D Glossary and Acronyms


accretion, accretion disk:

the process by which gas flows around and onto a compact gravitating object. Astronomical objects as diverse as protostars and active galaxies may derive their energy from the gravitational power released by the infall, or accretion, of material onto a central object. The combined effects of gravity and rotation often force the accreting material into an orbiting accretion disk.

active galactic nucleus (AGN):

the term active galactic nucleus refers to the existence of energetic phenomena in the nuclei, or central regions, of galaxies that cannot be attributed clearly and directly to stars.

air shower:

when a high-energy particle enters Earth’s atmosphere, the initial particle interacts with air atoms, producing many new particles. In turn, the newly produced particles produce additional particles. Some of the particles penetrate to Earth’s atmosphere. The resulting effect is known as an air shower.

ALICE:

a collaboration that is building a dedicated heavy-ion detector to exploit the unique physics potential of nucleus-nucleus interactions at LHC energies. The aim is to study the physics of strongly interacting matter at extreme energy densities, where the formation of a new phase of matter, the quark-gluon plasma, can occur.

anisotropy:

dependence of the properties of a system on the orientation or direction of observation. The distribution of galaxies in space is not uniform, whereas the intensity of the cosmic background radiation from the big bang is highly uniform in all directions—i.e., it is almost isotropic. Astronomers are using sensitive telescopes to study the small anisotropies in the cosmic background radiation that should be present given the nonuniform distribution of galaxies.

antimatter:

matter composed of antiparticles (e.g., antiprotons, antineutrons, antielectrons) instead of particles (e.g., protons, neutrons, electrons).



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D Glossary and Acronyms accretion, accretion disk: the process by which gas flows around and onto a compact gravitating object. Astronomical objects as diverse as protostars and active galaxies may derive their energy from the gravitational power released by the infall, or accretion, of material onto a central object. The combined effects of gravity and rotation often force the accreting material into an orbiting accretion disk. active galactic nucleus (AGN): the term active galactic nucleus refers to the existence of energetic phenomena in the nuclei, or central regions, of galaxies that cannot be attributed clearly and directly to stars. air shower: when a high-energy particle enters Earth’s atmosphere, the initial particle interacts with air atoms, producing many new particles. In turn, the newly produced particles produce additional particles. Some of the particles penetrate to Earth’s atmosphere. The resulting effect is known as an air shower. ALICE: a collaboration that is building a dedicated heavy-ion detector to exploit the unique physics potential of nucleus-nucleus interactions at LHC energies. The aim is to study the physics of strongly interacting matter at extreme energy densities, where the formation of a new phase of matter, the quark-gluon plasma, can occur. anisotropy: dependence of the properties of a system on the orientation or direction of observation. The distribution of galaxies in space is not uniform, whereas the intensity of the cosmic background radiation from the big bang is highly uniform in all directions—i.e., it is almost isotropic. Astronomers are using sensitive telescopes to study the small anisotropies in the cosmic background radiation that should be present given the nonuniform distribution of galaxies. antimatter: matter composed of antiparticles (e.g., antiprotons, antineutrons, antielectrons) instead of particles (e.g., protons, neutrons, electrons).

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antiparticle: 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. arcsecond: 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. axion: 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. baryon: 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.). BOOMERanG: 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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mental cosmological parameters such as the overall density of the universe. Bose-Einstein particles or bosons: subatomic particle with integral spin (i.e., angular momentum in quantum-mechanical units of 0, 1, etc.). Bosons, unlike fermions, do not obey the exclusion principle. Many bosons can occupy the same quantum state. Photons, gluons, pi-mesons, and nuclei of even mass number are all bosons. CDMS, CDMS II: the Cryogenic Dark Matter Search is a federally funded project involving some 50 scientists from 10 U.S. institutions in the search for the nature of dark matter. Cerenkov radiation: electromagnetic radiation (usually visible light) emitted by a charged particle when it passes through matter at a velocity exceeding that of light in the material. CERN: European Organization for Nuclear Research (originally the European Center for Nuclear Research), located near Geneva, Switzerland. Chandra x-ray satellite observatory (formerly Advanced X-ray Astrophysics Facility): NASA satellite observatory launched on the space shuttle in July 1999. It is imaging the x-ray sky over the energy range of 0.1 to 10 keV. charge-coupled device (CCD): an electronic image detector used in modern video cameras and astronomical instruments. charge-parity (CP) conservation: conservation of properties under a reflection in space and interchange of particle with antiparticle classical: a general term meaning nonquantum mechanical. closed universe: a finite-volume universe resulting from the gravitational pull of a high density of matter. It may be visualized as the three-dimensional analogue of the surface of a sphere—if one travels in any direction, one eventually returns to the same place. cluster of galaxies: a grouping of from tens to thousands of galaxies held together by gravity. COBE: NASA’s COsmic Background Explorer satellite was launched in November 1989; it made precision measurements of the spectrum of the microwave background radiation and discovered tiny variations in its intensity across the sky that arose due to small variations in the density of matter. compact object: compact objects are the remnants of stars that have burned all of their nuclear fuel, forming white dwarfs, neutron stars, or black holes. The extreme gravitational fields near the stars make them valuable as physical laboratories for studying the gravitational force itself.

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compactification: the latest versions of string theory involve an 11-dimensional space. In order for the theory to be consistent with the 4-dimensional space we experience, the extra space dimensions of string theory must curl up into a tiny geometrical space, whose size should be comparable to the string length. This process is known as compactification. condensate: the macroscopic occupation of a single quantum state, as in a low temperature trapped atomic gas of bosons. When used by particle physicists, it describes the lowest energy state of a system when it is not empty, but rather is filled with a particular nonzero value for some field. cosmic microwave background (CMB) radiation: the residual light from the big bang. Although the CMB is nearly uniform, there are tiny fluctuations in its temperature due to variations in the density of the early universe. These tiny fluctuations grew to form galaxies. cosmic rays: protons, nuclei of heavy atoms, and possibly other particles that have been accelerated to high energies by astrophysical processes in the universe and impinge upon Earth. cosmological constant Λ: the energy density associated with the vacuum (empty space). Recent astronomical observations suggest that there is a net energy associated with the vacuum. If there is a positive vacuum energy, then the expansion of the universe will eventually accelerate and our descendants will find themselves in a nearly empty universe. cosmology: the study of the contents, structure, and evolution of the universe from the beginning of time to the infinite future. critical density: the mean density that leads to a spatially uncurved (“flat”) universe; higher density universes are positively curved (like the surface of a sphere) and lower density universes are negatively curved (like the surface of a saddle). cross section: in nuclear or particle physics, the probability that a particular interaction will take place between particles. curvature: the bending or warping of space and time, predicted by general relativity and theories like it. DAMA: the DArk MAtter experiment based at the Gran Sasso laboratory in Italy, which has reported unconfirmed evidence for the signature of dark matter particles in the halo. dark energy: an as-yet-unknown form of energy that pervades the universe and whose gravity is repulsive. Its presence was inferred from the discovery that the expansion of the universe is accelerating, and these observations suggest that about 70 percent of the total density of matter plus energy is in this form. One explanation for dark energy is Einstein’s cosmological constant.

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dark halo: the roughly spherical distribution of dark matter that surrounds a galaxy. dark matter: matter that does not emit enough light or other radiation to be observed directly. Most of the matter in the universe is dark. Cold dark matter is made of particles (e.g., axions or neutralinos) that move slowly compared with the speed of light; hot dark matter is made of particles (e.g., neutrinos) that move at nearly the speed of light. density fluctuations: variations in the density of matter from place to place in the universe. The universe is not uniform. dragging of inertial frames: a general relativistic phenomenon predicted to occur near rotating masses, in which freely falling laboratories would be dragged slightly around the body. One consequence is that a gyroscope in such a laboratory would precess with respect to the direction it would point in empty space. Einstein’s equation: a mathematical equation written down by Einstein in 1915 to describe how matter and energy curve space and time. This curvature accounts for gravity, superseding Newton’s theory of a gravitational force, which remains a good approximation only when gravity is weak. electromagnetic spectrum: total range of wavelengths or frequencies of electromagnetic radiation. Radiation can be represented as electric and magnetic fields vibrating with a characteristic wavelength or frequency. Long wavelengths (low frequencies) correspond to radio radiation; intermediate wavelengths, to millimeter and infrared radiation; short wavelengths (high frequencies), to visible and ultraviolet light; and extremely short wavelengths, to x rays and gamma rays. Most astronomical observations measure some form of electromagnetic radiation. epoch: a period characterized by the dominance of a particular physical process, such as the formation of the light elements from protons and neutrons. epoch of photon decoupling: see epoch of recombination. epoch of recombination: the time when electrons and nuclei were combining to form atoms and the universe was 1,100 times smaller than its present size; also called the epoch of photon decoupling and the epoch of atom formation. equation of state: the equation that describes how the pressure and density of a substance are related. Stars remain in equilibrium by balancing the inward pull of gravity against the outward pressure force, so the equation of state must be known to construct theoretical models of stars.

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equivalence principle: a fundamental principle of general relativity, one of whose consequences is that all objects (and light) fall in a gravitational field in the same way independent of their internal structure or other properties. This universality of free fall is one of the most accurately verified principles in physics. eV: an electron-volt, a measure of energy equal to that gained by an electron passing through a potential difference of 1 volt; also a unit of particle mass when divided by the speed of light (c) squared. Electrons have a mass of about 0.511 MeV/c2 (million electron-volts); protons have a mass of about 938 GeV/c2 (billion electron-volts). event horizon: the surface of a black hole. It is a one-way membrane, allowing matter or signals to flow in but not out. Fermilab: The U.S. Department of Energy’s Fermi National Accelerator Laboratory, located in Batavia, Illinois. fermion: particles with the property that only one can occupy a quantum state (the Pauli exclusion principle). Such particles have half-integer values of spin. flat universe: a universe where space is uncurved and described by the geometry of Euclid. forces of nature: the four basic forces of physics: gravity, electromagnetism, and the weak and strong interactions. freeze-out: the disequilibrium by which relics are formed in the universe. galaxy: a large assemblage of stars. Our own galaxy, the Milky Way, contains 1011 stars. gamma ray: electromagnetic radiation more energetic than x rays. gamma-ray bursts: bursts of gamma rays from cosmic sources observed by detectors on satellites. Several hundred are detected per year, and they range in duration from fractions of a second to several seconds. Most gamma ray bursts come from objects at cosmological distances. general relativity: Einstein’s theory of gravity in which the gravity is the curved geometry of space and time. gluon: a massless particle that carries the strong force. grand unification era: the era when the universe cooled sufficiently for gravity to be described by Einstein’s general relativity theory, but where the temperature was still sufficiently high that the other remaining three forces of nature remained unified. grand unified theories: theories that combine the strong, electromagnetic, and weak interactions into one unified theory. gravitational collapse instability: the process whereby a small lump in an expanding universe can grow under gravity, pulling in surrounding

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matter and ultimately collapsing to form an object like a galaxy or cluster of galaxies. gravitational lens: an object in which rays of light from a distant astronomical source are deflected by the gravitational pull of an intermediate mass that may be a galaxy or a cluster of galaxies. The deflection causes a distortion in the image of the distant source and sometimes also leads to multiple images. gravitational lensing: a consequence of Einstein’s general relativity theory is that the path of light rays can be bent by the presence of matter. Astronomers have observed that the light from a distant galaxy or quasar can be “lensed” by the matter in an intervening galaxy to form multiple and often distorted images of the background object. gravitational wave: according to general relativity, a ripple in the geometry of space-time propagating as a wave. gravitational wave background: gravitational waves arriving from so many sources that the individual signals are indistinguishable. graviton: an as-yet-undetected massless particle that carries the gravitational force. Great Attractor: a region or structure of huge mass (equivalent to tens of thousands of galaxies) exerting a gravitational pull on the surrounding galaxies, including the Milky Way, proposed to explain the observed movement of these galaxies toward the Hydra-Centaurus superclusters in the southern sky with velocities significantly different from those predicted by Hubble law expansion. hadron: a strongly interacting particle such as a proton or neutron. halo: the matter surrounding a galaxy. Hawking radiation: when the effects of quantum mechanics are included in the analysis of black holes, it turns out that they are not, strictly speaking, black but rather radiate energy. This phenomenon is called Hawking radiation. horizon: edge of the portion of the universe visible to us. Light signals from beyond this point have not had time to reach Earth yet. HST: NASA’s Hubble Space Telescope, an optical/infrared telescope launched in 1990. Hubble’s law: the principle that any two distant celestial objects (e.g., galaxies) move away from each other at a speed that is proportional to the distance between them, due to the homogenous expansion of space. hyperon: a subatomic particle that is a quasi-stable member of the class of particles known as baryons and that is more massive than the nucleons (protons and neutrons).

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inflationary universe, inflationary paradigm: an extension of the big bang model characterized by a tremendous burst of expansions. The underlying cause of inflation is not known, though there are many models for it based upon particle physics. infrared: a region of the electromagnetic spectrum with wavelengths longer than visible light. Hot objects typically are very bright at infrared wavelengths. interferometer, interferometry: interferometer can be used on a single telescope to break up the light into its constituent colors. intergalactic medium: the material between galaxies. inverse square law: an interaction that becomes weaker as the inverse square of the distance between objects. ionized atom: an atom with an excess or deficit of electrons and thus with a net charge. Under terrestrial conditions, most matter has an equal amount of positive and negative charge, so that its net charge is zero. IRAS: NASA, British, and Dutch Infrared Astronomy Satellite, which was flown in 1983. isotope: two or more atoms of the same element that have the same number of protons in their nucleus but different numbers of neutrons are known as isotopes. Hydrogen, deuterium and tritium are isotopes of hydrogen. Most elements in nature consists of a mixture of isotopes. jet: a set of particles produced from the vacuum state by the movement of quarks and gluons with high momentum found in electron-positron annihilation. The energy associated with the quarks and/or gluons ultimately manifests itself in streams of elementary particles which can be detected. jet, astrophysical: stream of fast-moving material flowing outward from an object such as a young star or a massive central black hole in a galaxy. K meson or kaon: second least massive meson, made of one s quark and one u or d antiquark. Keck telescopes: the two largest ground-based (10-meter) optical telescopes, located on Mauna Kea, Hawaii. Kerr metric solutions: the Kerr metric describes space-time around a spinning mass. laser interferometer: a device that uses laser light to make accurate comparisons of the lengths of two perpendicular paths. Lense-Thirring effect: synonymous with dragging of inertial frames (q.v.). The effect is named after Josef Lense and Hans Thirring, Austrian physicists who first calculated the general relativistic predictions for dragging in 1918.

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LEP: the Large Electron-positron Project. A particle accelerator at CERN (q.v.). leptons: a class of elementary particles including electrons, muons, and tauons. LHC: Large Hadron Collider, a large, high-energy particle accelerator project under construction at CERN. It hopes to expose the Higgs particle and/ or supersymmetric partner particles. LIGO: the Laser Interferometer Gravitational-Wave Observatory is an NSF-sponsored project to build and operate two 4-kilometer laser interferometers (q.v.) to detect gravitational waves. local supercluster: a large flattened structure centered on the Virgo cluster, of which the Milky Way is a member. magnetars: neutron stars with the largest known magnetic fields in the universe. MAP satellite: NASA’s Microwave Anisotropy Probe, launched in June 2001, designed to accurately map the microwave sky with an angular resolution of 0.2 degrees. At MAP’s frequencies (22 to 96 GHz), most of the fluctuations in the microwave sky are due to variations in the cosmic microwave background (q.v.). matter-antimatter symmetry: when enough energy is concentrated to produce particles in an experiment, equal numbers of matter and antimatter (q.v.) particles are produced. When the antiparticles meet their matter counterparts, they disappear, returning to pure energy. Nothing is added. Nothing is lost. However, some subtle experiments have revealed that this symmetry is not perfect and that there exists a slight bias in matter’s favor. This bias translates to just a single proton surviving out of every billion that could have emerged from the big bang. It is from this one in a billion that the universe is made. MAXIMA: the Millimeter Anisotropy eXperiment Imaging Array (MAXIMA) is a balloon-borne millimeter-wave telescope designed to measure the angular power spectrum of fluctuations in the cosmic microwave background (q.v.) over a wide range of angular scales. Such measurements provide a powerful probe of the early universe. microlensing: if a small, dark body is directly in the line of sight to a bright background star, the brightness of the background star may appear to increase because of bending of the light rays by the dark body. neutralino: neutral particles with a spin of one-half, predicted by supersymmetry as counterparts to the photon, the W boson, and the neutral Higgs boson. neutrino: very light (possibly massless) particle emitted in the process of radioactive decay. There are three species, associated with electrons,

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muons, and tau-leptons. They interact with ordinary matter through the weak force. neutrino oscillation: a process whereby neutrinos of one type may be able to change into those of another type and back again if one or more of the types have mass. neutron star: a star at such a high density and pressure that its atoms have been completely crushed until the nuclei merge and most of the electrons have been squeezed onto the protons, forming neutron-rich material. Newton’s law: Newton’s law of gravity, which states that falling and orbiting of a mass in the vicinity of another mass are caused by an attractive force along a line joining them. This theory is the limit of general relativity when speeds are much less than the speed of light and gravitational fields are weak. nuclear density: the density at which neutrons and protons are packed together inside the nucleus of an atom. nucleon: neutron or proton. nucleosynthesis: the process by which the elements are built up from protons and neutrons. open universe: a negatively curved universe with mean density less than the critical density. Pauli exclusion principle: The quantum-mechanical principle, applying to fermions but not to bosons, that no two identical particles in a system, such as electrons in an atom or quarks in a hadron, can possess an identical set of quantum numbers. The origin of the Pauli exclusion principle lies in the spin-statistics theorem of relativistic quantum field theory. phase transition: a change in a feature that characterizes a system. Examples of phase transitions are changes from solid to liquid, liquid to gas, and the reverse changes. Phase transitions can occur by altering such variables as temperature and pressure. photon: quantum of electromagnetic energy; a unique massless particle that carries the electromagnetic force. Planck satellite: European Space Agency’s Planck satellite, scheduled for launch in 2007. It will measure the microwave sky over a wide range of wavelengths (22 to 900 GHz) with an angular resolution of 0.1 degrees. Planck scale: a scale related to the unique length that can be constructed from Newton’s gravitational constant, the velocity of light, and the quantum of action and that characterizes quantum-mechanical phe-

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nomena. Its value is 10−33 centimeters. There is a corresponding Planck energy (1019 GeV) and Planck time (10−43 seconds). plasma: consists of a gas heated to sufficiently high temperatures that the atoms ionize. The properties of the gas are controlled by electromagnetic forces among constituent ions and electrons, which results in a different type of behavior. Plasma is often considered the fourth state of matter (besides solid, liquid, and gas). Most of the matter in the universe is in the plasma state. polarization: the strength of the electric field associated with polarized light is stronger along one of the two directions perpendicular to the direction that the light is traveling. Scattered light is always polarized to some degree. The CMB is slightly polarized. positron: antiparticle of the electron. precess(ion): a form of motion that occurs when a torque is applied to a rotating body in such a way that it tends to change the direction of axis of rotation. A spinning top rotates, or precesses, around the direction perpendicular to the surface on which it spins. pulsar: a spinning neutron star that emits radiation in a beam. The sweeping action of the beam causes the object to pulse regularly when viewed by an observer, just as with a lighthouse. QCD: quantum chromodynamics, the theory that describes the strong force among quarks in a manner analogous to the description of the electromagnetic force by quantum electrodynamics. QED: quantum electrodynamics, the theory that describes the electromagnetic interaction in the framework of quantum mechanics (q.v.). The particle carrying the electromagnetic force is the photon. quantum cosmology: the area of physics and astrophysics concerned with a theory of the quantum initial state of the universe and its consequences for observations today. quantum mechanics: mathematical framework for describing the physics at atomic and smaller length scales, where energy exists in discrete quantum units. quark-gluon plasma: although today all quarks are bound together in nucleons (protons or neutrons), during the first 10 microseconds after the big bang the temperature of the universe was so high that unbound quarks moved freely in a state of matter called a quark-gluon plasma. It is also possible to artificially create a quark-gluon plasma by colliding two heavy nuclei at very high energies so that the nucleons dissolve into their quarks and gluons parts, such as is being done at RHIC.

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quarks: the elementary constituents of mesons and baryons (e.g., neutrons and protons). quasar: a very compact and extraordinarily luminous source of radiation in the nucleus of a distant galaxy. Quasars are believed to be powered by accretion (q.v.) of gas onto massive black holes. quasiperiodic oscillations: rapid not-quite-regular variations in the brightness of the x rays emitted by the accretion (q.v.) of matter onto a neutron star or black hole. The nearly periodic variations (that is, the variations are periodic but not perfectly so) are believed to reflect the dynamics of the disk of accreting matter. radio waves: electromagnetic waves with wavelengths that are very long compared with those of visible light. The radio band is usually considered to include all electromagnetic waves with wavelengths greater than about 1 millimeter. redshift, z : the shifting of light toward the red end of the spectrum that occurs due to the expansion of the universe. The wavelength of the light received is a factor (1 + z) larger, corresponding to the fact that the universe has grown in size by a factor (1 + z) since the light was emitted. relativistic: systems with particles moving with velocities close to the velocity of light. relativity: theoretical framework proposed by Einstein in the early part of the 20th century. There are two relativity theories: special and general. rest mass energy: the rest mass energy of a body is expressed by the relationship E = m0c2, where m0 is the rest mass of the body and c is the speed of light. Schwarzschild radius: the location of the “surface” of a black hole, from whose interior it is impossible to escape. shock waves: a very narrow region of high pressure and temperature formed in a fluid when the fluid flows supersonically over a stationary object or when a projectile flying supersonically passes through a stationary fluid. A shock wave may also be generated by violent disturbances in a fluid, such as occurs near a lightning stroke or a bomb blast. singularity: a region of infinite gravitational field and infinite space-time curvature. General relativity predicts that this is the ultimate result of gravitational collapse. SIRTF: NASA’s Space Infrared Telescope Facility, an orbiting infrared telescope, is scheduled for launch in 2003. SLAC: Stanford Linear Accelerator Center in Stanford, California; the electron and positron linear accelerator there has an energy of 50 GeV.

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SNO: the Sudbury Neutrino Observatory, located 6,800 feet underground in a mine in Ontario, Canada, is a heavy-water Cherenkov detector designed to detect neutrinos produced by fusion reactions in the Sun. solar flare: a bright eruption of hot gas in the Sun’s photosphere. solar mass: the mass of the Sun. solar neutrino: fusion reactions in the core of the Sun produce a huge flux of neutrinos called solar neutrinos. space-time: the four dimensional continuum in which we live, consisting of the three dimensions of space and one dimension of time. General relativity (q.v.) is concerned with the curvature (q.v.) of space-time. spatial interferometer: combines beams of light from different telescopes to synthesize the aperture of a single large telescope. Spatial interferometry is the main technique used by astronomers to map sources at high resolution and to measure their positions with high precision. special relativity: Einstein’s theory of space-time structure, in which Newton’s notion of absolute time is abandoned to account for the experimental fact that the speed of light is a universal constant and does not depend on the relative motion between the observer and the light source. spectroscopy: a technique whereby the light from astronomical objects is broken up into its constituent colors. Radiation from the different chemical elements that make up an object can be distinguished, giving information about the abundance of these elements and their physical state. SQUID: a Superconducting Quantum Interference Device is a very sensitive device for magnetic field detection developed for both traditional low-temperature superconductors and the new high-temperature superconductors. standard candle: a celestial object whose intrinsic brightness is known or can be estimated by some physical principle and whose observed brightness is therefore useful as a tool to measure distance. Standard Model: the theory that summarizes the current picture of the field of elementary-particle physics. It includes three generations of quarks and leptons, the electroweak theory of weak and electromagnetic forces, and the quantum chromodynamic theory of the strong force. It does not include answers to some basic questions such as how to unify electroweak forces with the strong or gravitational forces. string theory: a new physical theory that appears to be both a consistent quantum theory of gravity and a unified theory of all particles and forces.

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strong (color) force: one of the four fundamental forces, along with gravity, the electromagnetic force, and the weak nuclear force, that acts between elementary particles of matter. strong interaction (or strong nuclear force): the force felt by baryons and mesons that holds nucleons together in atomic nuclei. Once thought to be fundamental, the strong nuclear force is now described as a residual effect of the color force that binds quarks into mesons and baryons. superconductivity: the absence of measurable electrical resistance in certain substances. First discovered in 1911 in mercury, superconductivity is now known to occur in some 26 metallic elements and many compounds and alloys. The temperature below which a substance becomes superconducting is called the transition temperature (or critical temperature). superfluidity: the property of a low temperature system from liquid helium to atomic Bose-Einstein condensates to nuclear matter in neutron stars that enables fluid flow without friction. supermassive black hole: very large black holes (q.v.) with masses one million to one billion times the mass of our Sun that appear to be found at the core of most galaxies. Supermassive black holes are thought to be the engines that power quasars. Our own galaxy has a 2 million-solarmass black hole at its center. supersymmetry: a space-time symmetry that would imply the existence of partners to all elementary particles, with quantum spins of one-half a unit higher or lower. Often used in constructing theories that unify gravity with the three other forces. synchrotron radiation: electromagnetic radiation that is emitted by charged particles moving at relativistic speeds in circular orbits in a magnetic field. Much of the microwave radiation from celestial radio sources outside the galaxy is believed to originate from electrons moving in curved paths in celestial magnetic fields; it is also called synchrotron radiation. thermal spectrum: the characteristic distribution of radiation as a function of frequency that is emitted by a body at a well-defined temperature, also called a black-body or Planckian spectrum. time reversal invariance: or T symmetry, holds that the laws of physics should be the same when time is run backwards. topological defects: Symmetry-breaking phase transitions occur in many physical systems. In some of these systems, as the phase transition occurs, regions of space can become trapped in the wrong or unbroken phase. Examples include the vortices produced during the superfluid or

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superconducting phase transitions and also cosmic strings in the early universe. Such regions are called topological defects and give us an experimental handle on the nonequilibrium dynamics of the transition. Tully-Fisher relation: a method to determine galactic distances. Big, luminous galaxies rotate faster than small, faint ones. The connection between the two is given by the Tully-Fisher relation. Type Ia supernova: thermonuclear explosion of a white dwarf star caused by the accretion of material from a binary companion. Type IA supernovae (SneIa) can be used as standard candles to chart the universe. Type II supernova: a gigantic explosion that signals the death of a massive star. Often, the explosion leaves behind a neutron star; in other cases it may produce a black hole. uncertainty principle: the principle that it is not possible to know with unlimited precision both the position of a particle and its momentum. unification: the concept that two or more forces that seem distinct in today’s universe could, at higher energies (or temperatures), merge to become one force. universality of free fall: a central prediction of general relativity that the gravitational acceleration of a small object depends only on its location in space, not on any properties of the object itself. universe: all of space and time taken together. vacuum: a space in which there is a low pressure of gas, i.e., relatively few atoms or molecules. A perfect vacuum would contain no atoms or molecules but is unobtainable, as all the materials that surround such a space have a finite vapor pressure and give off atoms into the void. vacuum energy (sometimes called “Einstein’s cosmological constant”): quantum physics requires “empty space” to be filled with particles and antiparticles being continually created and annihilated. If the energy density associated with vacuum energy is not zero, it could be the dark energy that is causing the expansion of the universe to accelerate. virtual process or particle: one that is physically forbidden in classical mechanics but allowed by quantum mechanics. VLA: the Very Large Array, an array in New Mexico of 27 radio telescopes capable of adjustable spacing along a Y-shaped track, up to a radius of 27 kilometers. VLBA: Very Long Baseline Array, a newly completed radio interferometer operated by the National Radio Astronomy Observatory and capable of producing images with angular resolution of one-thousandth of an arcsecond (q.v.). W boson: particle that carries the charged weak force.

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weak interactions (force): the interactions of elementary particles that are responsible for radioactive decay. white dwarf star: a very small star that is the remnant core of a star that has completed fusion in its core. The Sun will become a white dwarf. White dwarfs are typically composed primarily of carbon, have about the radius of Earth, and do not significantly evolve further. x-ray binary: a double star in which one of the stars accretes matter from its binary companion (q.v.) and emits a copious quantity of x rays. The x-ray-emitting star is either a black hole (q.v.) or a neutron star (q.v.). XMM-Newton: a European x-ray space mission. Z boson: particle that carries the neutral weak force (q.v.).