Glossary of terms absorption line A dark line at a particular wavelength superimposed upon a bright, continuous spectrum. Such a spectral line can be formed when electromag- netic radiation, while travelling on its way to an observer, meets a substance; if that substance can absorb energy at that particular wavelength then the observer sees an absorption line. Compare with emission line. accretion disk A disk of gas or dust orbiting a massive object such as a star, a stellar-mass black hole or an active galactic nucleus. An accretion disk plays an important role in the formation of a planetary system around a young star. An accretion disk around a supermassive black hole is thought to be the key mecha- nism powering an active galactic nucleus. active galactic nucleus (agn) A compact region at the center of a galaxy that emits vast amounts of electromagnetic radiation and fast-moving jets of particles; an agn can outshine the rest of the galaxy despite being hardly larger in volume than the Solar System. Various classes of agn exist, including quasars and Seyfert galaxies, but in each case the energy is believed to be generated as matter accretes onto a supermassive black hole. adaptive optics A technique used by large ground-based optical telescopes to remove the blurring affects caused by Earth’s atmosphere. Light from a guide star is used as a calibration source; a complicated system of software and hardware then deforms a small mirror to correct for atmospheric distortions. The mirror shape changes more quickly than the atmosphere itself fluctuates. astrometry The branch of astronomy dealing with the precise measurement of the position and motion of celestial bodies. Historically, astrometry has played an important role in developing our understanding of cosmic distance scales, and it continues to play a key role in modern research; for example, many dark energy surveys require knowledge of the precise position and motion of galaxies. axion A hypothetical elementary particle, first postulated by Roberto Peccei and Helen Quinn in 1977 as a means of solving a problem in the theory of quantum chromodynamics. If axions exist then they have no electric charge and will react scarcely at all with matter: they are more elusive even than the neutrino. In some models, however, the Big Bang would have created vast numbers of low-mass axions that would still be around. This makes them a possible dark matter candidate. baryon acoustic oscillation (bao) The competing forces of gravitational attrac- tion and gas pressure caused acoustic oscillations – essentially sound waves – in the normal matter (baryons) of the primordial plasma. Peaks in the oscillations S. Webb, New Eyes on the Universe: Twelve Cosmic Mysteries and the Tools We Need to Solve Them, 313 Springer Praxis Books, DOI 10.1007/978-1-4614-2194-8, © Springer Science+Business Media, LLC 2012 314 New Eyes on the Universe correspond to regions of the Universe in which matter is denser than average. This clustering occurs on a well-understood length scale, and thus bao clustering gener- ates a standard length for use by astronomers. This standard ruler has a length of 480 million light years. baryonic matter A baryon is a subatomic particle composed of three quarks. The proton and the neutron are both types of baryon. (Particles such as the electron and the neutrino are not baryons; they are a type of fundamental particle called a lep- ton.) An atom of normal matter consists of some combination of protons, neutrons and electrons. Since protons and neutrons are much more massive than electrons, nearly all of the mass of an atom is contained in the baryons. In astronomy and cos- mology, then, ‘baryonic matter’ refers to matter that is composed mostly of baryons by mass; since that includes every type of atom, baryonic matter includes essen- tially all the matter that we observe. Most of the mass of the Universe appears to be in the form of non-baryonic matter and, in particular, some form of dark matter. Big Bang An event that took place 13.7 billion years ago in which the Universe was created. Initially the Universe was in an incredibly hot, dense state. As the Universe expanded, it cooled; the existence of the cosmic microwave background radiation, which indicates that the current temperature of the Universe is 2.7 K, is good evi- dence for the Big Bang theory. blackbody An idealized object that absorbs all the radiation that hits it and thus appears perfectly black. A blackbody is a perfect emitter, radiating the maximum amount of energy at any particular temperature. A blackbody spectrum has a dis- tinctive shape and maximum that depends only upon the blackbody temperature. black hole Any massive body possesses an escape velocity, the speed that some- thing must have in order to leave the gravitational pull of the body. For Earth, the escape velocity is just over 11 kilometers per second. If the mass of the body is crushed into a smaller volume the escape velocity increases. Eventually, when the escape velocity reaches 300 000 kilometers per second, not even light itself can travel fast enough to escape the pull of gravity. The result is a black hole. Although by definition it is impossible to see a black hole, it is possible to deduce its presence by its effects on surrounding matter. Astronomers have deduced the presence of several stellar-mass black holes and it is believed that a supermassive black hole lies at the heart of every galaxy. Blandford–Znajek mechanism An idea, proposed by Roger Blandford and Roman Znajek, for how energy can be extracted from a rotating black hole and be used to power the relativistic jets that shoot out from active galactic nuclei. The rough idea is that magnetic field lines will be threaded through the matter in the accretion disk surrounding a black hole. A rotating black hole drags space around it, and as matter falls into the black hole the field will become tightened, coiled and twisted. In these Glossary of terms 315 circumstances large amounts of energy, in the form of jets, will be emitted from the poles of the black hole. blazar A type of active galactic nucleus in which the relativistic jet from a cen- tral supermassive black hole is pointing more or less straight towards Earth. The name ‘blazar’, which is short for blazing quasi-stellar object, was coined in 1978 by Edward Spiegel. B mode polarization A distinctive polarization pattern in the cosmic micro- wave background radiation that would be caused by the interaction of primordial gravitational waves with the background radiation. If this polarization could be observed it would demonstrate that inflation did indeed occur in the very early history of the Universe. The signals are extremely weak, however, and so exquisitely sensitive observations of the microwave background are needed to search for them. bolometer An instrument, in essence a very sensitive thermometer, to measure infrared radiation. The first bolometer was invented in 1878 by Samuel Langley, who noted that when a thin, blackened strip of platinum absorbs radiation its elec- trical resistance changes in a way that can easily be measured. The sensitivity of Langley’s original bolometer was sufficient for him to discover absorption lines in the infrared part of the Sun’s spectrum. Modern bolometers, of course, are fantasti- cally more sensitive; they can pick up temperature differences on the sky of ten- millionths of a degree. Cerenkov radiation When a charged particle moves through certain types of material at a speed greater than light itself is able to move through the material, the charged particle can polarize the molecules in that material. When the molecules return to their ground state, they emit electromagnetic radiation – Cerenkov radia- tion. It has a continuous spectrum that is more intense at shorter wavelengths: most Cerenkov radiation is in the ultraviolet spectrum, while the visible radiation is seen to be a deep blue. CfA2 Great Wall A filament of galaxies, discovered in 1989 by Margaret Geller and John Huchra, that is more than 500 million light years long. Large, filamentary structures such as this are thought by astronomers to form part of the cosmic web. chirp The gravitational wave signal expected to come from two inspiraling neutron stars. As the stars spiral inwards, the frequency of the gravitational waves increases in a particular way; the crescendo, if it were converted into sound waves of the same frequency, would sound like the chirp of a large bird. coded aperture imaging A technique for taking images when the photons involved have high energies. The photons pass through an aperture mask containing holes arranged in some particular pattern. When the photons hit the detector, a computer applies algorithms that allow the image to be reconstructed. 316 New Eyes on the Universe cold dark matter Several lines of evidence lead to the notion that most of the mat- ter in the Universe cannot be seen by its electromagnetic effects (in other words, it’s dark) and that its constituent particles move relatively slowly (in other words, it’s cold). At the time of writing it is not known what cold dark matter consists of, but one favored idea is that it consists of some type of weakly interacting massive particle. collapsar Originally this referred to a collapsed star, the end result of a high-mass star that undergoes gravitational collapse to form a stellar-mass black hole. It is now often used to refer to a model in which a fast-rotating, high-mass Wolf–Rayet star undergoes gravitational collapse to form a black hole. Calculations suggest that relativistic jets shoot out along the poles of the rotating black hole.