You may have seen in the news that scientists have made a pretty big discovery relating to the cosmic microwave background, inflation, and the early universe with the BICEP2 experiment. Let's dig in and talk about what the BICEP2 experiment is, what it found, and what this means for our understanding of the universe.
BICEP2 is an experiment located at the South Pole. It tries to measure the polarization of the cosmic microwave background. Let's break that statement down a bit. The cosmic microwave background, or CMB, is radiation left over from the early universe that permeates through space. This is the oldest light that we can observe, and it gives us lots of clues about what the universe was like when the CMB radiation was released. Now for the polarization aspect. Have you ever looked through a pair of polarized sunglasses? You may have noticed that they reduce glare. This is because your sunglasses are only letting light with a certain orientation (or polarization) through the lens. Different processes polarize (change the orientation of) the CMB light in different ways. BICEP2 sought to measure the polarization of the CMB to find out which processes were occurring shortly after the Big Bang.
There are two different kinds of CMB polarization, called E-modes and B-modes. Each creates a different pattern in the orientation of the CMB light. The B-mode polarization is special, because its signature swirly pattern can only be created by gravitational waves (or ripples in the fabric of space) produced during a period called inflation. BICEP2 successfully found this B-mode polarization in the CMB light.
So why is it such a big deal that they found the B-modes? There are a few reasons.
- B-mode polarization lends support to the theory of inflation. Inflation was first proposed to explain why the distribution of matter in our universe is so smooth. When the universe was very young and relatively small, there would have been some regions that had a tiny bit more matter than others. Then, as the universe expanded, those regions would have gotten blown up, leading to a very lumpy universe. However, we observe that matter is distributed pretty much evenly in the universe. Inflation proposes that the universe went through a period, basically right after the Big Bang, where it expanded ridiculously fast (to one hundred billion billion times its original size in a teensy fraction of a second). This fast inflation smoothed out all those little bumps, so that we now have a pretty uniform universe. This rapid expansion would have produced gravitational waves - which in turn cause the B-mode polarization of the CMB.
- This gives another piece of evidence for the existence of gravitational waves. Although gravitational waves have been theorized to exist for some time, they are yet to be directly detected. Observing the B-mode polarization is another indicator that they do indeed exist, since they are the only way in which the B-mode pattern can be created.
- It gives us an example of gravity and quantum mechanics working together. Traditionally, quantum mechanics explains how tiny particles behave, a realm where gravity just doesn't cut it. But the theory of inflation comes out of quantum mechanical ideas, while gravitational waves come from Einstein's theory of general relativity. That the two (quantum mechanics and gravity) worked together in some way in the early universe to produce the B-mode polarization tells us that they may be connected in a fundamental way. A very exciting idea, but an area where there is still much more investigating to be done.
What's next? Since the implications of this result are so huge, scientists want to see them confirmed by other experiments. The POLARBEAR experiment in Chile, as well as the Planck space observatory are looking into the polarization of the CMB, and it'll be interesting to see how their results compare to the BICEP2 results. There are also many different theories and models for how inflation went down, and scientists will be looking to see which of those are consistent with these results. So keep your eyes peeled!