Second Round for POLARBEAR: Improving Measurements of CMB Polarization

Cosmology (the study of the origin and evolution of our universe) may seem to be one of the most daunting subfields of physics. We can’t travel back in time to see the Big Bang for ourselves or even any light that has escaped from its very beginning. Fortunately, the universe did leave us a trace of its childhood through the Cosmic Microwave Background (CMB). The CMB is made up of the oldest photons we will ever be able to detect.

Just after the Big Bang, our universe existed in a very hot and dense state. Atoms were not able to exist until the universe was about 300,000 years old and had expanded enough so that the temperature fell to about 3000 K (5000°F). After the formation of neutral atoms, photons were able to travel freely without scattering and their orientations or polarization changed.

Here at Cal, Prof. Adrian Lee’s experimental cosmology group is looking at the polarization of the CMB. In an interview in 2014, Adrian used a romantic metaphor to describe this polarization, “Think of it like this: the photons are bouncing off the electrons, and there is basically a last kiss, they touch the last electron and then they go for 14 billion years until they get to telescopes on the ground. That last kiss is polarizing.” He describes the stage in our universe’s history where photons can finally travel freely. More specifically, the POLARBEAR experiment looks at B-modes of the CMB. These B-mode polarization patterns formed at the photons’ last point of scattering, if the matter nearby was unevenly distributed. B-mode polarization also occurs after the photons have begun travelling freely when passing through large gravitational fields. Photons can be both B-mode and E-mode polarized, and these patterns differ geometrically as seen in the image below.


The current experiment POLARBEAR 1 (PB1) employs microwave detectors on the Huan Truan Telescope in a Chilean desert. POLARBEAR is a collaboration of over 70 researchers around the world and was first used for observations in 2012. PB1 was the first experiment to be successful in detecting pure B-modes that form through gravitational lensing. This experiment enabled the group to determine the total mass along the path of each photon, and their main focus so far has been to map the matter distribution of the universe back to the “inflationary” period of the universe. Inflation is a term used to describe the exponential expansion of the universe in extraordinarily little time very shortly after the Big Bang. POLARBEAR measurements have very wide astrophysical applications, including providing evidence for inflation, constraining the mass of the neutrino, and dark energy’s evolution.


POLARBEAR 2 (PB2), the newest addition to the POLARBEAR experiment, will include three different experiments and new telescopes with higher sensitivity. Its first update to PB1 will be released for observing sometime late next year. The three telescopes will be built to eventually completely replace PB1 in the next three years. “The biggest deal is that PB2 is much more sensitive than PB1. Our ability to measure the actual signal of the CMB is limited by our number of detectors. PB1 has only a small set of detectors, with only one detector attached to each antenna. Whereas on PB2, we’re making our focal plane, where all of our detectors sit, a lot larger. This enables us to more than double the amount of our detectors,” said Charles Hill, a third year graduate student working on the PB2 experiment.

This international team is working tirelessly to produce the second round of experiments for POLARBEAR.


If you would like to read more about the POLARBEAR telescope and see who makes up the team of collaborators from UC Berkeley and beyond, see the experiment’s website below:


You can read the 2014 UC Berkeley news article about POLARBEAR here:


You can read more about E-mode and B-mode polarization here:

Leave a Comment

Your email address will not be published. Required fields are marked *