A comparison of the sensitivity of WMAP with COBE and Penzias and Wilson's telescope. Simulated data.

There have been a variety of experiments to measure the Cosmic microwave background (CMB) radiation anisotropies and polarization since its first observation in 1964 by Penzias and Wilson. These include a mix of ground-, balloon- and space-based receivers. The most notable of these are COBE, which first detected the temperature anisotropies of the CMB, and showed that it had a black body spectrum; DASI, which first detected the polarization signal from the CMB; CBI that made high-resolution observations and obtained the first E-mode polarization spectrum and WMAP, which has provided the best full-sky CMB maps to date. Planned future experiments include the Planck spacecraft, which aims to produce high-resolution all-sky maps of both the temperature anisotropies and polarization signals, and various ground-based experiments primarily intended to investigate small-scale anisotropies and trying to detect the polarization caused by gravitational waves in the early universe.

The design of cosmic microwave background experiments is a very challenging task. The greatest problems are the receivers, the telescope optics and the atmosphere. Many improved microwave amplifier technologies have been designed for microwave background applications. Some technologies used are HEMT, MMIC, SIS and bolometers. Experiments generally use elaborate cryogenic systems to keep the amplifiers cool. Often, experiments are interferometers which only measure the spatial fluctuations in signals on the sky, and are insensitive to the average 2.7 K background.

Another problem is the 1/f noise intrinsic to all detectors. Usually the experimental scan strategy is designed to minimize the effect of such noise. To minimize side lobes, microwave optics usually utilize elaborate lenses and feed horns. Finally, the atmosphere is an issue because water absorbs microwave radiation (a fact utilized in the operation of microwave ovens), it is rather difficult to observe the microwave background with ground-based instruments. CMB research therefore makes increasing use of air and space-borne experiments. Ground-based observations are usually made from dry, high altitude locations such as the Chilean Andes and the South Pole.

The list below consists of a partial list of past, current and planned CMB experiments. The name, start and end years of each experiment are given, followed by the basis of the experiment—whether space, balloon or ground based—and the location where appropriate. The frequency and amplifier technologies used are given, as is the main targets of the experiments.

Image	Name	Start	End	Basis	Location	Frequency (GHz)	Amplifier technology	Targets	Ref(s)
	Advanced Cosmic Microwave Explorer (ACME) Also HACME: HEMT+ACME	1988	1996	Ground		26–35; 38–45	HEMT	Temperature anisotropies	[1]
	Antarctic Plateau Anisotropy Chasing Experiment (APACHE)	1995	1996	Ground	Antarctic	100, 150, 250	Bolometer	Temperature anisotropies	[1]
	Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE)	2001	—	Balloon		3, 5, 7, 10, 30, 90	HEMT	CMB Spectrum	[1]
	Archeops	1999	2002	Balloon		143, 217, 353, 545	Bolometer	Measured large and intermediate scale with improved precision at the larger scales.	[1]
	Arcminute Cosmology Bolometer Array Receiver (ACBAR)	2001	—	Ground		150, 219, 274	Bolometer	Temperature anisotropies	[1]
	Arcminute Microkelvin Imager (AMI)	2005	—	Ground	UK: Mullard Radio Astronomy Observatory	12-18	Interferometer	SZ effect, Temperature anisotropies	[1]
	ARGO	1988, 1990, 1993	1993	Balloon		150-600	Bolometer		[1]
	Array for Microwave Background Anisotropy (AMiBA)	2002	—	Ground	Hawaii: Mauna Loa	86-102	MMIC	SZ effect; Polarization	[1][2][3]
	Atacama Cosmology Telescope (ACT)	2007	—	Ground	Chile: Atacama Desert	145, 225, 265	Bolometer	Temperature anisotropies	[1]
	Atacama Pathfinder Experiment (APEX)	2005	—	Ground		150, 217	Bolometer	Temperature anisotropies; SZ effect	[1]
	Australia Telescope Compact Array (ATCA)	1991	1997	Ground		8.7	HEMT		[1]
	Background Emission Anisotropy Scanning Telescope (BEAST)	2000	—	Balloon, Ground		25-35; 38-45	HEMT	A ground single dish CMB observatory at the University of California's White Mountain Peak Research station.	[1]
	Background Imaging of Cosmic Extragalactic Polarization (BICEP)	2006	2008	Ground	South Pole	100, 150	Bolometer	Will measure large scale polarization with improved precision.	[1]
	Balloon-borne Anisotropy Measurement (BAM)	1995	1998	Balloon	UBC Balloon Expt	110-250	Spectrometer	Used differential Fourier Transform Spectrometer to measure degree scale anisotropy	[1]
	Balloon-borne Radiometers for Sky Polarisation Observations (BaR-SPoRT)	Future	—	Balloon		32, 90	Polarizer / OMT		[1]
	Berkeley Illinois Maryland Associations (BIMA)	1986	2004	Ground		70-116; 210-270	SIS		[1]
	BOOMERanG experiment	1997	2003	Balloon	Long-duration balloon above Antarctica	90-420	Bolometer	Measured intermediate scale fluctuations with improved precision.	[1]
	B-mode RAdiation INterferometer (BRAIN)	Never	—	Ground	Dome-C, Antarctica
	Clover	Never	—	Ground		97, 150, 230	Bolometer	Will measure the small scale fluctuations with improved precision, and the B-mode polarization.	[1]
	Cosmic Anisotropy Polarization Mapper (CAPMAP)	2002	—	Ground		40, 90	MMIC/HEMT		[1]
	Cosmic Anisotropy Telescope (CAT)	1994	1997	Ground	Mullard Radio Astronomy Observatory	13-17	Interferometer / HEMT	Measured the very small scale fluctuations in small regions of the sky.	[1]
	Cosmic Background Imager (CBI)	2002	2008	Ground	Llano de Chajnantor Observatory, Chile	26-36	HEMT	Measured the very small scale fluctuations with improved precision in small regions of the sky and polarization of CMB.	[1]
	COSMOSOMAS	1998	—	Ground	Teide Observatory, Tenerife, Spain	10-18	HEMT	Circular scanning experiments for CMB and foregrounds in Tenerife.	[1]
	Cosmic Background Explorer (COBE)	1989	1993	Space	Earth orbit			Temperature anisotropies	[1]
	Cosmological Gene	1999	—	Ground		0.6 to 32	HEMT		[1]
	Degree Angular Scale Interferometer (DASI)	1999	2003	Ground		26-36	HEMT	A temperature and polarization telescope at the South Pole.	[1]
	The E and B Experiment (EBEX)	Future	—	Balloon	Antarctica	150-450	Bolometer	Detection of the inflationary gravitational-wave background (IGB) signal is a primary goal of the EBEX experiment	[1]
	Far Infra-Red Survey (FIRS)	1989	1989	Balloon		170-680	Bolometer		[1]
	KU-band Polarization IDentifier (KUPID)	2003	—	Ground		12-18	HEMT		[1]
	Medium Scale Anisotropy Measurement (MSAM)	1992	1997	Balloon		150-650	Bolometer		[1]
	Millimeter Anisotropy eXperiment IMaging Array (MAXIMA)	1995, 1998, 1999	1999	Balloon	Near Palestine, Texas	150-420	Bolometer	Measured intermediate scale fluctuations with improved precision.	[1]
	Millimeter Interferometer (MINT)	Future	—	Ground		150	SIS		[1]
	Millimeter-Wave Bolometric Interferometer (MBI-B)	Future	—	Ground		90	Bolometer		[1]
	Mobile Anisotropy Telescope (MAT)	1997, 1998	1998	Ground		30-140	HEMT / SIS		[1]
	Planck	2009	—	Space	Lagrange 2	30-857	HEMT / Bolometer	Polarization; Temperature anisotropies; Foregrounds	[1]
	Polarization Observations of Large Angular Regions (POLAR)	2000	2000	Ground		26-46	HEMT		[1]
	Polarization of Background Microwave Radiation (POLARBeaR)	Future	—	Ground		90-240	Bolometer		[1]
	Polatron	Never	—	Ground		100	Bolometer		[1]
	Princeton I, Q, and U Experiment (PIQUE)	2002	2002	Balloon		90	Bolometer		[1]
	Python	1992	1997	Ground		30-90	HEMT / Bolometer		[1]
	QMAP	1996	1996	Ground		30-140	HEMT / SIS		[1]
	QUaD	2005	2007	Ground	South Pole	100, 150	Bolometer	Measured intermediate scale polarization with improved precision.	[1]
	Qubic	Future	—	Ground		97, 150, 230	Bolometer	Will measure the B-mode polarization on intermediate scale.
	Q/U Imaging ExperimenT (QUIET)	2008	—	Ground	Llano de Chajnantor Observatory, Chile	40, 90	HEMT		[1]
	RELIKT-1	1983	1984	Space	Earth orbit			Temperature anisotropies	[1]
	Saskatoon experiment	1993	1995	Ground	Saskatchewan	26-46	HEMT		[1]
	Sky Polarization Observatory (SPOrt)	Future	—	Space	International Space Station			Polarization	[1]
	South Pole Telescope	2006		Ground	South Pole			Will measure the small scale fluctuations and polarization.	[1]
	SPIDER	2011		Balloon		90, 150, 220	Bolometer	Will measure very large scale polarization.
	Sunyaev-Zeldovich Array (SZA)	Future	—	Ground		26-36; 85-115	Interferometer	SZ effect	[1]
	Sunyaev-Zeldovich Infrared Experiment (SuZIE)	1996	—	Ground		150, 220, 350	Bolometer	SZ effect	[1]
	Tenerife Experiment	1984	2000	Ground	Tenerife	10, 15, 33	HEMT		[1]
	TopHat	2002	—	Balloon		150-720	Bolometer		[1]
	Very Small Array	2002	2008	Ground		26-36	Interferometer / HEMT	Measured intermediate and small scale fluctuations with improved precision in small regions of the sky.	[1]
	Wilkinson Microwave Anisotropy Probe (WMAP)	2001	—	Space	Lagrange 2	23-94	HEMT	Temperature anisotropies; Polarization	[1]

[edit] References

v • d • e Cosmic microwave background radiation (CMB) Discovery of CMB radiation · List of CMB experiments · Timeline of CMB astronomy Effects Sachs–Wolfe · Sunyaev–Zel'dovich Full-sky temperature map taken by NASA's Wilkinson Microwave Anisotropy Probe (WMAP) Space-based experiments RELIKT-1 · COBE · WMAP · Planck · SPOrt  · CMBPol Balloon experiments QMAP · MAXIMA · BOOMERanG · Archeops · Spider · EBEX Ground-based experiments Saskatoon · MAT · COSMOSOMAS · Tenerife Experiment · DASI · CBI · CAT · ACBAR · CAPMAP · VSA · QUaD · SPT · SZA · ACT · AMI · Clover  · QUIET · AMiBA · OCRA · QUIJOTE · APEX-SZ · ABS · OVRO · BIMA · GUBBINS