SPT and the CMB

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


2 Responses to “SPT and the CMB”

  1. One lay person begging for a little simplification here: I have seen this sort of figure in reports of the SPT many times in the last few weeks but none of the write-ups either seem to understand it deeply enough or can be bothered to explain it in simple terms: So I guess these progressively more damped peaks in the figure are a sort of Fourier transform from (possibly) solid angle of the sky “domain” to this multipolar moment “domain”? OK they are getting damped at higher values of l (el) but what I really want to know is: what do the very regular peaks at increasing value of l (el) signify in terms of the structure of the early universe.

    Any volunteers?

    • telescoper Says:

      That’s an interesting challenge. My lazy reaction is to point you to Wayne Hu’s tutorial here:


      But here’s a much simplified version. The peaks in the power spectrum indicate that the initial perturbations were generated coherently, i.e that they were excited in phase with each other. Incidentally, this is a good argument for inflation as their origin, but that’s not really relevant). Since these perturbations are in the form of sound waves which propagate at a particular speed at some specific point later different frequency components will be at different points of their respective cycles. The hot matter-radiation plasma oscillates owing to these acoustic perturbations until the epoch of decoupling whence photons no longer interact with matter and just stream to us to form the CMB.

      The acoustic peaks are formed by wave modes that are just at the maximum (or minimum) phase of their cycle when decoupling happened. The first peak is the longest oscillating mode (wavelength corresponding to the sound horizon scale when decoupling happened); other peaks are like harmonics of this.

      Two things are important. One is that if the perturbations were excited incoherently then these peaks would not appear – the spectrum would be featureless. The other is that the CMB spectrum involves projecting the sound waves travelling in different directions onto the sky so there isn’t a clean correspondence between the 2D l and the 3D wavevector k. This smooths out some of the structure, and makes the acoustic peaks less pronounced, but doesn’t change their position.

      As you can probably imagine the relative heights and positions of these peaks tell us a huge amount about what was going on in the early Universe and also what happened to the photons between decoupling and now, i.e. what what happened to the Universe they travelled through to reach us.

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