University of Minnesota
School of Physics & Astronomy


Polarimetry at the South Pole

Stefan Fliescher
Stefan Fliescher in Antarctica

Stefan Fliescher is a post doc working with Clem Pryke on the BICEP2 and Keck Array experiments. Both these experiments are polarimetry telescopes located at the South Pole which track the Cosmic Microwave Background (CMB). The CMB is a remnant signal from the Big Bang. Pryke’s group is looking for B Mode patterns in the CMB which would be confirmation of the inflationary model of the early Universe. Cosmologists have long predicted that Universe underwent a period of rapid, exponential expansion in the first few fractions of a second after the big bang.

Confirmed B Mode patterns would also be evidence of gravitational waves, which were predicted by Einstein’s theory of general relativity, but have not yet been observed.

Pryke’s group uses a ground-based approach which has the advantage of being able to take continuous data over many years, whereas space or balloon-borne experiments have a limited flying time. The telescopes are located on the Antarctic plateau, on top of a large flat glacier, that is at 10,000 feet above sea level. The Antarctic climate is incredibly dry at that elevation the telescopes are situated above the vast majority of water vapor in the atmosphere.

Fliescher’s role in the experiment is doing data analysis. His recent work has focused on analyzing BICEP2 data and attempting to remove signal interference from cosmic dust. This dust can give a false positive for B Modes so it is very important to clean it away from the data. Fliescher combined BICEP2 data with Planck Space Telescope data taken in the same part of the sky. There, (referring to 'the same part of sky') the Planck telescope’s polarimetry equipment measurement is less sensitive than the ground based telescopes that Fliescher works with, but it has the advantage that the data is at several a bands of frequencies, whereas BICEP2 is tuned only to look for the polarized light from the CMB. After this virtual scrubbing which took place mostly on runs at different supercomputers, Fliescher and others determined that there is no evidence for gravitational waves within Bicep2's measured B Mode signal. While this was obviously a disappointing surprise, it has been an important step forward not just for BICEP2 and the Keck Array but for all experiments that are searching for B Modes. "We thought we had a clean window, now we see that the window was not as clean as we’d hoped."

Fliescher’s work is not limited to data analysis. BICEP2 and Keck are relatively small experiments and everyone involved needs to be able to do a bit of everything to keep the experiment running, which includes taking shifts in Antarctica. Fliescher has been to the Pole three times, always in the "summer" which is the only time that physicists can actually get to the telescope. He is also involved in designing and building BICEP3 which is the next generation that will allow the group to measure the CMB with even greater sensitivity. The next step is to produce more sensitive maps at different frequencies. In 2014 the Keck Array measured at two frequencies, and in 2015 it is working on three. “BICEP3 was installed with a huge effort over the summer, and we have now seen first light from this telescope.” Though Fliescher was not involved in installing BICEP3, he has the satisfaction of personally putting hardware into place for the Keck Array. "It is always fun to go into the lab, build something and then be able to go down to the pole to install it."