Collaborative Research: Imaging the Beginning of Time from the South Pole: Completing the BICEP Array Survey

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The theory of the "Big Bang" provides an established cosmological model for the origin of our Universe from its earliest known periods through its subsequent large-scale evolution. However, this theory leaves open the question of explaining the initial conditions. Current thoughts are consistent with the entire observable Universe being spawned in a dramatic, exponential "Inflation" of a sub-nuclear volume that lasted about one trillionth of a trillionth of a trillionth of a second. Following this short inflationary period, the Universe continues to expand, but at a less rapid rate. While this basic "Inflationary paradigm" is accepted by most cosmologists, the detailed physics mechanism responsible for inflation is still not known, but there is a testable prediction that this violent space-time expansion would have produced primordial gravitational waves now propagating through the expanding Universe and forming a cosmic gravitational-wave background (CGB). The CGB amplitude defines the energy scale of Inflation that imprints a faint signature in the polarization of the Cosmic Microwave Background (CMB) radiation. Therefore, detecting this polarization signature is arguably the most important goal in cosmology today. This award will continue addressing the oldest question ever posed by mankind "How did the Universe begin?", and it does so via observations made at one of the harshest places on Earth – the Amundsen-Scott South Pole Station in Antarctica. The most recent, community driven Decadal Survey Astro2020 report “Pathways to Discovery in Astronomy and Astrophysics for the 2020s” reaffirmed the importance of search for B-modes polarization signatures of primordial gravitational waves and Inflation, and specifically endorsed the CMB Stage-4 science to be pursued by systematically supported CMB experiments in Antarctica and Chile. The recently released BICEP results place stringent limits on Inflationary models which, for the first time, go well beyond what can be done with temperature data alone, and which rule out two entire classes of previously popular single-field models—natural Inflation and simple monomial potentials. This award aims to complete deployment of all four BICEP Array receivers and then operate them as the Stage-3+ generation observing system. BICEP Array will measure the polarized sky in six frequency bands to reach an ultimate sensitivity to the amplitude of PGW of σ(r) ≲ 0.003, extrapolating from achieved performance, and after conservatively accounting for the Galactic dust, Galactic synchrotron and CMB lensing foregrounds. These measurements will be a definitive test of slow-roll models of Inflation, which generally predict a gravitational-wave signal above approximately r=0.01. BICEP Array will therefore realize the goal set by the NASA/DOE/NSF Task Force for CMB Research in 2005 to achieve sensitivity at this level, and confirmed as “the most exciting quest of all” by the Astro2010, and advance the B-mode search strongly endorsed by the Astro2020 Decadal Survey. The project will continue to provide excellent training for undergraduate and graduate students and postdoctoral fellows including those from underrepresented groups in laboratories that have exceptional track records in this regard. Cosmology and research in Antarctica both capture the public imagination, making this combination a remarkably effective vehicle for stimulating interest in science. This project advances the goals of the NSF Windows on the Universe Big Idea.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.