Archive for South Pole Telescope

Newsflash: Direct Detection of B-mode Polarization

Posted in The Universe and Stuff with tags , , , , , on July 23, 2013 by telescoper

I’m not meant to be blogging these days but I thought I’d break radio silence to draw attention to a new paper on the arXiv by Hanson et al. from SPTpol, an experiment which aims to measure the polarization of the cosmic microwave background using the South Pole Telescope. One of the main aims of experiments such as this is to measure the so-called “B-mode” of polarization (the “curl” component of the polarization signal, which possesses a handedness) because this holds the key to direct detection of a number of interesting cosmological phenomena such as the existence of primordial gravitational waves.  However, primordial effects aren’t  the only way to generate B-mode polarization. Other “foreground” effects can do the job too, especially gravitational lensing can also generate a signal of this form. These “late-time” effects have to be understood before the primordial contribution can be isolated.

Before today there was no direct measurement of B-mode polarization at all, primordial nor not.

The abstract basically says it all:

Gravitational lensing of the cosmic microwave background generates a curl pattern in the observed polarization. This “B-mode” signal provides a measure of the projected mass distribution over the entire observable Universe and also acts as a contaminant for the measurement of primordial gravity-wave signals. In this letter we present the first detection of gravitational lensing B modes, using first-season data from the polarization-sensitive receiver on the South Pole Telescope (SPTpol). We construct a template for the lensing B-mode signal by combining E-mode polarization measured by SPTpol with estimates of the lensing potential from a Herschel-SPIRE map of the cosmic infrared background. We compare this template to the B modes measured directly by SPTpol, finding a non-zero correlation at 7.7 sigma significance. The correlation has an amplitude and scale-dependence consistent with theoretical expectations, is robust with respect to analysis choices, and constitutes the first measurement of a powerful cosmological observable.

This measurement is not unexpected. Indeed, the B-mode contribution from lensing by the known distribution of galaxies can be calculated fairly straightforwardly because the physics is well understood; failure to find the expected signal would therefore have been somewhat embarrassing.  It’s a different story for the primordial B-mode because that depends strongly on what is going on in the very early universe, and that is much less certain. Although the new result doesn’t itself tell us anything new about the very early Universe it is definitely an important step on the way, and it’s a fairly safe prediction that there will be a great deal of activity and interest in CMB polarization over the next few years, including next year’s planned release of polarization data from Planck.

I’ll also note the use of Herschel-SPIRE images in tracing the galaxy images, in deference to my former colleagues in Cardiff who played a key role in developing that instrument!


SPT and the CMB

Posted in The Universe and Stuff with tags , , , , , , , , , on November 30, 2012 by telescoper

I’ve been remiss in not yet passing on news  from the South Pole Telescope, which has recently produced a number of breakthrough scientific results, including:  improved cosmological constraints from the SPT-SZ cluster survey (preprint here); a new catalogue of 224 SZ-selected cluster candidates from the first 720 square-degrees of the survey (preprint here); the first measurement of galaxy bias from the gravitational lensing of the CMB (preprint here); the first CMB-based constraint on the evolution of the ionized fraction during the epoch of reionization (preprint here); the most-significant detection of non-Gaussianity induced from the gravitational lensing of the CMB (preprint here); and the most precise measurement of the CMB damping tail and improved constraints on models of Inflation (preprint here).

Here’s the graph that drew my eye (from this paper). It shows the (angular) power spectrum of the cosmic microwave for very high (angular) frequency spherical harmonics; the resolution of SPT allows it to probe finer details of the spectrum that WMAP (also shown, at lower l).


This is an amazing graph, especially for oldies like me who remember being so impressed by the emergence of the first “acoustic peak” at around l=200 way back in the days of Boomerang and Maxima and gobsmacked by WMAP’s revelation of the second and third. Now there are at least six acoustic peaks, although of progressively lower amplitude. The attenuation of the CMB fluctuations at high frequencies is the result of diffusion damping – similar to the way high-frequency sound waves are attenuated when they pass through a diffusive medium (e.g. a gas).  The phenomenon in this case is usually called Silk Damping, as it was first worked out back in the 1960s by Joe Damping Silk.

Anyway, there’ll be a lot more CMB news early (?) next year from Planck which will demonstrate yet again that cosmic microwave background physics has certainly come a long way from pigeon shit