New Tricks from Old Light
By Josh Roberts and Ryan Wyatt
In cosmology (the study of the origin and evolution of the Universe), inflation refers to a period in the very, very, very early history of the Universe (like, in the first 10–32 seconds), when space expanded faster than the speed of light.
That sentence should make your head hurt.
So let’s take a step back. Way back! About 13.8 billion years…
You’ve probably heard of the Big Bang. (If you haven’t, or even if you have, we invite you to visit the Academy for a showing of our latest planetarium show, Dark Universe, which deals with some of these topics.) The phrase “Big Bang” often calls to mind a tremendous explosion, but physicists have learned to think of it as an expansion, a change of state, or a transition in phase for the entire Universe. However we describe the process, the Universe started out small and is getting bigger: we can study our expanding universe and extrapolate backward to understand what it must have been like in its hotter and denser state.
The earliest light we can detect dates to an epoch 380,000 years after the first moments of the Universe, and while that might seem like an astoundingly long time to us humans, it’s dwarfed by the 13,800,000,000 year age of the Universe. This signal, known as the Cosmic Microwave Background Radiation (aka CMBR or CMB to its friends), is the ambient glow of the dense particle soup that filled our universe when it had just become spacious enough to allow light to move through it. The expansion of the Universe has stretched the once-visible light to longer and longer wavelengths, so today it appears in the microwave portion of the electromagnetic spectrum (hence the “M” in “CMB”).
For the past several years, astronomers have studied the CMB using state of the art instruments at the BICEP2 facility located near the South Pole. They seek to measure a key attribute of the CMB’s light: its polarization. Yes, much like the sunlight blocked by your high-quality sunglasses, this ancient light is polarized!
As it turns out, light can be polarized in many different ways—and different phenomena can cause this polarization—but luckily, we understand the types of polarization and their causes pretty well. The “swirly” pattern (we’re quoting a Stanford researcher on that) is caused not by the density of matter, as expected from light coming from the early and dense universe, but from a phenomenon predicated by Albert Einstein called a gravitational wave. Gravitational waves have proven difficult to observe directly, although their effects are inferred from the behavior of orbiting pulsars (and inferred well enough to merit the 1993 Nobel Prize in Physics).
And this is where inflation comes in.
Cosmologists had predicted that the rapid expansion of space during inflation would cause gravitational waves to propagate through the (at the time) tiny, young universe. The polarization of the CMB allows us to probe the strength of these gravitational waves, which is in turn directly related to the energy scale of inflation. In a phone conversation, Academy Fellow Joel Primack commented on today’s announcement, explaining that “most of us have taken inflation seriously for long time,” but the enormous strength of the signal—and thus the energy of inflation—came as a bit of a surprise.
If the BICEP2 result (clocking in at about 1016 GeV) is correct, then the energy scale is “just about as high as possible,” said Primack, quite close to the grand unification energy (the energy at three of the four known forces become equal in strength, also around 1016 GeV) and not far from the Planck energy scale (considered a “maximum” energy of sorts, around 1019 GeV). As Clem Pryke of the University of Minnesota and co-leader of the BICEP2 team put it, “This has been like looking for a needle in a haystack, but instead we found a crowbar.”
By poring over their data for years, the BICEP2 team is confident that the rather robust signal they are seeing in the data can be attributed to the attributes of the early Universe—and not, for example, to a galaxy cluster that may have warped the passing light (it’s been traveling for billions of years, after all), which might cause similar distortion. Since their data fit the structure of the CMB quite well, the scientists remain optimistic that their findings will soon be confirmed by other similar projects already underway—for example, POLARBEAR, headquartered just across the Bay at UC Berkeley.
So, yes, today’s announcement, in the words of the press release, provides the “first direct evidence of cosmic inflation.” But more importantly, it gives cosmologists actual data to refine their theories! As Primack put it, “inflation isn’t a theory so much as a strategy,” with specific manifestations that vary widely in their details. Today’s announcement provides significant constraints on what theorists can invent, narrowing the search for future inspiration.
Josh Roberts is a Senior Presenter and astronomer at the California Academy of Sciences. He also contributes to Morrison Planetarium productions and is involved in Bay Area astronomy outreach. Ryan Wyatt is Director of Morrison Planetarium and Science Visualization at the Academy.
Image: Steffen Richter, Harvard