Counterpart to a particle with properties identical to those of the particle except that the antiparticle’s electrical charge and a few other properties are opposite those of the particle. When a particle and its antiparticle meet, they can annihilate each other.


a unit of angle corresponding to 1/3600th of a degree. An arcsecond is approximately the size of a dime viewed from the distance of 1 mile.


a hypothetical elementary particle whose existence might explain certain particle physics experiments; a candidate for cold dark matter.

B meson:

meson that contains one b quark and one u, d, or s antiquark.

B factory:

specialized accelerator facility that produces large numbers of B mesons.


a massive, strongly interacting elementary particle, such as a proton or a neutron. Ordinary matter as we know it consists largely of baryons.

big bang:

the theory that the universe began with all matter and energy concentrated to very high density and temperature some 13 billion years ago. The present universe expanded from that epoch and is still expanding.

big bang nucleosynthesis:

the process by which during the first 3 minutes after the big bang, protons and neutrons fused together to form the nuclei of the lightest elements in the periodic table, hydrogen, deuterium, helium, and lithium. The relative abundance of these elements, particularly deuterium, is sensitive to the density of ordinary matter and provides the first method for estimating the amount of baryonic matter.

binary companion:

a star that is gravitationally bound to another star. Bi-nary companions orbit around their common center of gravity. A high proportion, perhaps one-half, of all stars in the Milky Way galaxy are binaries or members of more complex multiple systems.

binary pulsar:

a radio pulsar (q.v.) that is gravitationally bound to a companion star and orbits it. The signals from such a system can be used to test some aspects of general relativity to great precision.

black hole:

a region of space where the gravitational pull is so strong that, classically, nothing can escape. The boundary of this region is called the black hole’s event horizon (q.v.). Black holes can form when a massive star undergoes gravitational collapse (q.v.).


the Balloon Observation Of Millimeteric Extragalactic Radiation and Geophysics (BOOMERanG) maps the cosmic microwave background (CMB) using a balloon-borne telescope that circumnavigates Antarctica. The data gathered are analyzed to create images of the early universe, test models of cosmology, and measure funda-

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