The BICEP Array is a planned array of four microwave telescopes to be installed at the geographic South Pole, replacing the now-decommissioned Keck Array. From there, the telescopes will observe the cosmic microwave background (CMB), the oldest detectable light in the Universe, at six different frequencies. The CMB dates from the Epoch of Recombination (about 380,000 years after the Big Bang) when the Universe cooled enough that neutral atoms could form, releasing photons from their plasma cage to travel unimpeded through space.

The BICEP Array will search for evidence within the CMB of cosmic inflation, the name given to a popular theory governing the first fraction of a second in the existence of the Universe. According to inflation, during this time our observable Universe expanded exponentially from a dense, hot, subatomic volume before transitioning to the slower expansion with which we're more familiar. This process would have left its mark on current cosmological structure, and the existence of this process (or something similar) is backed up by several pieces of observational evidence. That's not the end of the matter, though—experiments like the BICEP Array and others with KIPAC involvement will look for more evidence for inflation and further test its predictions.

Clues hidden in ancient light

The evidence of inflation contained within the CMB is a very faint pattern of polarized light called B-modes, which are imprinted onto the CMB by gravity—either gravitational lensing, which could have occurred at any time after the CMB was created, or primordial gravitational waves, which could have happened during inflation.

Detecting B-modes is challenging; CMB radiation is a nearly perfect, uniform 2.7 K with variations in temperature of a mere 0.1 mK (milli-kelvins) on degree scales. Only about ten percent of it is polarized, and most of that light is patterned by a different sort of polarization called E-modes.

To add to the challenge, certain foreground processes can also create B-modes, such as signals from dust and synchrotron radiation from sources within our own galaxy, through which researchers must image the CMB.

The BICEP Array: More wavelengths and greater sensitivity

Detecting the faint B-mode signature of primordial gravitational waves requires the ability to subtract signals from polarized Galactic dust and synchrotron foregrounds and remove the effects of gravitational lensing, using next-generation instruments with higher sensitivity that observe in multiple wave bands. When complete, the four 550mm telescopes of the BICEP Array will train more than 30,000 detectors on 600 square degrees of the southern sky to look for polarization of the CMB at 30/40GHz, 95 GHz, 150 GHz, and 220/270 GHz. This is a more than ten-fold increase in sensors over BICEP 3, upon which the BICEP Array telescopes are based. BICEP 3 will continue to observe at 95 GHz.

The BICEP Array telescopes take advantage of advances in RF multiplexing to enable efficient readout of data, especially from the approximately 20,000 sensors on the 220/270 GHz telescope. It uses SLAC Microresonator Radio Frequency (SMuRF) technology, developed in large part by KIPAC researchers, to read out data from 2000 sensors using a single line.

The first BICEP Array telescope is scheduled to be installed and begin observations during the 2019-2020 season, with an additional telescope installed during each subsequent season, followed by three years of observations with the completed array.