Research Highlight
April 23, 2012
The South Pole Telescope Measures Small-scale Structure of the Cosmic Microwave Background to Unprecedented Precision
by The SPT Team

The 10-meter South Pole Telescope (<a target='_blank' href=''>SPT</a>) is a millimeter/submillimeter-wave telescope designed to make low-noise, high-resolution images of the cosmic microwave background (CMB) radiation. The first camera on the SPT was a three-color, ~1000-element bolometer array, one of the largest and most sensitive millimeter-wave receivers ever constructed. This ultra-sensitive receiver, combined with the large collecting area, large field of view, and low-noise design of the telescope---and with the pristine observing conditions available from the South Pole---enabled the SPT team to map 2500 square degrees of the southern sky at previously unattainable levels of sensitivity and angular resolution. The primary science goal of this first receiver was to make a census of distant, massive galaxy clusters and use that information to constrain the properties of Dark Energy, the mysterious agent behind the accelerating expansion of the universe. The same data used for the cluster survey, however, also provides the most precise measurement yet of the small-angular-scale CMB.

In a paper published in December in the Astrophysical Journal (R. Keisler et al., 2011 ApJ 743 28), the SPT team used approximately one third of the total 2500-square-degree survey data in just one of the three colors to make the most precise measurement yet of the region of the CMB angular power spectrum known as the damping tail. The CMB power spectrum (which measures fluctuations in CMB temperature as a function of angular scale) is predicted to have a characteristic series of peaks, caused by oscillations in the tightly coupled plasma of charged particles and photons in the early universe.

The first peaks were discovered by ground- and balloon-based experiments and have now been measured extremely well by the WMAP satellite. Subsequent peaks are more difficult to measure, both because they trace smaller angular scales, which are difficult to resolve with single-dish microwave telescopes, and because the amplitude of these peaks is predicted to decrease. This decrease in amplitude is due to the not-quite-perfect coupling between matter particles and photons, and this region of the CMB power spectrum is commonly known as the damping tail. The combined data from all previous instruments had clearly begun to resolve the third peak and hint at further peaks, but the power spectrum measured by SPT clearly shows at least seven peaks (see Figure 1).

Figure 1.

The new SPT measurements have led to improved cosmological constraints as well as hints of some surprising results. The added cosmological power is due to the sensitivity of the amplitude and shape of the damping tail to physics from the very early universe through the time of emission of the CMB. Any process that changes the amplitude of small-scale fluctuations or that affects the balance of free and bound electrons near the time of the emission of the CMB will show its effects in the damping tail. For example, the period of exponential inflation that we believe expanded a tiny patch of spacetime into our observable universe leaves an imprint on the CMB in the ratio of large-to-small-scale fluctuations, characterized as n<sub>s</sub>, the spectral index. When combined with WMAP data (see Figure 2), SPT data improves the constraint on n<sub>s</sub> and gives strong evidence that a period of inflation did, in fact, occur.

Figure 2.

A slightly surprising result from the SPT measurement is that there appears to be stronger damping than the simple 'concordance' cosmological model would predict. This excess damping could be explained by a higher fraction of primordial helium than is indicated by local measurements, a non-standard number of relativistic particle species in the early universe, or a non-standard shape of the primordial power spectrum. If any of these scenarios is borne out, it would have profound implications for fields of physics well beyond CMB studies and cosmology.

The SPT team is currently working on a measurement of the CMB power spectrum from the full 2500-square-degree survey, and the team recently installed a new, even more sensitive camera on the telescope (one capable of measuring not just the intensity of the CMB radiation but also its polarization properties). Results from the SPT in the near future will further exploit the promise of cosmological observations to directly constraining fundamental physics.

The SPT is a collaboration among scientists at several institutions including the University of Chicago / KICP, Argonne National Laboratory, Cardiff University, Case Western Reserve University, Harvard University, Ludwig-Maximilians-Universit, Smithsonian Astrophysical Observatory, McGill University, University of California at Berkeley, University of California at Davis, University of Colorado at Boulder and the University of Michigan.

The SPT is funded primarily by the NSF Office of Polar Programs. Partial support is also provided by the NSF-funded Physics Frontier Center of the Kavli Institute for Cosmological Physics, the Kavli Foundation and the Gordon and Betty Moore Foundation.

KICP members participating in the South Pole Telescope collaboration include KICP faculty John Carlstrom (PI), Mike Gladders, Wayne Hu, Andrey Kravtsov, and Steve Meyer; senior researchers Clarence Chang, Tom Crawford, Erik Leitch, and Kathryn Schaffer; postdocs Brad Benson, F. William High, Steven Hoover, Ryan Keisler, Jared Mehl, and Tom Plagge; and graduate students Lindsey Bleem, Abby Crites, Monica Mocanu, Tyler Natoli, and Kyle Story.