, and at low energy in experiments at deep-underground laboratories where rare collisions occur between normal atoms and the sea of galactic dark matter particles through which Earth swims. Already, important constraints have been set on the nature of dark matter through the failure to detect it using underground detectors and the Fermi Gamma-ray Space Telescope. This is a great period of interdisciplinary convergence in the quest to understand the nature of dark matter.

The Nature of Neutrinos

Neutrinos (a type of elementary particle) interact very weakly with other matter. Because of this property, even massive bodies such as stars are transparent to neutrinos. The detection of neutrinos produced in the center of the Sun provided a direct confirmation of the nuclear reactions occurring there, and the ~20 neutrinos detected from a supernova explosion in a nearby galaxy in 1987 confirmed that the core of this massive star had collapsed to densities comparable to that of an atomic nucleus (likely forming a neutron star). More remarkably, over the past decade, observations of neutrinos produced by cosmic rays striking Earth’s atmosphere, and more refined detections of solar neutrinos, demonstrated that the three known types of neutrinos can oscillate from one type to another. This discovery implies that the neutrino mass, though small, is non-zero and offers direct proof that the



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