Fig. 1: large-scale polarization structure of the CMB superimposed on the temperature anisotropy map. Click here to enlarge
. NASA/WMAP Science Team.
Our story so far:
WMAP first impressions
WMAP and the Axis of Evil
WMAP looks through a galaxy
And now a topic near and dear to my heart, the polarization of the CMB. The WMAP team claims to have detected polarization, although it's a pretty weak and ratty signal and there's massive foreground contamination to deal with. Which is good for me, since there's no shortage of ground left for my mission to cover, although it also emphasizes just how difficult this field is going to be.
First off, let's tackle what I've labeled as Figure 1, a press image that's been widely disseminated. When the 3-year data release appeared, I heard from a number of quarters a sort of "my word, is that their data?" exclamation upon seeing it. Now the underlying temperature map is essentially a linear combination of the temperature data in the five bands that minimizes the foreground contribution (principally from the galaxy), and except perhaps in the plane of the galaxy is highly robust. The big white lines do represent polarization angles, but are far removed from raw data.
Compare to the image I've labeled Figure 2.
According to the Page et al Polarization Analysis paper (available here), the 61 GHz band was the least contaminated with polarized foreground signal, with synchrotron emission rising at lower frequencies and thermal radiation from warm dust kicking in at the highest band. Even so, the polarized part of the CMB amounted to around 0.3 μK of the entire signal, while even in the cleanest band the foreground polarization averaged twice that much. In Figure 2 the color indicates the total polarized intensity. The white lines again indicate the polarization angle measured on the sky, but this time at something like the resolution of the instrument1. A line is only drawn where the polarization signal-to-noise (S/N) ratio is greater than one, and even after some binning there's not a whole lot of lines that aren't part of the galactic plane.
Increasing the number of samples that go into a single measurement boosts your signal faster than the noise (as the square root of the number of samples, generically). Thus three years of WMAP data are able to reveal details that couldn't be detected in a single year. Or, given a single set of data, it's possible to improve S/N by sacrificing resolution and averaging pixels together, just as is done in Figure 1. As it happens, it's possible to get a handle on reionization using just polarization data at the largest angular scales, so the WMAP team averaged over large enough patches of the sky to leave just the l=2 through l=6 multipole modes (I touched on multipoles earlier in this series). That's what was chosen for display in the nice press release image.
Fig. 2: V-band (61 GHz) polarization intensity and averaged S/N>1 vectors. Click here to enlarge
. WMAP Science Team
An l=6 multipole has a characteristic scale of 30° or so, which at the distance of the CMB corresponds to lengths far greater than the horizon size at that epoch. These sorts of large-scale patterns cannot be intrinsic to the CMB; they must arise from physics going on closer to us. To get a feel for what this means, imagine this: shortly after the Big Bang, a magical indestructible transmitter sends out a signal. About 300,000 years later it is received by a moderately less magical station during the epoch of recombination. If you did this experiment today the two receivers would be very nearly 300,000 light-years apart, but thanks to the expansion of the universe that isn't true in any useful sense. However, it turns out that if you observed both of these stations while taking measurements of the CMB, they would appear to be roughly 1° (two full moon's widths) apart on the sky. Anything more widely separated than that hadn't yet had a chance to exchange information, so there can't be any coherent structure in the CMB on larger scales2.
In fact, according to the current models, we do expect to see some power in the l=6 and larger polarization multipoles due to scattering during reionization. Post-recombination, the universe evolved in relative darkness for nearly a half-billion years, until it was fully a tenth of its present size. Then the light from the first stars and quasars began to permeate space, creating a patchwork of ionized plasma interspersed with the neutral soon-to-be intergalactic medium. Although few and far between compared to CMB photons, liberated electrons managed to scatter a small portion of the microwave energy, and this led to polarization writ large, if faintly. The amount of scattering is encapsulated in the τ term that I mentioned; now that we know it's value moderately well, it is possible to say that the first generation of stars was born when the universe was approximately 400 million years old.
This is just the beginning, though. There's an abundance of new science to be done with the polarization of the CMB, but it'll take experiments more sensitive than WMAP. The next steps will require raw sensitivity at least ten times better than WMAP, accompanied by clever techniques for removing the foreground noise, and at higher resolution to boot. This generation of experiments is already in design and under construction, so expect results on this front in the next handful of years.
1 The beam size at V-band is about 30 arcminute. In the sort of high-S/N regime that, say, optical astronomers work in you could do image deconvolution and get down to 10' or better pixel resolution. Here, you instead resort to averaging many beams into a single pixel to push the S/N ratio up. So in Figure 2, the polarization has been smoothed to 2 degree pixels.
2 That's probably not true. Blame inflation, which can and does create structures this large and more, right up to waves longer than the observable universe today. However, I'm talking about the polarization of the CMB as seen by WMAP, in which case inflation doesn't play a detectable role. So I'm doing like your high school physics teacher did, and sticking to the simple story.
Tags: CMB, WMAP, astronomy, Cosmology, space