Over the past 12 years, the BICEP (Background Imaging of Cosmic Extragalactic Polarization) collaboration has deployed four generations of increasingly powerful instruments to observe the early universe from the Amundsen-Scott South Pole Station. BICEP1, BICEP2, the Keck Array, and BICEP3 were designed to measure the polarization of the cosmic microwave background in search of the faint signature of primordial gravitational waves. The collaboration is currently constructing an even more ambitious instrument called the BICEP Array.
Reading the cosmic microwave background for evidence of primordial gravitational waves
A Q&A with Professor Jeff Filippini, astrophysicist
Over the past 12 years, the BICEP (Background Imaging of Cosmic Extragalactic Polarization) collaboration has deployed four generations of increasingly powerful instruments to observe the early universe from the Amundsen-Scott South Pole Station. BICEP1, BICEP2, the Keck Array, and BICEP3 were designed to measure the polarization of the cosmic microwave background in search of the faint signature of primordial gravitational waves. The collaboration is currently constructing an even more ambitious instrument called the BICEP Array.
The collaboration recently published findings based on all data taken through the 2015 season, providing the most stringent constraint to date on the amplitude of primordial gravitational waves in our universe. Professor Jeff Filippini is an astrophysicist at the University of Illinois at Urbana-Champaign, interested in observational searches for signatures of new fundamental physics. He joined the BICEP collaboration while BICEP2 was being developed and contributed to its superconducting detectors and readout systems. Filippini is a coauthor on the BICEP collaboration’s latest result.
Q: What generally has the BICEP collaboration achieved in this latest published research?
A: Through several years of observation with BICEP2 and the Keck Array, the collaboration has made the most precise (lowest noise) map to date of the polarization of the cosmic microwave background (CMB). The map covers about 1 percent of the sky, chosen to be relatively (though not completely!) free of contaminating polarized emission from our own galaxy. The remaining galaxy glow is cleaned from the data by combining maps made at multiple observing frequencies (90, 150, and 220 GHz), as well as noisier maps at 23 GHz–353 GHz from the WMAP and Planck satellites. The main result is a new world-leading upper limit on the amplitude of primordial gravitational waves in the early universe, a key prediction of the theory of cosmic inflation.
Q: Why is it important to study the CMB?
A: The CMB has proven to be an incredibly versatile probe of cosmology, teaching us an enormous amount about the contents, history, and workings of our universe. The CMB is the black-body glow of the hot early universe, emitted about 380,000 years after the big bang, when the universe transitioned from opaque plasma to transparent neutral gas. The faint anisotropies in this glow—about 1 part in 100,000—carry the imprint of sound waves in the primordial plasma, and through those, information about the universe’s contents and geometry. The CMB radiation also carries information about subsequent cosmic history, through the absorption and gravitational lensing it experienced on its long journey to our telescopes.
Q: What is B-mode polarization, and how does it relate to primordial gravitational waves?
A: Doppler shifts within the plasma imprint a weak polarization on the CMB via Thomson scattering. By symmetry, sound waves can generate only a “curl-free” (E-mode) pattern of polarization. Gravitational waves have no such restriction, however, and generate a mix of E-modes and twisty B-modes. If such B-mode polarization were observed it would be evidence that our universe was born with a primordial hum of gravitational waves, imperceptibly faint in the modern era, but detectable via their effect on the CMB.
Q: What does this measurement mean for the theory of cosmic inflation?
A: Cosmic inflation postulates that the universe expanded extremely rapidly in its early moments, and that this epoch seeded spacetime with quantum “noise” in the form of both sound waves and gravitational waves. The sound waves are detectable in the CMB—they formed the seeds of the vast cosmic structures we see through our telescopes. The gravitational waves remain undetected, and this result constrains their power to be no more than 6 percent of that in the sound waves. Different specific models for the physics behind inflation predict different amounts of gravitational and sound waves, so this measurement disfavors certain models while leaving others viable.
Q: In what ways has the instrumentation of the BICEP experiment improved over the years?
A: Each new iteration of the program adds more sensitivity by cramming in more detectors, as well as fanning out to new observing frequencies to better characterize galactic contamination. BICEP1 observed with 98 individually wired bolometric detectors, maintained at 300 mK using a 3He refrigerator. BICEP2 brought that count to 512 detectors using monolithic arrays of superconducting transition-edge sensors (TESs) and multiplexed SQUID readout. Keck Array consisted of five such telescopes spanning several observing frequencies. BICEP3 pioneered a new 2500-sensor receiver design, which BICEP Array will deploy in quantity at several frequencies.
Q: You said the new measurement has improved only slightly over the team’s previous result —why is it important?
A: The key advance is the addition of new Keck Array data at 220 GHz, which map galactic dust emission with a sensitivity similar to Planck’s. Removing this dust contamination accurately is currently the key limiting factor on the analysis. Right now the Keck map gives only an incremental improvement over using Planck’s dust map alone, but it’s a turning point: in coming seasons we’ll be doing a lot better.
Q: What are the next steps for the collaboration and the field?
A: BICEP is completing another big analysis of more recent data, including first observations from BICEP3. Starting next year, the team will begin fielding BICEP Array, a massive new instrument with more than 30,000 detectors. In parallel, I’m excited about observations from SPIDER, an instrument pursuing similar science with observations from a stratospheric balloon over Antarctica. My group at the U of I is working hard to analyze data from SPIDER’s 2015 flight and to prepare new telescopes for a second flight in 2019. On a longer time horizon, we and CMB scientists worldwide are developing technologies for a future generation of instruments on the ground (CMB Stage IV), from balloons, and from space.
The BICEP collaboration’s latest results are published in the November 27, 2018 issue of Physical Review Letters. This work is supported by the National Science Foundation and by the Keck Foundation. Additional support came from the JPL Research Technology Development Fund, NASA, the Gordon and Betty Moore Foundation, the Canada Foundation for Innovation, the U.K. Science & Technology Facilities Council, and member institutions. The conclusions presented are those of the researchers and not necessarily those of the funding agencies.