Precision measurements of the Cosmic Microwave Background (CMB) radiation have revolutionized our understanding of cosmology, and there is still much that we can learn from it. Many of the features observed in the Universe today can be explained by a period of rapid expansion less than one second after the Big Bang. During this inflationary period, the Universe grew by many orders of magnitude in a small fraction of a second, resulting 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. While the magnitude of the energy scales involved and the complexity of the subsequent evolution make inflation a difficult epoch to study, there is one observable consequence that offers new opportunities for understanding early universe physics.

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. On degree angular scales, this signature could dominate the B-mode CMB power spectrum, providing an important test of inflationary physics. 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. The ratio of the power in the inflationary gravitational waves to the CMB temperature anisotropies is known as the tensor-to-scalar ratio, r, and it is directly related to the energy scale at which inflation occurred in a given model. In this way, either a detection or a better upper limit on r would transform our understanding of early universe physics by ruling out viable inflation models from the vast parameter space. Although B-mode CMB polarization from gravitational lensing has been measured on small angular scales, inflationary B-modes 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. Although the level of polarized foregrounds varies widely across the sky, recent measurements have revealed that even in the cleanest regions, the amplitude will be problematic for increasingly sensitive CMB experiments. 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 galaxy. Although the Planck satellite has made full sky polarization maps in many relevant frequency bands, this measurement is not sensitive enough for the current generation of experiments, which will need to incorporate more precise foreground measurements.

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 10% of the sky over the course of two Antarctic flights. The ballooning platform is well-suited to this measurement because it gives access to large sky coverage and high frequency channels while significantly reducing atmospheric photon loading and noise fluctuations relative to ground-based observations. 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 will include 285 GHz receivers in addition to the two lower frequencies. The B-mode polarization results from SPIDER’s 2015 flight are currently in preparation for publication, and this dataset was also used to publish a new upper limit on CMB circular polarization by exploiting non-idealities of the polarization modulators.


SPIDER’s Homepage

A High-Flying Web May Catch the Beginning of Time, Scientific American

Big Bang to be Investigated from Balloon in Antarctica, The New York Times

Canadian scientists on Antarctic mission aim to reveal cosmic origins, The Globe and Mail 

Big Bang by Balloon, Starts with a Bang!