B modes detected!

A paper just came out describing the detection of B modes in the cosmic microwave background polarization. This is a great example of how specialized and technical science can get: to someone like me, who works on this stuff, it’s a big deal, but it takes quite a bit of effort to explain what those words even mean to a non-specialist. Peter Coles has a description. Here’s mine.

Let’s start with the cosmic microwave background, which is the oldest light in the Universe. It consists of microwaves that have been traveling through space ever since the Universe was about 1/30,000 of its present age. (If it’s made of microwaves, why did I call it light? Because microwaves are light! They’re just light with wavelengths that are too long for us to see.)

Maps of the varying intensity of this radiation across the sky give us information about what the Universe was like when it was very young. People have been studying these maps in detail for over 20 years, and were trying hard to make them for decades before that. For many years, studies of the microwave background focused on making and analyzing maps of the variations in intensity, but in recent years a lot of effort has gone into trying to map the polarization of the radiation.

Microwave radiation (like all light) consists of waves of electric and magnetic fields. Those fields can be oriented in different ways — for instance, a light wave coming right at you could have its electric field wiggling up and down or left and right. We say that this wave can be either vertically or horizontally polarized. Radiation can be unpolarized, meaning that the direction of the wiggles shifts around randomly among all the possibilities, but people predicted a long time ago that the microwave background radiation should be weakly linearly polarized, so that, in some parts of the sky, the electric fields tend to be oriented in one direction, while in other areas they tend to be oriented another way.  Lots of effort these days is going into detecting this polarization and mapping how it varies across the sky.

These polarization maps provide additional information about conditions in the young Universe, going beyond what we’ve gotten out of the intensity maps. In fact, if we can tease out just the right details from a polarization map, we can learn about entirely new phenomena that might have taken place when the Universe was extremely young. In particular, some varieties of the theory known as cosmic inflation predict that, when the Universe was very young, it was filled with gravitational waves (ripples in space). These waves would leave a characteristic imprint in the microwave background polarization. If you saw that imprint, it’d be totally amazing.

The problem is that the really amazing things you’d like to see in the polarization maps are all very very faint — much fainter than the “ordinary” stuff we expect to see. The “ordinary” stuff is still pretty cool, by the way — it’s just not as cool as the other stuff. Fortunately, we have one more trick we can use to see the exotic stuff mixed in with the ordinary stuff. There are two kinds of patterns that can appear in a polarization map, called E modes and B modes (for no particularly good reason). The E modes have a sort of mirror-reflection symmetry, and the B modes don’t. A lot of the “ordinary” polarization information shows up only in the E modes, so if you can measure the B modes, you have a better chance of measuring the exotic stuff.

That’s why people have spent a long time trying to figure out how to (a) make maps of the polarization of the microwave background radiation and (b) extract the “B modes” from those maps. The new paper announces that that goal has been accomplished for the first time.

This detection does not show anything extraordinary, like gravitational waves crashing around in the early Universe. The B modes they found matched what you’d expect to find based on what we already knew. But it shows that we can do the sorts of things we’ll need to do if we want to search for the exotica.

The fact that this experiment found “only” what we expected may sound kind of uninteresting, but that’s just because we’ve gotten a bit jaded by our success up to this point. The signal that these people found is the result of photons, produced in the early Universe, following paths through space that are distorted by the gravitational pull of all the matter in their vicinity. The fact that we can map the Universe well enough to know what distortions to expect, and then go out and see those distortions, is amazing. It’s another bit of evidence that we really do understand the structure and behavior of the Universe over incredibly large scales of distance and time.

Incidentally, the lead author on this paper, Duncan Hanson, is a former collaborator of mine. When he was an undergraduate, he and I and Douglas Scott coauthored a paper. It’s great to see all the terrific work he’s done since then.

(Just to avoid any misunderstanding, as much as I would like to, I can’t claim Duncan for the University of Richmond. Although he and I worked together when he was an undergraduate, he wasn’t an undergraduate here. He was at the University of British Columbia.)

2 Responses to “B modes detected!”

  1. Brent Follin says:

    And Duncan’s analysis was so brilliantly simple–no one was thinking about getting at the B-mode through CIB cross-correlation, but once Duncan showed the plots, it was so completely obvious how smart it was.

  2. Ted Bunn says:

    I wasn’t expecting this discovery to come yet, partly because I too wasn’t thinking in terms of a cross-correlation.

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