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Physics + MathPhysics & Math

The B-Mode Story You Haven’t Heard

The BICEP2 experiment's discovery of polarized light from the time of inflation has not help up to scrutiny, but B-modes are still revealing cosmic secrets.

BySarah ScolesThe Nature of RealityThe Nature of Reality
The B-Mode Story You Haven’t Heard

It’s a tough time to be a B-mode.

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B-modes rocketed to fame in March 2014 , when a team of scientists working on an experiment called BICEP2 announced the first direct evidence for primordial gravitational waves rippling out from the earliest moments of the universe: a distinctive imprint in the cosmic microwave background (the light left over from the Big Bang) called “B-mode polarization,” or B-modes for short. Polarization describes the way that light-waves are oriented, and B-mode waves are twisted into a swirling pattern. The detection of these swirls was a stunning confirmation of the theory of cosmic inflation. Suddenly, B-modes were electromagnetic celebrities.

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Polarized emission from Milky Way dust, which complicates the search for B-modes from the era of inflation, as seen by the Planck telescope. Credit: ESA and the Planck Collaboration

But the story went sour just weeks later, when doubt arose that the B-mode signal was really coming from the era of inflation. Eleven months after their first, much-buzzed-over announcement, the researchers conceded that some or all of the CMB swirling might actually be due to nearby galactic dust. They had not necessarily detected gravitational waves at all, but perhaps just flecks of dirt floating in the space around us.

And so, deserved or not, the B-mode story became a cautionary tale. But in reality, there are a variety of different astrophysical processes that create B-modes—processes better understood and tested than inflation—and B-modes can reveal a great deal about the universe. Telescopes like POLARBEAR , now being expanded into a three-telescope set called the

Simons Array , can study not just the birth of the cosmos but also the galaxies it has produced, the dark matter it tries to hide, the oldest magnetic fields it made, and more. They have been returning results this whole time, and what they discover about the universe is only going to get more interesting as they continue to spy into space.

Mapping the Milky Way’s Magnetism

After the controversy, dust seemed like the enemy. But, as BICEP2 team member Brian Keating of the University of California, San Diego says, “One scientist’s trash is another scientist’s treasure.” And this dusty trash tells us about our own galaxy. The grains, left behind from star formation and spewed out in supernova explosions, line up like soldiers along the Milky Way’s magnetic field lines. “They become a swirling pattern of microwave emission that looks like B-mode polarization,” says Keating.

By tracing out the dust’s swirls, scientists can make a better map of the galaxy’s magnetic field. But that’s not a trivial pursuit, says Jamie Bock of Caltech, because it’s hard to make a 3-D map of a structure that encloses you. Imagine trying to make a realistic map of a forest without leaving your campsite! The cosmic cartographers have their work cut out for them, even with the help of B-modes.

Weighing the Universe

By the time a CMB photon, or particle of light, hits the Milky Way, it has already journeyed billions of light-years. And along the way, it has passed a lot of roadside attractions. “Each photon encodes the properties of everything it encountered on the way to us,” says Keating. “The most significant thing it encounters is gravitational potential,” that is, the stretching of space-time under the gravity of massive objects.

You can imagine space-time as a trampoline. Massive objects warp the trampoline more than tiny ones. For example, if you place a pencil on the trampoline, it hardly makes a dent. But if you put a penguin there, the stretchy material will dip down around the animal. Now, if you roll a marble near the penguin, the dip will change its path. In a similar way, the space-time dip from a galaxy cluster will change the path of a CMB photon.

The wonkiness of the photon’s travel changes the B-modes. And from the size and shape of those shifts, scientists can figure out how much stuff the photons encountered on their trip. “You can measure mass in the entire universe without holding it,” says Keating.

Cosmic Mousetrap

That mass is some 80 percent dark matter. We often think of dark matter as a single thing — some mysterious particle we haven’t discovered yet. But there’s more to it. “There are many types of ordinary matter,” says Keating. “So why should you expect that dark matter would be any different? There could be hundreds of different flavors of dark matter.” But we only know of one flavor for sure: the neutrino, a tiny neutral particle that travels nearly the speed of light and earned two scientists the 2015 Nobel Prize in Physics .

Like cosmic mice, they’re hard to find because they are small, move fast, and don’t interact directly with us. However, they do imprint a specific “signature” on the B-modes, and that signature depends on their mass as a whole and as individuals—both quantities we don’t know yet.

Once our B-mode map is good enough, we will learn about neutrinos — no matter what. “It’s a guaranteed signal,” says Keating, “whereas the B-modes of inflation may not exist at all.”

On the Weird Side

On a larger scale, magnetic fields beyond the galaxy also cause B-modes to swirl out of the static of the cosmic microwave background. Scientists are specifically interested in the most ancient magnetism. Magnetic fields guide the formation and evolution of galaxies and stars, helping to sculpt the raw material of the cosmos into the universe we see today. The POLARBEAR collaboration, a project specifically designed to detect B-modes, exists in part to figure out where magnetic fields come from in the first place. But before it can find an answer to origin, it has to measure this original magnetism. So far, the team members know it can’t be more than a few billionths of a gauss, or a billion times smaller than the mere Earth’s magnetic field. But the specifics are still to come.

B-modes can also show us stranger stuff, too, such as cosmic strings. These “strings” aren’t like those in string theory but are defects in spacetime, like one-dimensional versions of fault lines. They would have formed when the cosmos first cooled off, right after the hot Big Bang.

Most interesting, though, are the total unknowns. “B-modes will tell us if there’s anything in the universe that prefers one polarization versus another,” says Keating. And anytime the universe has a preference, that reveals something fundamental about physics, just like your preferences reveal something fundamental about your personality. On Earth, for example, all life depends on amino acid molecules that face one direction (called “left-handed” molecules), while right-handed molecules also exist in space. And the universe is made mostly of matter rather than antimatter. We don’t yet understand the full reason behind either of those preferences, but we know they are fundamental.

Such fundamentals interest Keating most. “I always wanted to study things that were the purest, most elementary things in nature,” he says. And he believes others want to know about those things too. “It traces back to mankind’s oldest pursuit,” he says. “Where did we come from? We came out of caves and looked up. … I think there’s something primordial in people just like the primordial universe. It’s an urge people have to want to know, ‘How does it all work?’”

We will have to wait to find out how it all works, but the science is moving steadily forward, with BICEP3 now online and POLARBEAR collecting its own photons. “The most exciting CMB project isn’t in the future,” says Bock. “It is right now.”

Go Deeper
Editor’s picks for further reading

Nature: Dust to dust
The editors of Nature ask, what can we learn from the twists and turns of the BICEP2 story?

NOVA Next: From Discovery to Dust
Science writer Amanda Gefter takes a deep look at inflation and the science of the BICEP2 experiment.

University of California, San Diego: POLARBEAR detects B-modes in the cosmic microwave background: Mapping cosmic structure, finding neutrino masses
This press release highlights discoveries from the POLARBEAR experiment, which showed in 2014 that it is possible to isolate B-modes produced by gravitational lensing.

This project/research was supported by grant number FQXi-RFP-1822 from the Foundational Questions Institute and Fetzer Franklin Fund, a donor-advised fund of Silicon Valley Community Foundation.