BICEP2, Primordial Gravity Waves, and Cosmic Inflation

“Like the microscopic strands of DNA
that predetermine the identity of a macroscopic species
and the unique properties of its members,
the modern look and feel of the cosmos
was writ in the fabric of its earliest moments,
and carried relentlessly through time and space.
We feel it when we look up.
We feel it when we look down.
We feel it when we look within.”
~Niel Degrasse Tyson

BICEP2 sunset
The BICEP2 telescope on the South Pole. This is the device which may have finally discovered primordial gravitational waves. (Credit: the BICEP2 collaboration)

There was some very big news today! If you haven’t already heard, the BICEP2 research group at Harvard has found evidence of ancient gravitational waves in the sky.

A lot of news outlets are touting this as a big discovery because it is indirect evidence for gravitational waves or because it is proof of the Big Bang. But the former reason is misleading and the latter is simply wrong. It’s big news because, if true, it’s very definitive evidence for something called cosmic inflation.

There’s already a lot of news out there on the BICEP2 discovery, but I figured I’d explain my take on it, too. Hopefully I’ll be more accurate than the standard popular-science article and less technical than the standard science blogger.

Our Universe: The Early Years

(or, more accurately, The Early Millionths of a Second)

Most cosmologists believe that in the very early universe, about 133.6 billion years ago, the universe underwent a period of extremely rapid expansion called cosmic inflation. (Although the opinion is still controversial, there are good reasons to believe inflation occurred.) In the standard story, inflation is caused by a particle called the inflaton, which–much like today’s dark energy–has some strange properties like negative internal pressure that are only possible due to quantum mechanics.

The history of the universe
A modern picture of the history of the universe, including inflation. That dramatic widening of the universe right after the Big Bang is inflation. (Image credit goes to Rhys Taylor of Cardiff University, via the Planck collaboration.)

Since the inflaton is quantum mechanical, it is a wave as well as a particle. And that wave is wobbling rapidly. The quantum wobbles can be quite large in amplitude–the amplitude of a quantum wave is analogous to the height of, say, a water wave–but they only happen over very short distances. But the rapidly expanding universe stretches these wobbles out to enormous scales.

After inflation ends, the inflaton dumps all of its energy into more typical particles like electrons and protons. This process is called reheating. (“Reheating” is a bit of a misnomer. Perhaps it should be called first-heating.) And those stretched-out quantum wobbles matter. In places where the inflaton wave had a large amplitude, we got more, faster-moving normal particles. In places where the amplitude of the inflaton wave was small, we got fewer, slower-moving particles. Thermodynamically, this meant that certain parts of the early universe were much hotter than others.

Of course, the inflaton had a lot of energy, and everywhere in the universe was so absurdly hot that no atoms could form–electrons, protons, and neutrons were all torn apart by the intense temperatures. So we had lots of charged particles flying around very fast. But accelerating charged particles emit light (which happens to be how radios work). So the early universe was very, very bright. To this day, that primordial light remains. Over time it has gotten much dimmer and much redder (thanks to cosmic redshift), but still permeates the universe everywhere. We call it the cosmic microwave background, or CMB for short.

And the quantum inflaton fluctuations are still there, too. Because they affected the temperature of the early universe, these fluctuations affected the spectrum of the CMB. The light has a higher frequency where the inflaton waves had a high amplitude and a lower frequency where the inflaton fields had a low amplitude.

Although the effect is very, very small (about one part in one hundred thousand!), we can actually observe these fluctuations in the CMB, which is why the theory of cosmic inflation has become fairly mainstream among cosmologists. The figure below is a recent map of CMB by the Planck Collaboration, which shows relative temperatures of the early universe.

Planck Survey of the Sky
Planck survey of the CMB. The oval is the observable sky. Orange areas are hotter and blue areas are colder. The scale is exaggerated to make the contrast between high and low temperatures more obvious. However, the difference is actually only about one part in one hundred thousand. Image from the Planck Collaboration.

Wibbley Wobbley Spacetime

As I’ve discussed before (at great length, since it’s one of my favorite topics), gravity is caused by the warpings and wigglings of space and time. We think of space as stretching, shrinking, and warping based on the mass and energy in the universe. This means that the shortest possible path between two points may not be what it appears. And so, even though particles all travel along the shortest possible paths through space and time, the paths can appear curved to our simplistic three-dimensional Euclidean eyes.

If you like, you can think of mass as actually causing empty space to be added or removed. Distances are shrinking or growing and angles are changing. Even though we can attribute this to the stretching or warping of space, I describe it as empty space being created or destroyed because I think it helps us understand exactly what a primordial gravity wave is. And the term itself is actually a bit misleading; the gravitational waves recently detected by the Harvard research team, are nothing like the ones which we hope to detect with laser interferometers like LIGO and LISA.

Quantum Wobbles

Quantum mechanics tells us that the world is inherently probabilistic, so even highly improbable things happen. Empty space is no exception: space is only empty on average. Because of quantum fluctuations, particles are constantly appearing and disappearing in so-called empty space. Indeed, space is buzzing with particles that only exist for a short period of time. These are called virtual particles. It sounds crazy, but it’s true. We even have experimental evidence.

You’ve probably heard that bit before, but here’s the clincher: Just like particles constantly appear and disappear, so does empty space. Even if there’s no mass in the universe to warp or stretch spacetime, it warps and stretches all by itself because of quantum fluctuations. This is what people mean when they talk about primordial gravity waves.

(Actually, there’s another way to think about this, related more closely to virtual particles. We already learned that every wave has a particle associated with it. Space and time can warp in a wave-like way, similar to an electromagnetic wave. And just as electromagnetic waves have the photon as an associated particle, gravitational waves have the graviton. And there are virtual gravitons that fluctuate in and out of existence, just like other virtual particles.)

And spacetime itself is affected by inflation in the same way that the inflaton is. Although the fluctuations in spacetime occur on extremely short distances, when the universe undergoes inflation, the quantum fluctuations in spacetime get expanded to enormous scales.

These fluctuations are now essentially impossible to see directly, but we can look for their signature in the CMB. As the CMB photons pass through areas where spacetime is warped, they change polarization depending on how extreme the warping is. I won’t go into detail about what polarization is right now (in a later article, I promise!), but suffice to say that it is a property of light and we can represent it as an arrow perpendicular to the direction a photon is traveling, as shown below. And when we talk about the direction of polarization, we are talking about the direction this arrow is pointing.

This is how polarization works!
We can represent polarization as an arrow (blue) perpendicular to the direction a photon is moving (red).

As a further test of inflation (and a probe into what caused inflation), we can try to observe the polarization of the light in the CMB. At each point in the sky, we would measure the polarization and describe it as an arrow pointing in the direction of polarization. Unfortunately, this measurement is fiendishly difficult… much more difficult than measuring the frequency and intensity of the light from the CMB.

However, this is precisely what the BICEP2 team claims to have achieved. They couldn’t measure the entire sky like Planck did for the temperature associated with the CMB, but they did measure a small piece of the sky (plotted below). The lines represent the directions of polarization. The colors represent the polarization’s “B-mode pseudoscalar,” which measures how much the lines form a spiral shape. The pseudoscalar patterns that BICEP2 observed is characteristic of primordial gravity waves.

BICEP2 polarization map
The polarization of the cosmic microwave background due to primordial gravity waves, as measured by the BICEP2 team. The lines represent the directions of polarization and the colors represent the B-mode pseudoscalar of the polarizations.

Implications

First and foremost, the BICEP2 results are not the first indirect measurement of gravitational waves. The first indirect measurement of gravitational waves won the Nobel prize. They are also not evidence for the Big Bang theory. The CMB and the expanding universe are evidence enough for that.

The BICEP2 results are extremely strong evidence that our understanding of the universe after the Big Bang is correct and that cosmic inflation did indeed happen. Up until this point, inflation has been somewhat controversial. It successfully makes predictions, but it has some conceptual problems. Observation of primordial gravitational waves would put this controversy to rest. These observations can also offer insight into how inflation started. Understanding how the inflaton grabbed the tiny fluctuations in spacetime and expanded them will help us understand the inflaton a lot better.

Finally, the BICEP2 results are the first real measurement we’ve ever made of quantum gravity. Describing the quantum fluctuations in spacetime is tricky business and we really don’t have a good method for it. This is a huge issue in physics at the moment, called the “problem of quantum gravity.” However, in some special cases, where space and time are relatively well behaved (in a technical sense) and where the fluctuations are small, we can come up with a good mathematical description. This kind of math leads to some pretty mind-boggling things, such as Hawking radiation. But if we try to go beyond the simplest cases, the math blows up in our faces. A measurement of primordial gravitational waves tells us that, at least in the simplest cases, we’re on the right track.

Tensions?

By assuming a cause for inflation, cosmologists have been able to analyze temperature measurements made in the past (like the Planck map above) and propose a rough upper bound on how much of a signal we should see from primordial gravity waves. It looks like the BICEP2 results violate this upper bound. This isn’t necessarily a bad thing–indeed, it makes inflation more certain, not less, and perhaps implies new physics. But it does mean that the scientific community is fairly skeptical of the BICEP2 results.

Fortunately, BICEP2 isn’t the only telescope on the job. A huge number of other collaborations are trying to study the polarization of the CMB. (My good friend Sara Simon is part of the Atacama B-Mode Search team, for example.) BICEP2 is just the first group to gather and analyze their data. Once the other teams finish gathering and analyzing their data, we’ll be able to say for sure whether or not BICEP2’s conclusions were correct.

Further Reading

There is a lot of information related to the BICEP2 results out there. If you’re curious, here’s some more to read about them.

 Related Articles

If you’re confused, here are some articles I’ve written in the past:

Questions? Comments? Insults?

Although my research is in gravity, I don’t do active research in cosmology. So if you know better, please correct me! (And, as always, please ask any questions you may have.)

About jonah

Jonah is the author of The Physics Mill.
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