How can a difference in light be detected? Isn’t light just light?
What’s going on is that light has polarization. The electric field vibrates in a certain direction. You wear polarized sunglasses because the light bouncing off the highway vibrates horizontally and the glasses only allow light to pass through that is vertical. So, the glasses cut off the glare.
What these guys did was essentially build a telescope which looks for that kind of linear polarization in the cosmic microwave background. It’s not very big (the change in polarization); it’s only a weak thing. But they’re able to make a map, saying in this part of the sky, the light is polarized in this direction, and this part of the sky is polarized in a different direction.
Photo: A graduate student tests the BICEP2 readout electronics from the warm environment of the Dark Sector Lab. (Steffen Richter/Harvard University)