by Shane L. Larson
The fact that the curtain of the Cosmic Microwave Background exists is a huge boon to astronomers, giving us confidence in our understanding of Big Bang cosmology. But is that all? We have all this microwave light coming at us from every direction in the Cosmos — what can we do with it? Is there anything else we can learn?
As it turns out, there is more to the light than meets the eye. Astronomers excel at taking information from the Cosmos and digging deeper, to tease out secrets and information that at first glance are not easy to see. If you look closely at the Cosmic Microwave Background you discover some Very Important Things. One thing is that it is not absolutely, perfectly smooth. Imagine picking up a ball off a pool table — it looks and feels very smooth; that is analogous to the uniform temperature of the Cosmic Microwave Background looking the same no matter where we look on the sky. But if you take that same pool ball and look at the surface closely with a magnifying glass, you find that it has some scratches and roughness to it — they are so small as to not be detectable by your fingertips, nor to your naked eye.
The Cosmic Microwave Background is very similar. If we use a highly accurate microwave telescope to look, we find that there is a small scale “roughness” to the sky; astronomers think this roughness is an indicator of clumpiness at the time atoms formed, ultimately leading to what you and I see today — galaxies and clusters of galaxies in the Universe. This roughness was first detected by the COBE satellite in 1992. It was later mapped at even finer precision by the WMAP mission, and by the Planck mission.The spots on this map can be thought of as hot and cold spots on the sky. The red spots are the hottest, at only about 2.7 Kelvin (1 Kelvin is 1 degree Celsius) above absolute zero. The blue spots are the coolest, about 0.00001 Kelvin cooler than 2.7 Kelvin — a difference almost so small as to be undetectable except with the most exquisite astronomical instruments! Before the microwave background broke free from matter, the small variations in the density of matter were spread across the Cosmos. When the matter condensed to form neutral atoms, these variations imprinted themselves in the Cosmic Microwave Background. The spotted map of the Cosmos seen by COBE, WMAP and Planck is a message from the other side of the curtain!Is there any other property of the Cosmic Microwave Background that might be measured? Is there any other physical imprint that might tell us something about the early days of the Cosmos? Of course there is! It is called polarization. Light is known to scientists by its proper name, “electromagnetic radiation.” As the name suggests, it has an electric part and a magnetic part, called “fields.” The way we visualize light is as a wave of electric fields and magnetic fields travelling together, at right angles to each other. For any light you receive, the direction the electric field is waving defines the polarization of the wave.
Polarization encodes many kinds of information, related to how light and matter interact with one another. The most common way you encounter this every day is with sunglasses. Reflected light is polarized, so we build sunglasses that block polarized light, cutting down glare off of reflected surfaces. You might have noticed that the display of your smartphone is polarized too — you can’t see it in certain orientations through your polarized sunglasses.Light from the Cosmic Microwave Background is expected to be polarized, imprinted with patterns because during decoupling (the time when electrons were binding to nuclei to form atoms) the light bounced off of the free electrons (in physics-speak: it “scattered”) giving it a definite polarization. Astronomers call these polarization patterns “E-modes” and they are recognizable because they make symmetric patterns in the sky — the polarization pattern looks exactly like itself if you look at a reflection of the pattern in a mirror. This kind of pattern was discovered by the DASI experiment in 2002.
So what does this all have to do with inflation? Inflation happened shortly after the Big Bang, before anything that you and I might recognize as a particle had formed. The forces of Nature spontaneously appear as the Universe cools, enabling different kinds of physical interactions to appear. Gravity had appeared sometime before inflation, during a time cosmologists call the “Planck epoch.” Inflation was the sudden, rapid expansion of everything — the energy soup, and spacetime itself — from the incredibly tiny point that expanded to become the Observable Universe. Think of spacetime like a balled up bundle of wrapping paper being unfolded, flattened and smoothed out by inflation. On the smallest scales, spacetime is changing — expanding, stretching, flexing of the folds in our bundle of paper. Physicists call these happenings “quantum fluctuations.” The stretching and unfolding of spacetime means the gravitational field is changing. But that’s exactly what we said causes gravitational waves! As the Universe inflates, the quantum fluctuations in spacetime itself generate gravitational waves. This has physicists really excited, because detecting these gravitational waves would be the first hint of the quantum nature of gravity itself.
As we noted last time, gravitational waves interact with matter very weakly, so they propagate through the slowly evolving soup of the Cosmos, pushing matter here and their until the formation of the Cosmic Microwave Background. What is the net result? The net result is that gravitational waves leave a polarization imprint in the microwave light. Just as with the expected polarization from electron scattering, there is a pattern to the polarization made by the gravitational waves. Astronomers call this pattern “B-modes” — they have a twist to them. You can recognize a B-mode pattern, a twist, because in a mirror the pattern appears reversed.
Which brings us back to the story of BICEP2. Astronomers have been looking for the tell-tale twist of polarization in the Cosmic Microwave Background for some time. Major experiments have been slowly gearing up to look for and characterize the unique, gravitational-wave signature of inflation. The science team at BICEP2 won the race. What has all of us so excited is the measured value is larger than we anticipated, indicating the relative importance of the gravitational waves is large. This has provided sudden and unexpected guidance for theoretical physicists trying to model inflation, and for future experimenters attempting to build new experiments to probe the early Universe.
So what now? Astronomy is a spectator sport — we keep looking! Now that our colleagues at BICEP2 have made the initial detection, we are gearing up to look more closely at the result, and to dissect it for clues that confirm we are on the right track to understanding the Cosmos. New papers are appearing rapidly (I counted about 100 at the time of this writing). The result doesn’t precisely agree with other results we already have (there is “tension” between the results, in physics-speak), and some of us still have reserved skepticsm. The field has been thrown into a big mess, and now we have to figure out what happened. It’s a bit like coming home to find your living room in a disastrous state, and trying to figure out if it was the kids, the cat, the dog, or some combination thereof that made the mess! But astronomers don’t mind — it’s the figuring out of the mess that is so rewarding. But make no mistake — it was an outstanding achievement, a triumph of the human intellect and human ingenuity. And onward we go.————————————————–
This is the last installment in a 3-part series about the March 2014 BICEP2 announcement about the detection of a putative signal from inflation in the Cosmic Microwave Background. Part 1 can be found here.