The Sciences

South Pole Telescope and CMB Constraints

Cosmic VarianceBy Sean CarrollNov 5, 2012 6:12 PM


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The South Pole Telescope is a wonderful instrument, a ten-meter radio telescope that has been operating at the South Pole since 2007. Its primary target is the cosmic microwave background (CMB), but a lot of the science comes from observations of the Sunyaev-Zeldovich effect due clusters of galaxies -- a distortion of the frequency of CMB photons as they travel through the hot gas of the cluster. We learn a lot about galaxy clusters this way, and as a bonus we have a great way of looking for small-scale structure in the CMB itself.

Now the collaboration has released new results on using SPT observations to constrain cosmological parameters.

A Measurement of the Cosmic Microwave Background Damping Tail from the 2500-square-degree SPT-SZ survey K. T. Story, C. L. Reichardt, Z. Hou, R. Keisler, et al. We present a measurement of the cosmic microwave background (CMB) temperature power spectrum using data from the recently completed South Pole Telescope Sunyaev-Zel'dovich (SPT-SZ) survey. This measurement is made from observations of 2540 deg^2 of sky with arcminute resolution at 150 GHz, and improves upon previous measurements using the SPT by tripling the sky area. We report CMB temperature anisotropy power over the multipole range 650<ell<3000. We fit the SPT bandpowers, combined with the results from the seven-year Wilkinson Microwave Anisotropy Probe (WMAP7) data release, with a six-parameter LCDM cosmological model and find that the two datasets are consistent and well fit by the model. Adding SPT measurements significantly improves LCDM parameter constraints, and in particular tightens the constraint on the angular sound horizon theta_s by a factor of 2.7...[abridged]

Here is the first plot anyone should look for in a paper like this: the CMB power spectrum, giving the amplitude of fluctuations from large scales (left) to small scales (right). This graph shows both the most recent results from the WMAP satellite, and the new SPT numbers. The dashed line is a theoretical prediction that includes only the CMB, while the solid line is the prediction when foregrounds (our galaxy as well as others) are included. Not bad! I remember when it was considered amazing to find a single identifiable peak in the CMB spectrum, and the second one was also big news. Now we have what, nine or ten visible wiggles?

With data like these, you can do an unprecedentedly good job at constraining cosmological parameters. Here is a plot with the density of matter on the horizontal axis, and the cosmological constant on the vertical axis, using only CMB data. Show this to any remaining friends of yours who are still skeptical about the supernova observations that revealed the acceleration of the universe (if you have such friends). This is an independent detection of the cosmological constant at better than five sigma.

Since most of us have already accepted the existence of some form of dark energy, the interesting questions going forward have to do with what evidence we can extract about inflation -- did it happen, and if so what form did it take? SPT can help there, by improving our limits on the tilt of the primordial spectrum -- that is, the strength of fluctuations on small scales compared to large scales. A naive guess would be that it's perfectly flat, with equal fluctuations on all scales. But most inflationary models predict a tiny variation from perfect flatness, usually in the "red" direction -- more power on larger scales. That corresponds in this graph to


being less than one, which is indeed what the data seem to be indicating. The vertical axis is the amount of primordial gravitational waves, which is still consistent with zero according to our best current data. As you can see, different kinds of inflationary models tend to make predictions that lie in different regions of parameter space, so we are using data to constrain what might have been happening in the first 10^-35 seconds after the Big Bang. (Some of the points are labeled "chaotic inflation," where what is really meant is "power-law inflation," but that's a mistake everybody makes.)

It's almost tempting to say that we can conclude there definitely is a detectable deviation from


= 1, but I'm less ready than most cosmologists to accept that at face value. ("BAO" refers to observations of baryon acoustic oscillations in the large-scale structure of galaxies, which improve the constraints.) As the paper says, there are a number of ways to skew that result, including if neutrinos have the right kind of mass. I think cosmology should be as careful as particle physics in declaring things to be discovered, and I'm not sure we're there yet on spectral tilt. But that's certainly the way the data seem to be leaning -- it might just be a matter of time.

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