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Cosmology Glossary



A Glossary for Aspiring Cosmologists
compiled by Roger Hildebrand

A Glossary for Aspiring Cosmologists

Cosmologists converse mainly in acronyms or in words like dark energy and quintessence that sound like alchemists' concoctions. Would you like to know what they are talking about? Would you like to work a cosmo-sounding word into your cocktail conversation? If so, this glossary is for you.

If you are a cosmologist but don't know all the words, you are in good company. You will know a lot about some of the words so correct my mistakes and fill in my omissions. If you are not a cosmologist, tell me what is unintelligible. I will try harder or will try to find a cosmologist who also speaks English. Some actually are bilingual and have already helped.

Please join our distinguished board of contributors. Those who have contributed thus far are: Sean Carroll, Juan Collar, Wayne Hu, Josh Frieman, Randy Landsberg, Stephan Meyer, Angela Olinto, Simon Swordy, Monica Valluri, Bruce Winstein, Joan Winstein.

The mistakes are mostly mine. Just send in your correction, question, addition, or complaint to

Roger Hildebrand




Arcminute Cosmology Bolometric Array Receiver. Probably contrived to sound like Akbar, the emperor of the Mogul dynasty in India, whose name means "great." ACBAR is a 16 element bolometer array operated on the Viper telescope at the South Pole. It is designed for observations of CMB anisotropy and the Sunyaev-Zel'dovich effect. It has four frequency bands in submillimeter atmospheric windows. The beam size is 4 arcmin. ACBAR will be sensitive to a range of multipole moments from l=60 to 2700. The project is headed by W. Holzapfel (UCB)


Accelerated Strategic Computing Initiative. See Flash Center.


Antarctic Submillimeter Telescope and Remote Observatory. AST/RO is a 1.7 meter telescope that has been in operation at the South Pole since January 1995. The PI is Tony Stark. It is a general-purpose telescope for astronomy and aeronomy studies at wavelengths between 200 and 2000 microns. The telescope operates continuously through the austral winter, and it has been available for use on a proposal basis by all astronomers since 1998. For more information, see CARA Research and Harvard AST/RO.


The Pierre Auger Project (named for the famous French physicist). Auger will consist of two giant air shower arrays. One is under construction in Argentina. The second will be in the northern hemisphere. Each will cover 3000 km2 with 1600 water Cherenkov tanks (and associated fluorescence detectors) that can record about 6000 cosmic ray showers per year with energies above 1020eV. The main goal is to study the origin of ultra-high-energy cosmic rays. This international project is led by Jim Cronin. For more information, see J. W. Cronin, Nucl. Phys. Proc. Suppl. B471, 135 (1996) A. A. Watson, Phys. Rept. 333-334, 309 (2000).


A hypothetical weakly-interacting boson of mass << 200 keV. Its existence would resolve what is known as the strong-CP problem in quantum chromodynamics. Such a particle would efficiently transport energy out of stars or out of supernova cores. Axions are also prime "cold dark matter candidates." See Chapter 10 of Kolb & Turner, The Early Universe, Addison-Wesley Publishing Company, 1990.

The two most sensitive ongoing experimental searches for axions rely on the so-called "Primakov effect," i.e., the coupling of the axion to two photons, one of them virtual or "man-made" provided by a strong electric or magnetic field, and the second being the detectable signal. A high-Q resonant microwave cavity immersed in a strong superconducting magnet (a left over from a "star wars" project) is operated at Lawrence Livermore National Laboratory (LLNL), where it slowly scans microwave frequencies looking for axions in the narrow mass region over which they would constitute good galactic dark matter candidates. Juan Collar is involved in the second experiment, CAST, which is presently being assembled at CERN. CAST will use a 10 meter, 20 ton decomissioned LHC (Large Hadron Collider) dipole magnet as a telescope to track the sun looking for axions emitted from its core.




B-modes (don't confuse with B-vectors)

Linear polarization, described by the Stokes parameters Q and U, or more generally by any symmetric trace-free tensor field on the sky, can be represented by a "headless" vector whose amplitude and direction vary as a function of position on the sky. The Q and U representation of the direction (e.g., north-south/northwest-southeast) is an inherently coordinate dependent representation since Q and U mix into each other under a rotation of the coordinate system. The so-called "E" and "B"-mode representation of the linear polarization is a coordinate independent representation. It is the tensor equivalent of the vector decomposition into a curl-free and divergence-free piece.

The geometric meaning of the representation is best seen in a simple plane-parallel geometry from which the general case can be constructed by Fourier superposition. For the vector case, the curl-free piece is the component of the vector pointing along the direction of the wavevector and the divergence-free piece is the perpendicular component. For the tensor case, the "E"-mode, the polarization is aligned parallel or perpendicular to the wavevector, whereas the "B"-mode is aligned at 45 degree angles to the wavevector -- the difference is that polarization is "headless" vector which maps back onto itself under a 180 degree rotation. Because the "B"-mode must then choose "left" from "right," it is said to have a handedness, just like the curl field. The "E" and "B" amplitudes may be represented as scalar and pseudoscalar pieces respectively -- much like the velocity potential of a curl-free vector.

The virtue of the "E" and "B" decomposition is that linear processes that generate polarization from the scalar gravitational potential (e.g., the primary CMB polarization) and the cosmic shear of weak gravitational lensing only generate the "E"-mode. Detection of the "B"-mode would represent new physics (e.g., primordial gravitational waves) or allow useful checks on systematics and contaminants. The drawback is that like the velocity potential, it is a non-local representation of the field and thus complicates data analysis.


B-vectors (don't confuse with B-modes)

B-vectors give the orientation of the projected magnetic field as derived from polarization measurements. Polarization of starlight by absorption in magnetically aligned grains give B-vectors directly (long axes of spinning grains B, hence transmitted radiation parallel B). Measurements of polarized emission from magnetically aligned dust grains give E-vectors parallel long axes and hence perpB. Neither E- nor B-vectors are true vectors. They have no heads or tails. To add them you must use Stokes parameters.


A millimeter-wave telescope and bolometric receiver system designed for long-duration balloon flights from Antarctica. The instrument is being used to measure the angular power spectrum of the CMB at degree and subdegree scales. See Piacentini et al, ApJS 138, 315 (2002) and Crill et al, 2002 (in preparation).


Cosmological constant   


A new multi-horn initiative based upon the techniques of PIQUE to measure the polarization power spectrum of the microwave background and its cross-correlation with the temperature anisotropy (90 GHz). The area to be mapped is a small cap centered on the north celestial pole (hence CAPMAP). The leading figures in this endeavor are Suzanne Staggs (Princeton), Bruce Winstein (Chicago), and Josh Gundersen (Miami). They expect good sensitivity in the multipole range from 500 to 1500.


Center for Astrophysical Research in Antarctica. Its existence as a Center ended in February 2002 but the experiments are continuing under other NSF funding. This Center, initiated by Al Harper, was designed to exploit the advantages of the Polar Plateau for research on the origin of structure in the universe. CARA has been a National Science Foundation Science and Technology Center with headquarters at the University of Chicago.


Juan Collar explains that this is "a totally discombobulated acronym meaning 'CERN Axion Solar Telescope'. The original was the much more melodious SATAN for 'Solar Axion Telescopic ANtenna' which everyone loved and was approved as a CERN experiment. Hence the cute logo. Reactionary forces subsequently took control: the collaboration momentarily considered switching to HELMS (HELioscopic Magnetic Search) to leave no doubt that they were really, truly, pulling for the dark side. For what CAST is about, see axion or CAST.


Cold Dark Matter. Non-relativistic and non-luminous matter that likely contributes significantly to the density of galaxies and of the universe as a whole (but does not close the universe). (See WIMPs and neutralino.)


Center for Cosmological Physics. Director: Bruce Winstein. One of several Physics Frontier Centers funded by NSF. The CfCP is funded for five years beginning October 1, 2001. There should be a prize for inventing a catchy nickname (as in ASCI "Flash Center").


Cosmic Infrared Background Radiation. The CIBR was predicted to account for the energy from production of metals in the nearby universe. The radiation was detected in the sub-millimeter and its average spectrum measured by two COBE experiments. The CIBR average surface brightness and spectrum are consistent with models of the high redshift protogalaxies detected by SCUBA. The models have the bulk of the energy coming from the thermal emission from dust heated by UV from hot young stars. The CIBR can be used as a probe of 1) the galaxy and star formation history to high redshift, and 2) the cosmic density fluctuation spectrum at intermediate redshift.


Cosmic Microwave Background Radiation. Thompson-scattered photons from the surface of last scattering. [You can find it in text books.]


COsmic Background Explorer. NASA launched the COBE satellite on November 18, 1989 to measure the diffuse infrared and microwave radiation from the early universe, to the limits set by our astrophysical environment. It carried three instruments: a Far Infrared Absolute Spectrophotometer (FIRAS) to compare the spectrum of the cosmic microwave background radiation with a precise blackbody, a Differential Microwave Radiometer (DMR) to map the cosmic radiation precisely, and a Diffuse Infrared Background Experiment (DIRBE) to search for the cosmic infrared background radiation. The FIRAS has shown that the cosmic microwave background spectrum matches that of a blackbody of temperature 2.726K +/- 0.002K with a precision of 0.03%. The COBE DMR had a nominal beam size of 7 degrees and the analysis of data from this instrument showed for the first time that the background radiation has an intrinsic anisotropy at a level of a part in 10^5. The first four years of data were released in January 1996.

Cosmological constant

Cosmological constant (vacuum energy)

A concept introduced by Einstein in 1917 to reconcile his theory of General Relativity with the prevailing assumption that the universe was static. In fact, the resulting static solutions were later shown to be unstable to collapse or expansion. Einstein's cosmological constant was later interpreted as the energy density of the vacuum. The observational upper bound on the value of the vacuum energy density is 40 to 120 orders of magnitude smaller than that predicted from quantum field theories of elementary particles -- the so called cosmological constant problem. The cosmological constant may be the cause of the acceleration of the universe recently inferred from observations of Type Ia supernovae, but again there is as yet no theoretical understanding of why it would have the small, non-zero value needed to explain these observations (see S.M. Carroll, Living Rev. Rel. 4 (2001) 1 [astro-ph/0004075]).


Dark energyDark matterDART 

Dark energy

A form of energy that is gravitationally repulsive, due to a negative effective pressure. Dark energy is spread almost uniformly throughout space and appears to contribute about 70% of the present energy density of the universe. Its existence was recently inferred from observations of distant Type Ia supernovae (see supernova) and from combining CMBR anisotropy measurements with measurements of the density of dark matter. The cosmological constant is a particular form of dark energy other possibilities for dark energy include a very light scalar field (also known as quintessence east of the Delaware River).

Dark matter

Any form of matter which does not emit, absorb, or otherwise interact with electromagnetic radiation. By "matter" we mean some form of particles which can cluster under the force of gravity this is to be distinguished from "dark energy" which is smoothly distributed throughout space. The existence of dark matter has been inferred from its gravitational effects on the dynamics of luminous tracers in galaxies and galaxy clusters and on the bending of light rays (gravitational lensing). Recent evidence indicates that dark matter contributes about 25% of the critical density for a flat universe, with the remainder, except ~5% ordinary matter, in dark energy. Much of the dark matter resides in halos encompassing luminous galaxies and in clusters of galaxies where it appears to be distributed more smoothly. Evidence from Big Bang Nucleosynthesis (BBN) and the CMBR indicates that the bulk of the dark matter is non-baryonic, that is, not composed of quarks. Particle physics theories predict the existence of exotic weakly interacting elementary particles (see WIMPs), such as supersymmetric neutralinos and axions, which are hypothetical candidates for non-baryonic dark matter. Dark matter plays a critical role in shaping the formation of large-scale structure. The constraints from BBN and the CMBR, combined with the density of luminous matter, also indicate that the bulk of the baryons in the universe are also dark, perhaps in the form sub-stellar objects (MACHOs).


Dual Anamorphic Reflector Telescope. Also known as Dragovan's Awesome Reflector Telescope. An ultralight telescope concept with reflections from two successive cylindrical paraboloids. The first prototype is diffaction-limited beyond 40 µ. The goal is to produce telescopes with large apertures that are practical for far-IR space observatories.




See B-modes.


See B-vectors.


Explorer of Diffuse high-z Galactic Emission. A balloon-borne experiment being developed by Stephan Meyer and collaborators to survey the far-infrared background emission with 6' resolution and frequency coverage from 150 GHz to 1.3 THz. The primary goal of EDGE is to characterize the power spectrum of fluctuations in the CIBR at large angular scales. The instrument will also provide a detailed characterization of the high galactic latitude dust emission. The addition of polarization sensitivity, now under study (I would say an essential feature of any respectable continuum instrument in this millenium), would give the best characterization of the diffuse emission from the Milky Way and external galaxies. Differences in flux and polarization spectra from one external galaxy to the next are also predicted (by RH) and could be studied with a polarization-sensitive EDGE measurement.


Energetic Gamma Ray Experiment Telescope. An instrument on the Compton Gamma Ray Observatory (CGRO), launched 1991. EGRET made a complete survey of the sky from 30 MeV to 10 GeV. EGRET showed the high-energy gamma-ray sky to be surprisingly dynamic and diverse, with sources ranging from the sun and moon to massive black holes at large redshifts. Most of the gamma-ray sources detected by EGRET remain unidentified. EGRET will be succeeded by GLAST.


Flash CenterFUSE  

Flash Center

The University of Chicago Center for Astrophysical Thermonuclear Flashes. The Flash Center is sponsored by the Department of Energy ASCI/Alliances Program (ASCI = Accelerated Strategic Computing Initiative). The aim of the Center is to solve the problem of thermonuclear flashes on the surface of compact stars such as neutron stars, white dwarfs, and in the interior of white dwarfs (i.e. Type 1 supernovae). The problem involves accretion flow on to the surfaces of these compact stars, shear flow and Rayleigh-Taylor instabilities on the stellar surfaces, ignition of nuclear burning under conditions leading to convection, and either deflagration or detonation, and stellar envelope expansion. Solutions should lead to understanding of x-ray bursts, radii of neutron stars, classical novae, abundances of intermediate mass elements, evolution of white dwarf masses in close binary systems, and calibration of Type 1 supernovae as standard candles. The center is managed by Don Lamb (Director), Robert Rosner (former Director), Todd Dupont (validation and basic physics), Bruce Fryxell (Code Group), Rusty Lusk (Computer Science), Jim Truran (Astrophysics), and Rick Stevens (ANL ex officio).


Far-Ultraviolet Spectroscopic Explorer: A satellite for high spectral resolution in the range 905 to 1187 Å. See ApJL, Vol 538, Number 1 Part 2 (~15 letters on FUSE).




Gamma-ray Large Area Space Telescope. GLAST is an international and multi-agency space mission that will study the cosmos in the energy range 10 keV - 300 GeV. GLAST will provide a factor of 30 or more advance in sensitivity over EGRET and will provide capability for study of transient phenomena. The GLAST Burst Monitor (GBM) will have a field of view several times larger than the LAT and will provide spectral coverage of gamma-ray bursts that extends from the lower limit of the LAT down to 10 keV. NASA plans to launch GLAST in late 2005.


The Greisen-Zatsepin-Kusmin cutoff. A computed limit to cosmic ray energies above which interactions with the microwave background should prevent transmission over cosmological distances.




HAWC, a High-resolution Airborne Wideband Camera (D. A. Harper, PI, is a first-light instrument for SOFIA (qv). HAWC is a photometer with four passbands in the range 53µm (5.2 arcsec resolution) to 215µm (21 arcsec resolution) and a 12 x 32 array of detectors. An upgrade is under study to convert HAWC to a polarimeter (SuperHAWC) with >5000 pixels on the sky.




Long Duration Balloon. Current Long Duration Balloons use conventional zero pressure balloon technology but are launched at polar latitudes near midsummer when the day/night cycle is gone. This permits these flights to remain at altitude for over a month. The polar stratosphere near midsummer has a strong circum-polar vortex which keeps the flights at a nearly constant latitude. Experiments that benifit from extended observation time are candidates for LDB flights. Currently, LDB flights are launched from McMurdo Antarctica in the southern hemisphere and Alaska in the northern hemisphere. In the near future, a new type of balloon which is sealed will permit ultra long duration flights lasting over 100 days. These flights would not be restricted to polar latitudes. Boomerang and TopHat have been southern LDB flights. BoomPol and EDGE are being designed for future LDB flights.




Massive Astrophysical Compact Halo Objects. These are candidates for the dark matter in galaxy halos, with masses in the range from brown dwarfs to white dwarfs. With the possible exception of primordial black hole MACHOs, they would likely be baryonic dark matter. Several experiments have been carried out and others are underway to detect MACHOs through their gravitational microlensing (amplification) of the light from background stars, first using the LMC as a background and more recently using Andromeda. A number of microlensing events toward the LMC were detected, but their interpretation is uncertain. As much as 30% of the dark matter in the Milky Way halo could be in the form of MACHOs.






A candidate for the non-baryonic dark matter particle. Supersymmetry, a hypothetical symmetry relating bosons to fermions, predicts the existence of hitherto undetected "partners" for each type of particle in the Standard Model of particle physics. Under certain assumptions, the lightest such partner particle would be stable, and if it is neutral (a "neutralino"), it would make a good dark matter candidate. Reasonable neutralino masses range from 30 GeV to 10 TeV. If they make up the dark matter of the Milky Way, then they have nonrelativistic velocities and hence the annihilation would give rise to gamma rays with unique energies. Experiments are underway to directly detect halo neutralinos via their scattering with nuclei (and deposition of energy) in underground detectors. The neutralino signal in its various guises has been discussed in the literature (Jungman et al, 1996, and references therein Bergstrom, Ullio & Buckley, 1998). GLAST can search for gamma-ray lines in the mass range above 30 GeV.




Office of Polar Programs. An office under the NSF director for the management of the US science program in both the Arctic and in Antarctica. This office has funded Antarctic LDB ballooning instruments, CARA, and many non-astronomy science projects. The US Amunsen-Scott South Pole Station is managed through OPP as is all the logistical support for US Antarctic operations.


Office of Special Programs, at the University of Chicago. The OSP is a major partner in the Astronomy & Astrophysics Department's outreach efforts, specifically with the Space Explorers Program. The Office of Special Programs/College Prep (OSP) utilizes the life of the University of Chicago community in its efforts to prepare educationally disadvantaged minority youths for college. Through several programs -- Upward Bound, Upward Bound Math-Science, the Pilot Enrichment Program, the Young Astronomers, and the Institute for Athletics and Education -- OSP encourages South Side youths, ages 10 through 18, to consider, prepare for, and succeed in postsecondary education.




NSF Physics Frontier Center. The Center for Cosmological Physics is one of several.


A completed effort that has provided a sensitive limit on the polarized microwave anisotropy, in both Q and U, on a small ring around the North Celestial Pole. It used a correlation polarimeter and served as a validation of the technique and the basis for a new expanded effort, CAPMAP.  PIQUE -- usually pronounced as in "My interest is piqued" -- stands for the Princeton I, Q, and U Experiment. (For information about I, Q, and U, see Stokes parameters.




QUIET is a collaboration of Chicago with Princeton, Caltech, JPL, Columbia and Miami. QUIET proposes to make very sensitive measurements of the polarization of the cosmic microwave background radiation, using the technology of coherent correlation polarimeters. It takes advantage of a breakthrough developed at JPL for the packaging of the polarimeters ("radiometer on a chip") which allows their mass production so that thousands of detectors can be used.

QUIET stands for Q/U Imaging ExperimenT (where Q and U are the stokes parameters for the polarization).


Classically, this means "fifth element" (after earth, air, fire, and water). It was the ephemeral substance that prevented the planets from falling to the center of the celestial sphere. East of the Delaware River, this term has been used in recent years to refer to scalar field models for the dark energy (A semi-popular introduction to "quintessence" is available at These models may be distinguishable from the cosmological constant as dark energy by future experiments which will probe the effective equation of state (ratio of pressure to energy density) of the dark energy (see SNAP, for example). Unlike the cosmological constant, the energy density in scalar field dark energy models is, in general, time-dependent."


Relic neutrinos   

Relic neutrinos

The ultimate teaser for the pigheaded astroparticle experimentalist. Originating in the Big Bang much like their CMB photonic counterpart, their existence and characteristics are non-controversial: we are able to predict their local present density (about 110 per nerutrino species per cm3 and some of their expected phenomenology. Even if ~1012 of them traverse any cm2 of space per second, no viable method of direct detection exists (so far all methods have proved to be flawed in concept or to require Buck Rogers XXII century technologies). This is due to the miniscule momentum exchange (~10-6 eV/c) expected from their coherent interactions with target materials. The physics behind this mechanism of interaction is very similar to optical scattering of ultracold neutrons off smooth containment vessel walls (see a beautiful description of this in E. Fermi's Nuclear Physics course notes [U of C press]). Astroparticle experimentalists generally grow interested in relic neutrinos once they have been granted tenure or have gone completely cuckcoo, whichever happens first.

Recent solar and atmospheric neutrino experiments have established that at least some of the neutrinos have mass and that they mix (oscillate) between themselves. Based on these experiments, it appears that the cosmic density of massive neutrinos is at least as large as that of luminous matter. Neutrino experiments as well as the large-scale clustering of galaxies indicate that light stable neutrinos have a mass less than a few electron volts.


SPAROSPIFISPTStokes parameters
Strong lensingSZA  


Spouse Acceptance Factor. The amount of time married scientists are allowed to spend away from home, in the office, at the lab, talking about physics at social events, going to the South Pole, attending conferences, etc., before the spouse says "ENOUGH!!!!!!" (from a cosmo girl now on our editorial staff).


Sloan Digital Sky Survey. (A key resource for the Cosmological Center. Project organized by Don York). SDSS is making a survey of pi steradians of the North Galactic Cap. The survey will record > 108 celestial objects. It will measure distances to a million of the nearest galaxies to give a 3D picture of the universe in a volume 100 times greater than that explored in any previous survey and will record distances to 100,000 quasars. The SDSS uses a dedicated 2.5 M telescope at the Apache Point Observatory, in Sunspot, New Mexico. The SDSS and the 3.5 meter telescope are managed by the Astrophysical Research Consortium (ARC).


Super Nova/Acceleration Probe. A space-based experiment to measure the properties of the accelerating universe and to study both dark matter and dark energy. With its wide-field imager and spectrograph it is expected to discover and follow > 2000 Type Ia supernovae per year. The resulting data set will be used to measure the mass density, omega-m, the vacuum energy density, omega-lambda, and the curvature, omega-K, and to estimate the equation of state of the dark energy with sufficient precision (۪.05) to differentiate between a cosmological constant and alternatives such as Quintessence.


SOFIA is a 2.7 m telescope is mounted in a Boeing 747-HP aircraft based at Mo ett Field, California with the capability to fly almost anywhere in the world. It will fly at altitudes up to 45,000 feet, above 99% of the Earth's water vapor and can observe at wavelengths from the optical to submillimeter. The first test flights are scheduled for November 2006. SOFIA, the Stratospheric Observatory for Infrared Astronomy, is a collaborative project of NASA and the German Space Agency, DLR. The science missions will be carried out by the Universities Space Research Associoation (Eric Becklin, PI).


Submillimeter Polarimeter for Antarctic Remote Observing. A 450 µ polarimeter built at Northwestern University (Giles Novak, PI) for large scale mapping of southern objects, especially the Galactic center. SPARO operates on the Viper telescope.


South Pole Imaging Fabry-Perot Interferometer. SPIFI is an imaging Fabry-Perot interferometer designed for use in the 350 and 450 µ windows available at the JCMT on Mauna Kea, and for the 200 and 350 µ windows available at the AST/RO telescope at the South Pole. SPIFI currently employs a 5 x 5 array of silicon bolometers held at 60 mK by an ADR. Through 2 crogenic scanning Fabry-Perots in series, SPIFI achieves a resolving power that can be tuned from R ~1000 to 10,000. Primary science is investigating the warm dense neutral interstellar medium associated with star formation in galaxies via its 370 µ [CI] fine-structure line and 372 µ CO (7-6) rotational line emission. On AST/RO SPIFI will map the inner galaxy and nearby external galaxies in these lines and the 205 µ [NII] line as well. The system is being upgraded to include a much larger (12 or 16 X 32 pixel) format TES bolometer array from GSFC.


South Pole Telescope. An 8-meter off-axis telescope recently funded by the NSF to be built at the NSF Amundson-Scott South Pole Station. This telescope is to be used for a large (4000 deg2 SZE survey to measure the abundance and evolution of galaxy clusters from redshift z >= 2 to the nearby universe. The goal is to measure the equation of state of the dark energy with high precision. The Center will build a state-of-the-art 1000-element bolometer receiver operating at 140 GHz in collaboration with the UC Berkeley detector development group.

Stokes parameters

Stokes parameters (named for Sir George Stokes)
A set of four parameters, I, Q, U, V, that describe completely the state of polarization of a beam of radiation.

Define a coordinate system. Align dipole and loop antennas with respect to that coordinate system. Record the signals received by the antennas and take sums and differences as indicated by the figure at right.

Reduced Stokes parameters are q Q/I, u U/I, v V/I.

The degree of polarization, P, is related to q and u by P2 = q2 + u2.

The direction is given by formula .

For more information, see Hildebrand et al, PASP, 112, 1215 (2000) or van de Hulst, H. C., Light Scattering by Small Particles. New York: Dover (1957).

Strong lensing

An effect due to the gravitational bending of light by mass concentrations which results in 1) the strong distortion of background galaxy images (arcs) behind some massive foreground galaxy clusters, and 2) multiple images of the same background quasar behind a foreground galaxy. Over 50 lensed QSOs are known. The Sloan Digital Sky Survey (see SDSS) may find hundreds more. Lensed QSOs provide a probe of dark matter halos and perhaps of the dark energy as well.


Sunyaev-Zel'dovich Array. An array of six 3.5 meter telescopes operating at 1 cm. The Center will enhance this array by adding two additional telescopes and associated electronics to double the speed of the array and will add a 3 mm band to improve the spatial resolution. John Carlstrom will be responsible for the Center's contributions to the array. The new survey will begin operation in 2004�.




An innovative hat-shaped astronomical experiment that sat on top of a Long Duration Balloon (LDB) that was launched from McMurdo Station, Antarctica on January 4, 2001 and circled the continent for nearly two weeks collecting data on the CMBR. The TopHat experiment included a spinning telescope and a detector system. It mapped a 48 degree diameter disk of the sky above the southern polar cap. To accomplish this, the telescope spun at a constant rate about its vertical axis, mapping the entire polar cap each day as the earth rotated below it. TopHat was built by a collaboration consisting of NASA's Goddard Space Flight Center, the University of Chicago, the University of Wisconsin, and the Danish Space Research Institute.




Ultra High Energy Cosmic Rays. A somewhat vague term. (100 MeV used to be high energy.) In current cosmospeak, it usually means >~ 1017eV, i.e., close to and above the point where the spectrum steepens. The Auger experiment is aimed directly at this UHE component.


Vacuum energyVERITASViper 

Vacuum energy

See cosmological constant.


Very Energetic Radiation Imaging Telescope Array System (an acronym made to spell truth). VERITAS is a new major ground-based gamma-ray observatory with an array of seven 10 meter optical reflectors for gamma-ray astronomy in the energy range of 50 GeV – 50 TeV. The design will be based on the design of the existing 10 meter gamma-ray telescope of the Whipple Observatory at Mt Hopkins. The telescopes will be deployed such that they permit the maximum versatility and will give the highest sensitivity in the 50 GeV – 50 TeV band (with best sensitivity from 100 GeV to 10 TeV). In this band, critical measurements will be made of SNRs (super nova remnants) and AGNs (active galactic nuclei). This VHE (very high energy) observatory will effectively complement GLAST. The Center's interest in VERITAS is mainly as a dark matter annihilation detector -- neutralino annihilations generate a clear gamma-ray signal and will serve as a "constrainer" of theories beyond the standard model such as changes in the speed of light, violations of Lorentz Invariance, and the existence of large extra dimensions.


The Viper telescope is a 2.5 meter off-axis telescope designed for CMBR studies at the South Pole. Currently, ACBAR (a small angular scale CMBR anisotropy experiment) and SPARO (a submillimeter polarimeter) are the instruments used on the telescope. Viper was built and maintained by CARA.


Weak lensingWIMPsWMAP 

Weak lensing

The small distortion and amplification of background galaxy images by foreground mass concentrations. In the WL regime, the distortion of an individual image is undetectably small (unlike the case of strong lensing), and the effect must be detected by searching for correlated distortions of thousands of source images. WL has been used to map the projected mass distribution of a number of galaxy clusters and has been used statistically to constrain the masses and sizes of dark galaxy halos. The SDSS has made the most precise measurements of WL by galaxy halos to date, using only 2% of its eventual dataset.


Weakly Interacting Massive Particles (see also their nemeses, MACHOs). A broad class of particles which were once in thermal equilibrium with the early universe but were "cold," i.e., moving non-relativistically at the time of structure formation. If the dark matter is composed mainly of WIMPs, then their density today is determined roughly by their annihilation rate, with weak-scale interactions. WIMPs are expected to have collapsed into a roughly isothermal, spherical halo within which the visible portion of our galaxy resides, consistent with measurements of spiral galaxy rotation curves. The original WIMP was simply a heavy Dirac neutrino, now experimentally ruled out. The neutralino predicted by supersymmetric extensions of the Standard Model is everyone's favorite WIMP nowadays. This may be due to its non-ad hoc qualities: it is predicted in a separate context (particle physics) and actively sought in accelerator experiments, yet makes a nearly perfect cold dark matter particle.

Direct detection of WIMPs is possible through their elastic scattering from nuclei. The contributions of individual nucleons are summed coherently to yield a WIMP-nucleus cross section still typically smaller than 10-6 pb down to many orders of magnitude below neutrino couplings. The nuclear recoil energy is no more than a few keV up to tens of keV since WIMP velocities relative to earth should be typical of galactic velocities. This presents fabulous problems for their direct detection: interaction rates for some neutralinos can be lower than one recoil event per ton of detector material per year. Detectors must be operated underground, away from natural radioactive backgrounds, and must be able to reject or be intrinsically insensitive to all other types of particle interactions. Present emphasis is on development of new large-mass detector technologies able to face these challenges. Juan Collar is involved in several of these efforts.


Wilkinson Microwave Anisotropy Probe. A NASA Explorer mission launched in June 2001, WMAP is the first mission to use an L2 orbit as its permanent observing station. (L2 is a semi-stable region of gravity that is about four times farther than the Moon). WMAP has mapped the temperature fluctuations of the CMB radiation with much higher resolution, sensitivity, and accuracy than COBE. It is currently scheduled to observe for four years. In February 2003, the WMAP team announced the results of the first complete year of measurements of the temperature of the CMB over the full sky. WMAP's main science goals were 1) to improve the precision of the measurement of various cosmological parameters 2) shed light on the process by which galaxies and other structures formed in the universe and 3) more accurately deduce the epoch at which the first objects formed in the universe. In addition to producing a map of the CMB sky of unprecedented quality, the WMAP results have provided better estimates of various fundamental cosmological parameters. It has also enabled astronomers to infer when the first objects were formed that were capable of ionizing the gas in the universe. WMAP has detected this ionization signal at 200 million years after the Big Bang.


UChicago Department of Physics
UChicago Department of Astronomy & Astrophysics
Enrico Fermi Institute

Other Kavli Institutes

Last update: February 26, 2011