BICEP (Background Imaging of Cosmic Extragalactic Polarization) and the Keck Array are a series of

cosmic microwave background In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...

(CMB)

experiments An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs when ...

. They aim to measure the polarization of the CMB; in particular, measuring the ''B''-mode of the CMB. The experiments have had five generations of instrumentation, consisting of BICEP1 (or just BICEP), BICEP2, the Keck Array, BICEP3, and the BICEP Array. The Keck Array started observations in 2012 and BICEP3 has been fully operational since May 2016, with the BICEP Array beginning installation in 2017/18.

Purpose and collaboration

The purpose of the BICEP experiment is to measure the polarization of the cosmic microwave background. Specifically, it aims to measure the ''B''-modes ( curl component) of the polarization of the CMB. BICEP operates from

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...

, at the Amundsen–Scott South Pole Station. All three instruments have mapped the same part of the sky, around the south celestial pole. The institutions involved in the various instruments are

Caltech The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...

, Cardiff University,

University of Chicago The University of Chicago (UChicago, Chicago, U of C, or UChi) is a private university, private research university in Chicago, Illinois. Its main campus is located in Chicago's Hyde Park, Chicago, Hyde Park neighborhood. The University of Chic ...

, Center for Astrophysics Harvard & Smithsonian,

Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA an ...

, CEA Grenoble (FR),

University of Minnesota The University of Minnesota, formally the University of Minnesota, Twin Cities, (UMN Twin Cities, the U of M, or Minnesota) is a public land-grant research university in the Twin Cities of Minneapolis and Saint Paul, Minnesota, United States. ...

and Stanford University (all experiments);

UC San Diego The University of California, San Diego (UC San Diego or colloquially, UCSD) is a public land-grant research university in San Diego, California. Established in 1960 near the pre-existing Scripps Institution of Oceanography, UC San Diego is t ...

(BICEP1 and 2);

National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical s ...

(NIST),

University of British Columbia The University of British Columbia (UBC) is a public research university with campuses near Vancouver and in Kelowna, British Columbia. Established in 1908, it is British Columbia's oldest university. The university ranks among the top thre ...

and

University of Toronto The University of Toronto (UToronto or U of T) is a public university, public research university in Toronto, Ontario, Canada, located on the grounds that surround Queen's Park (Toronto), Queen's Park. It was founded by royal charter in 1827 ...

(BICEP2, Keck Array and BICEP3); and Case Western Reserve University (Keck Array). The series of experiments began at the

California Institute of Technology The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...

in 2002. In collaboration with the Jet Propulsion Laboratory, physicists

Andrew Lange Andrew E. Lange (July 23, 1957 – January 22, 2010)Janette WilliamsAndrew Lange, noted universe researcher at Caltech, dies ''Pasadena Star-News''. Retrieved on January 26, 2010. was an astrophysicist and Goldberger Professor of Physics at th ...

, Jamie Bock,

Brian Keating Brian Gregory Keating (born September 9, 1971) is an American cosmologist. He works on observations of the cosmic microwave background, leading the BICEP, POLARBEAR2 and Simons Array experiments. He received his PhD in 2000, and is a Distinguishe ...

, and William Holzapfel began the construction of the BICEP1 telescope which deployed to the Amundsen-Scott South Pole Station in 2005 for a three-season observing run. Immediately after deployment of BICEP1, the team, which now included Caltech postdoctoral fellows John Kovac and Chao-Lin Kuo, among others, began work on BICEP2. The telescope remained the same, but new detectors were inserted into BICEP2 using a completely different technology: a printed circuit board on the focal plane that could filter, process, image, and measure radiation from the cosmic microwave background. BICEP2 was deployed to the

South Pole The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on Earth and lies antipod ...

in 2009 to begin its three-season observing run which yielded the detection of B-mode polarization in the cosmic microwave background.

BICEP1

The first BICEP instrument (known during development as the "Robinson gravitational wave background telescope") observed the sky at 100 and 150 GHz (3 mm and 2 mm wavelength) with an angular resolution of 1.0 and 0.7 degrees. It had an array of 98 detectors (50 at 100 GHz and 48 at 150 GHz), which were sensitive to the polarisation of the CMB. A pair of detectors constitutes one polarization-sensitive pixel. The instrument, a prototype for future instruments, was first described in Keating et al. 2003 and started observing in January 2006 and ran until the end of 2008.

BICEP2

The second-generation instrument was BICEP2. Featuring a greatly improved focal-plane transition edge sensor (TES) bolometer array of 512 sensors (256 pixels) operating at 150 GHz, this 26 cm aperture telescope replaced the BICEP1 instrument, and observed from 2010 to 2012. Reports stated in March 2014 that BICEP2 had detected ''B''-modes from

gravitational waves Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...

in the

early universe The chronology of the universe describes the history and future of the universe according to Big Bang cosmology. Research published in 2015 estimates the earliest stages of the universe's existence as taking place 13.8 billion years ago, wit ...

(called

primordial gravitational wave In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all space ...

s), a result reported by the four co-principal investigators of BICEP2: John M. Kovac of the Center for Astrophysics Harvard & Smithsonian; Chao-Lin Kuo of Stanford University; Jamie Bock of the

; and Clem Pryke of the

. An announcement was made on 17 March 2014 from the Center for Astrophysics Harvard & Smithsonian. The reported detection was of B-modes at the level of , disfavouring the

null hypothesis In scientific research, the null hypothesis (often denoted ''H''0) is the claim that no difference or relationship exists between two sets of data or variables being analyzed. The null hypothesis is that any experimentally observed difference is d ...

() at the level of 7 sigma (5.9''σ'' after foreground subtraction). However, on 19 June 2014, lowered confidence in confirming the cosmic inflation findings was reported; the accepted and reviewed version of the discovery paper contains an appendix discussing the possible production of the signal by

cosmic dust Cosmic dust, also called extraterrestrial dust, star dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 micrometers). Larger particles are c ...

. In part because the large value of the tensor to scalar ratio, which contradicts limits from the

Planck Max Karl Ernst Ludwig Planck (, ; 23 April 1858 – 4 October 1947) was a German theoretical physicist whose discovery of energy quanta won him the Nobel Prize in Physics in 1918. Planck made many substantial contributions to theoretical p ...

data, this is considered the most likely explanation for the detected signal by many scientists. For example, on June 5, 2014 at a conference of the American Astronomical Society, astronomer

David Spergel David Nathaniel Spergel is an American theoretical astrophysicist and the Emeritus Charles A. Young Professor of Astronomy on the Class of 1897 Foundation at Princeton University. Since 2021, he has been the President of the Simons Foundation ...

argued that the B-mode polarization detected by BICEP2 could instead be the result of light emitted from dust between the stars in our

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

galaxy. A preprint released by the

team in September 2014, eventually accepted in 2016, provided the most accurate measurement yet of dust, concluding that the signal from dust is the same strength as that reported from BICEP2. On January 30, 2015, a joint analysis of BICEP2 and

data was published and the European Space Agency announced that the signal can be entirely attributed to

dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ho ...

in the Milky Way. BICEP2 has combined their data with the Keck Array and Planck in a joint analysis. A March 2015 publication in ''

Physical Review Letters ''Physical Review Letters'' (''PRL''), established in 1958, is a peer-reviewed, scientific journal that is published 52 times per year by the American Physical Society. As also confirmed by various measurement standards, which include the ''Journa ...

'' set a limit on the tensor-to-scalar ratio of . The BICEP2 affair forms the subject of a book by Brian Keating.

Keck Array

Immediately next to the BICEP telescope at the Martin A. Pomerantz Observatory building at the South Pole was an unused telescope mount previously occupied by the

Degree Angular Scale Interferometer The Degree Angular Scale Interferometer (DASI) was a telescope installed at the U.S. National Science Foundation's Amundsen–Scott South Pole Station in Antarctica. It was a 13-element interferometer operating between 26 and 36 GHz (Ka band ...

. The ''Keck Array'' was built to take advantage of this larger telescope mount. This project was funded by $2.3 million from W. M. Keck Foundation, as well as funding from the

National Science Foundation The National Science Foundation (NSF) is an independent agency of the United States government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National ...

, the

Gordon and Betty Moore Foundation The Gordon and Betty Moore Foundation is an American foundation established by Intel co-founder Gordon E. Moore and his wife Betty I. Moore in September 2000 to support scientific discovery, environmental conservation, patient care improvements a ...

, the James and Nelly Kilroy Foundation and the Barzan Foundation. The Keck Array project was originally led by

. The Keck Array consists of five polarimeters, each very similar to the BICEP2 design, but using a

pulse tube refrigerator The pulse tube refrigerator (PTR) or pulse tube cryocooler is a developing technology that emerged largely in the early 1980s with a series of other innovations in the broader field of thermoacoustics. In contrast with other cryocoolers (e.g. Sti ...

rather than a large liquid helium

cryogenic storage dewar A cryogenic storage dewar (named after James Dewar) is a specialised type of vacuum flask used for storing cryogens (such as liquid nitrogen or liquid helium), whose boiling points are much lower than room temperature. Cryogenic storage dew ...

. The first three started observations in the

austral summer Summer is the hottest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, the earliest sunrise and latest sunset occurs, daylight hours are longest and dark hours are shortest, ...

of 2010–11; another two started observing in 2012. All of the receivers observed at 150 GHz until 2013, when two of them were converted to observe at 100 GHz. Each polarimeter consists of a

refracting telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and a ...

(to minimise systematics) cooled by a pulse tube cooler to 4 K, and a focal-plane array of 512 transition edge sensors cooled to 250 mK, giving a total of 2560 detectors, or 1280 dual-polarization pixels. In October 2018, the first results from the Keck Array (combined with BICEP2 data) were announced, using observations up to and including the 2015 season. These yielded an upper limit on cosmological B-modes of

r<0.07

(95% confidence level), which reduces to

r<0.06

in combination with

data. In October 2021, new results were announced giving

r<0.036

(at 95% confidence level) based on BICEP/Keck 2018 observation season combined with Planck and

WMAP The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP and Explorer 80), was a NASA spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic mic ...

data.

BICEP3

Once the Keck array was completed in 2012, it was no longer cost-effective to continue to operate BICEP2. However, using the same technique as the Keck array to eliminate the large liquid helium dewar, a much larger telescope has been installed on the original BICEP telescope mount. BICEP3 consists of a single telescope with the same 2560 detectors (observing at 95 GHz) as the five-telescope Keck array, but a 68 cm aperture, providing roughly twice the optical throughput of the entire Keck array. One consequence of the large focal plane is a larger 28° field of view, which will necessarily mean scanning some foreground-contaminated portions of the sky. It was installed (with initial configuration) at the pole in January 2015. It was upgraded for the 2015-2016 Austral summer season to a full 2560 detector configuration. BICEP3 is also a prototype for the BICEP Array.

BICEP Array

The Keck array is being succeeded by the BICEP array, which consists of four BICEP3-like telescopes on a common mount, operating at 30/40, 95, 150 and 220/270 GHz. Installation began between the 2017 and 2018 observing seasons. It is scheduled to be fully installed by the 2020 observing season. According to the project website: "BICEP Array will measure the polarized sky in five frequency bands to reach an ultimate sensitivity to the amplitude of IGW nflationary gravitational wavesof σ(r) < 0.005" and "This measurement will be a definitive test of slow-roll models of inflation, which generally predict a gravitational-wave signal above approximately 0.01."

References

External links

BICEP2 winter-over (2009–2012) Steffen Richter (9 winters at the South Pole).Keck winter-over (2010-current) Robert Schwarz (12 winters at the South Pole).
{{DEFAULTSORT:Bicep Radio telescopes Physics experiments Cosmic microwave background experiments Astronomical experiments in the Antarctic Inflation (cosmology)