SPIDER – Physics

Many of the features observed in the Universe today can be explained by a period of rapid expansion much less than one second after the Big Bang. During this inflationary period, the Universe grew by many orders of magnitude in a tiny fraction of a second. This resulted in an extraordinarily flat Universe that is homogeneous and isotropic on large scales. Although more than 13.8 billion years have elapsed since inflation ended, all of the structure that we observe today is thought to originate from the quantum fluctuations in the field driving inflation.

An important prediction of the inflationary paradigm is the existence of a stochastic gravitational wave background, which would leave an imprint in the polarization pattern of the CMB. Roughly 380,000 years after inflation, the Universe had expanded and cooled sufficiently for photons to decouple from the surrounding baryonic matter. At decoupling, gravitational waves would impart a polarization pattern in the CMB with a non-zero curl component, known as a B-mode pattern. The detection of this signal would revolutionize early Universe cosmology, revealing physics at energy scales far beyond those accessible to terrestrial particle accelerators and providing a direct probe of quantum gravity. Although B-mode CMB polarization from gravitational lensing has been measured on small angular scales, the inflationary B-mode that peak around degree angular scales have yet to be observed.

A major challenge in the search for inflationary B-modes is separating the cosmological signal from the polarized foreground emission produced by our galaxy. Foregrounds can mimic the polarization pattern of the CMB signal, overshadowing a small amplitude primordial spectrum. Nevertheless, since foregrounds have a different frequency scaling than the cosmological signal, multi-frequency measurements can be used to distinguish between radiation from the CMB and the Milky Way Galaxy.

SPIDER is a balloon-borne telescope that is searching for the inflationary B-mode signature by making polarized maps with degree-scale angular resolution across roughly 5% of the sky over the course of two Antarctic flights. SPIDER consists of six monochromatic refracting telescopes housed in a large liquid helium cryostat. Stepped cryogenic half-wave plate polarization modulators are used to mitigate instrumental systematics. The first SPIDER flight took place in January of 2015 and featured observing bands at 90 and 150 GHz. A second flight, which launched in December 2022, added 285 GHz receivers in addition to the two lower frequencies.