My research is experimental cosmology: measuring the polarization of the cosmic microwave background to constrain the physics of the very early universe.
The measurement
Inflation — a period of accelerated expansion in the first fraction of a second — predicts a background of primordial gravitational waves. Those waves would leave a curl-like, or B-mode, pattern in the polarization of the CMB that no amount of ordinary density perturbation can produce. The amplitude of that signal, relative to the scalar perturbations we have already measured well, is the tensor-to-scalar ratio r. Measuring r, or bounding it tightly, is the most direct observational handle we have on physics at grand-unification energies.
The signal is faint. It is buried under polarized emission from galactic dust and synchrotron radiation, contaminated by gravitational lensing of the much larger E-mode signal, and observed through an atmosphere that is itself a source of noise. Progress therefore comes less from cleverness about cosmology than from control of instrumental systematics, multi-frequency observation, and detector counts measured in the tens of thousands.
Experiments
Simons Observatory
Co-founded in 2016 and located on the Chajnantor Plateau in Chile’s Atacama Desert at 5,200 metres. A 6-metre Large Aperture Telescope and an array of Small Aperture Telescopes observe across six frequency bands from roughly 30 to 280 GHz. The small apertures target primordial B-modes in clean, low-foreground sky patches; the large aperture targets neutrino mass, the Sunyaev-Zel’dovich effect in galaxy clusters, gravitational lensing, and millimetre-wave transients. I serve as Principal Investigator.
POLARBEAR-2 and the Simons Array
Three telescopes in the Atacama measuring CMB polarization at degree and sub-degree angular scales, with a strong emphasis on lensing B-modes and on the systematics control required to separate them from a primordial signal.
BICEP
I conceived and led the first BICEP telescope at the Amundsen-Scott South Pole Station — the instrument that established the small-aperture, refracting, degree-scale approach that most primordial B-mode searches have used since. The 2007 Presidential Early Career Award for Scientists and Engineers recognized that work.
Instrumentation
Nearly every result above rests on hardware: cryogenic receivers operating at sub-kelvin temperatures, superconducting transition-edge sensor and MKID detector arrays, cold half-wave-plate polarization modulators, broadband anti-reflection coatings, and the calibration sources — including polarized astrophysical sources and drone-borne emitters — needed to characterize a telescope well enough to trust a part-in-a-million polarization measurement. My group works across this whole chain, and holds U.S. patents arising from it.
A complete and current list of results is on the publications page.
Brian Keating 