University of Minnesota
School of Physics & Astronomy


Mapping the Cosmic Microwave Background

Francois Aubin

Physicists at the University of Minnesota are getting close to having workable maps of the Cosmic Microwave Background (CMB). The CMB is a remnant of the Big Bang and understanding it in depth will help cosmologists piece together how the Universe began and evolved. One of the outstanding questions about the Universe involves an inflationary period right after the Big Bang, in which the Universe expanded at a much faster rate than it is expanding today. This period has been theorized but direct evidence for it has not yet been found.

Maps of the fluctuations of the CMB, a near perfect 2.716 kelvin blackbody, have already been measured. Physicists are working towards mapping the polarization of the CMB which can be mathematically decomposed into E and B-mode components. The E-mode component has also been well measured. The predicted B-mode signals are expected to be one part in 10 millions the magnitude of the CBM signal and would be the signature left by the primordial gravity waves, created in the inflationary period. Francois Aubin is a post-doctoral researcher with Shaul Hanany’s experimental cosmology group working to piece together the data to make these CMB maps possible.

Aubin is working data from EBEX, a balloon-borned telescope that was designed to fly a circuit around Antarctica searching the sky for evidence of B-modes. B modes are predicted to evidence of gravity waves produced during the inflationary period. By using a balloon the experiment was able to get above the atmosphere and reach frequencies that ground-based telescopes cannot reach. There was however a malfunction with the telescope in its flight and one of the control motors failed. The telescope was left to oscillate like a torsion pendulum, searching a wide band of the sky rather than the narrow pre-selected portion that they had wanted to scan. This has made the analysis more challenging and delayed the group's results by several years, but they are finally getting to the point where they have a picture of the CMB and the galaxy pop through.

Aubin says that the group is unlikely to find evidence of B-modes in their data because of the malfunction. “We are still going to get polarization measurements. We are going to be most competitive at characterizing foregrounds at our frequencies.” Ground-based telescopes are limited by water vapor in the air which absorbs and emits lots of light. A ground based telescope is constrained to observing at frequencies below approximately 300 GHZ. By going above the atmosphere, EBEX was able to observe the sky at frequencies that a ground telescope could not observe. EBEX’s polarization maps will be very useful for ground-based telescopes to verify what they are seeing. In the last few years it’s been discovered that ground-based telescopes need to remove all the foreground signal in order to get an accurate picture of the CMB. For example Clem Pryke’s experiment (BICEP2/Keck Array) used data from the Planck satellite to determine that B-mode signals they initially thought were from primordial gravity waves were actually from dust. Aubin says that EBEX’s data will be competitive with Planck because its instrument has been optimized for polarization only, unlike the Planck satellite.

Aubin has been with the Hanany group since just after the last balloon flight in Antarctica (December 2012) and is the only member besides Hanany, of the current group at Minnesota that was around to see the telescope in action. Aubin’s graduate work was to build the read-out for the EBEX detectors. Now he finds himself analyzing the data from the telescope. He also worked on a proposal for the next generation of EBEX telescope.