BICEP (Background Imaging of Cosmic Extragalactic Polarization) and the Keck Array are a series of cosmic microwave background (CMB) experiments. 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, 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, Cardiff University, University of Chicago, Center for Astrophysics  Harvard & Smithsonian, Jet Propulsion Laboratory, CEA Grenoble (FR), University of Minnesota and Stanford University (all experiments); UC San Diego (BICEP1 and 2); National Institute of Standards and Technology (NIST), University of British Columbia and University of Toronto (BICEP2, Keck Array and BICEP3); and Case Western Reserve University (Keck Array).

The series of experiments began at the California Institute of Technology in 2002. In collaboration with the Jet Propulsion Laboratory, physicists Andrew Lange, Jamie Bock, Brian Keating, 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 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 in the early universe (called primordial gravitational waves), 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 California Institute of Technology; and Clem Pryke of the University of Minnesota.

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 () at the level of 7 sigma (5.9σ 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. In part because the large value of the tensor to scalar ratio, which contradicts limits from the Planck 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 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 galaxy.

A preprint released by the Planck 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 Planck data was published and the European Space Agency announced that the signal can be entirely attributed to dust 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 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 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 Gordon and Betty Moore Foundation, the James and Nelly Kilroy Foundation and the Barzan Foundation. The Keck Array project was originally led by Andrew Lange.

The Keck Array consists of five polarimeters, each very similar to the BICEP2 design, but using a pulse tube refrigerator rather than a large liquid helium cryogenic storage dewar.

The first three started observations in the austral summer 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 (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  (95% confidence level), which reduces to  in combination with Planck data.

In October 2021, new results were announced giving  (at 95% confidence level) based on BICEP/Keck 2018 observation season combined with Planck and WMAP 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 [inflationary gravitational waves] of 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."

See also

Cosmology
Inflation (cosmology)
Atacama Cosmology Telescope
South Pole Telescope
Cosmology Large Angular Scale Surveyor
POLARBEAR
LiteBIRD, space-based CMB B-mode polarization search project
Spider, balloon-based CMB B-mode polarization project

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).

Radio telescopes
Physics experiments
Cosmic microwave background experiments
 Astronomical experiments in the Antarctic
Inflation (cosmology)
BICEP과 Keck Array는 우주배경복사(CMB)와 관련한 관측실험으로, 우주배경복사의 B-모드 편광 측정을 목적으로 한다. 우주배경복사의 B-모드 편광은 초기 우주 급팽창의 증거라고 여겨진다. 이 관측의 장비는 지금까지 BICEP, BICEP2, Keck Array의 단계를 거쳐 왔으며, 2014년 현재 BICEP3를 구축하고 있다. 2014년 3월 17일, 관련 연구 그룹은 BICEP2가 B-모드 편광 신호를 관측했다고 발표했지만 우주배경복사로부터 온 것인지는 논쟁중에 있다.

목표와 역사 

이론에 따르면, 빅뱅 직후 빛보다 빠르게 공간이 팽창함에 따라(급팽창 이론) 중력파가 발생한다. BICEP의 목표는 이 중력파에 의해 발생한 우주배경복사의 편광 중, 특히 B-모드(컬 성분:Curl component)을 측정하는 것이다. Amundsen-Scott 남극기지에 설치되어 관측중이며, 지금까지의 세 장비 모두 천구의 남극을 중심으로 관측해 오고 있다.

이 관측과 관련된 기관들은 다음과 같다. Caltech, Cardiff University, University of Chicago, Harvard–Smithsonian Center for Astrophysics, Jet Propulsion Laboratory, CEA Grenoble (FR), University of Minnesota, Stanford University (이상 모든 장비). UC San Diego (BICEP1과2). National Institute of Standards and Technology (NIST), University of British Columbia and University of Toronto (BICEP2, Keck Array와 BICEP3). Case Western Reserve University (Keck Array).

BICEP은 2002년에 California Institute of Technology(Caltech)에서 시작되었다. Jet Propulsion Laboratory의 도움으로, 물리학자 Andrew Lange, Jamie Bock, Brian Keating와 William Holzapfel는 BICEP1 장비를 만들었고, 2005년에 Amundsen-Scott 남극기지에 설치해 관측을 수행했다. BICEP1을 남극에 설치한 직후, Caltech의 박사후과정이던 John Kovac 과 Chao-Lin Kuo을 비롯한 그룹은 BICEP2를 만들기 시작했다. BICEP2는 BICEP1과 비교했을 때 망원경은 그대로였지만, 새로운 방법으로 만든 측정기를 탑재했다. 이 새 측정기는 초점면에 필터, 처리, 영사, 그리고 측정 역할을 하는 인쇄 회로 기판(PCB)을 두었다. BICEP2는 2009년에 남극에 설치되어 관측을 수행했다.

BICEP1 

첫 BICEP 장비(개발 중에는 ‘Robinson 배경 중력파 망원경’이라 불렸다)는 주파수 100GHz와 150GHz(파장으로 3mm와 2mm)를 관측했으며, 각분해능은 각각 1.0°와 0.7°이었다. 우주배경복사의 편광에 민감한 98개의 측정기 배열(100GHz 50개, 150GHz 48개)을 가지고 있었으며, 한 쌍의 측정기는 편광 측정의 한 픽셀을 이루었다. 이 장비는 후에 만들 장비의 시범작이었고, 2006년 1월부터 관측을 시작해 2008년 말까지 진행되었다.

BICEP2 

두 번째 장비는 BICEP2이다. 초점면에 볼로미터인 초전도 전이단 센서(Transition-Edge Sensor: TES) 512개의 배열을 이용하여 256 픽셀을 구성하여, 이전 장비보다 훨씬 좋은 성능을 가지게 되었다. 26cm 구경의 망원경으로 150GHz를 관측하였으며, BICEP1을 대체하여 2010년부터 2012년까지 작동하였다.

2014년 3월, 네 명의 공동연구자인 John M. Kovac(Harvard-Smithsonian Center for Astrophysics), Chao-Lin Kuo(Stanford University), Jamie Bock(California Institute of Technology), Clem Pryke(University of Minnesota)는 BICEP2가 초기 우주의 중력파(primordial gravitational waves)로부터 생긴 배경복사의 B-모드를 관측했다고 발표했다. 2014년 3월 17일 Harvard–Smithsonian Center for Astrophysics에서 진행된 이 발표에 따르면, BICEP2는  수준의 B-모드를 관측하였다. 그러나, 2014년 6월, American Astronomical Society학회에서 천문학자 David Spergel은 이 관측 결과가 배경 복사에 의함이 아니라 우리 은하 내부의 성간먼지에 의한 빛의 산란 때문일 것이라고 주장했다. 측정된 텐서-스칼라 비율이 Planck 관측에 따른 범위를 벗어나기 때문에, 많은 과학자들은 이 관측결과가 성간먼지에 의한 결과라고 생각하고 있다.

Planck 연구진은 2014년 9월 발표한 논문에서 성간먼지를 정밀하게 측정했고, 이 성간먼지로 인한 신호가 BICEP2에서 얻어졌던 신호와 같은 세기라는 결론을 내렸다. 2014년 10월엔 POLARBEAR 연구진이 성간먼지와 기타 잡음의 영향을 제거한 후에도 97.2%의 신뢰도의 새로운 B-모드 편광을 관측했다고 발표했다.

Keck Array 

남극의 Martin A. Pomerantz Observatory에 있는 BICEP 망원경 바로 옆에, 사용되지 않고 있던 Degree Angular Scale Interferometer가 있었다. 이 자리에 Keck Array를 만들었다.

Keck Array는 다섯 개의 편광계로 이루어져 있다. 이 편광계는 BICEP2의 디자인과 비슷하지만, 큰 냉각용 액체 헬륨 저장 듀어(dewar) 대신 펄스 튜브 냉동기(pulse tube refregirator)를 이용한다.

2010-11 남반구 여름에 첫 세 개가 가동되었으며, 나머지 두 개는 2012년에 관측을 시작했다. 모든 편광계가 2013년까지 150GHz로 관측하다가, 두 개는 100GHz로 전향하였다. 각 편광계는 펄스 튜브 냉동기로 4K로 냉각되는 굴절 망원경(규모를 작게 만들기 위해)과, 초점면에 위치한 250mK로 냉각되는 512개의 초전도 전이단 센서로 이루어져 총 2560의 센서(1280 픽셀)로 구성된다.

이 프로젝트는 W. M. Keck Foundation로부터 230만 달러를 비롯해 National Science Foundation, the Gordon and Betty Moore Foundation, the James and Nelly Kilroy Foundation, the Barzan Foundation로부터의 자금을 지원받았다

BICEP3 

2012년 Keck array가 완성된 후, BICEP2는 더 이상 투자금 대비 효율이 좋지 않게 되었다. 그러나, Keck array와 같이 큰 액체헬륨 듀어를 제거하는 기술을 적용하여, 사용되지 않는 BICEP 망원경의 자리에 새로운 훨씬 큰 BICEP3 망원경을 건설중에 있다.

BICEP3은 Keck Array와 같이 2560개의 측정기(95GHz 이용)로 이루어져 있지만, 하나의 큰 55cm 구경 망원경으로 Keck Array보다 두 배에 가까운 광학적 성능을 가지게 된다. 2014-15 남반구 여름에 설치될 예정이다.

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