POLARBEAR-2 Telescope Sees First Light

Addition to Simons Array in the Chilean high desert could help detect evidence of quantum gravity

April 9, 2019 | By Stuart Volkow

Wide view of the Simons Array site in the Atacama region of Chile. Photo courtesy of the UC San Diego Cosmology Lab

The Simons Array has deployed the first of three new POLARBEAR-2 telescopes at its 17,000 ft. high site on Cerro Toco Mountain in the Atacama Desert region of Chile. UC San Diego’s Lindsay Lowry and Tucker Elleflot, both PhD candidates, along with collaborators from UC Berkeley and Japan’s KEK Institute, installed a receiver and optical components in the telescope mount. The activity kicked off the process of testing and calibrating the new scope. When fully assembled, the Simons Array will be the world’s most advanced terrestrial observational cosmology experiments dedicated to studying the cosmic microwave background (CMB)—the universe’s oldest light.

UC San Diego Professor of Physics and Director of the Simons Observatory Brian Keating, who along with Professor Kam Arnold co-leads the Simons Array, explained the significance of the project. “This experiment could lead to a better understanding of the physics of the big bang, the nature of dark energy, dark matter and neutrinos,” said Keating. “The detection of these elementary particles might be the first experimental evidence of quantum gravity.”

Each of the three POLARBEAR-2 (PB-2) telescope receivers have 7,500 exquisitely sensitive detectors, called bolometers, for measuring minute microwave energies, characteristic of the fossil “light” which makes up the CMB. The bolometers along with their Transition Edge Sensors (TES) are fabricated into silicon wafers at UC Berkeley’s Marvell Nanolab exclusively for the Simons Array.

“Assembling these telescopes requires lots of physical labor and attention to detail,” explained Tucker, who along with colleagues required extra oxygen while installing the component parts on site in Chile. “Working at 17,500 feet is taxing, but the results are rewarding.”

Students take in oxygen as they work at the site. Photo courtesy of the UC San Diego Cosmology Lab

Keating describes the POLARBEAR-2 telescopes as having a unique design that combines features of reflecting and refracting telescopes. “It tickles me that the first experimental scientist in history, Galileo, used some of the same principles we’re using in his first refracting telescopes over 400 years ago.”

According to the scientists, the POLARBEAR-2 telescopes allow cosmologists to see microwave radiation that is invisible to the human eye. To achieve their unprecedented levels of sensitivity, however, the POLARBEAR-2 receivers must filter out all irrelevant energies and incorporate superconducting detectors and amplifiers that work at ultra-low temperatures.

“It takes 7 to 10 days to cool down all the stages of PB-2, from 90 Kelvin in the warmer optical stages, to the 4K superconducting amplifiers, to 275 Milli Kelvin at the focal plane of the detector, much colder than the CMB itself,” said UC San Diego graduate student Logan Howe. “Only after successful cool-down can we start collecting data. We’ve now begun analyzing data to calibrate and fine tune the telescope.”

The process of calibration and debugging will continue for several months until experimental quality data can be collected.

The earlier POLARBEAR telescope collected data from 2012 to 2016, resulting in numerous publications and dissertations. Additional funding from the Simons Foundation enabled the construction of two additional telescopes, as well as upgrades for dramatically improved sensitivity, resolution and electronics to create the Simons Array.

Along with the nearby Atacama Cosmology Telescope operated by Princeton University, POLARBEAR-2 became part of the Simons Observatory in 2016. Keating and Arnold also co-lead the UC San Diego Cosmology Lab, where the second POLARBEAR-2 receiver and optics are currently being validated at the Center for Astrophysics and Space Sciences. It will arrive at the Simons Array site in Chile in July of this year.