A Window into New Physics
Bursts of radio waves lasting just milliseconds could reveal the physics behind dark matter or quantum gravity theories.
In 2007, David Narkevic was using a new algorithm to chug through 480 hours of archived data collected by the Parkes radio telescope in Australia. The data was already six years old and had been thoroughly combed for the repeating drumbeat signals that come from rapidly-rotating dead stars called pulsars.
But Narkevic, a West Virginia University undergrad working under the supervision of astrophysicist Duncan Lorimer, was scouring these leftovers for a different animal: single pulses of unusually bright radio waves that are known to punctuate the rhythm of the most energetic pulsars.
Radio astronomers have an arsenal of well-honed tricks for teasing out faint signals including correcting for “dispersion.” Dispersion is when signals traveling through space arrive slightly earlier at high frequencies than they do at low frequencies according to a precise formula that describes how electromagnetic radiation is delayed by free-floating electrons. The more interstellar stuff the signals have to traverse, the more dispersed they are, so “dispersion measure” functions as a rough proxy for distance.
Distant, and therefore highly dispersed signals, are difficult to pick up because their energy is smeared out across frequency and time. So, astrophysicists design search algorithms that apply one correction factor after another, with the hope that, by trial and error, they might hit on the right one and pluck a signal out from the noise. The process requires a lot of computing time, so astronomers typically only use common-sense dispersion corrections. But with all the common-sense results already wrung out from the data set, Narkevic was trying out correction factors corresponding to distances far beyond the Milky Way and its neighboring galaxies.
To his surprise, it worked: He discovered a bright burst of radio waves, lasting less than five milliseconds, coming from a point on the sky a few degrees away from the Small Magellanic Cloud but that seemed to originate from far beyond it. It was impossible to pin down its precise location and distance but, based on the dispersion, Lorimer and his team calculated that it had to be far: billions of light years beyond the Milky Way.
Lorimer’s team trained the Parkes telescope on the site for 90 more hours but never picked up another burst. Whatever Narkevic had found, it didn’t look like one of the pulsar pulses Lorimer had originally set out to find.
It had to be far: billions of light years beyond the Milky Way.
That left plenty of other possibilities. It could be some human-made interference masquerading as a mysterious cosmic object: military radar, microwave ovens, bug zappers, and even electric blankets all produce electromagnetic radiation that can confuse readings from radio telescopes. But the “Lorimer burst” didn’t look like it was coming from one of these sources. For one thing, the dispersion was by-the-book: that is, the signal “swept” in at high frequencies first, and low frequencies later. For another, it was picked up by just three of the telescope’s 13 “beams,” each of which corresponds to a single pixel on a sky map, suggesting that it was localized out there , somewhere in the sky, rather than coming from a nearby source of interference, which would swamp the whole telescope.
“We couldn’t think of any radio-frequency interference that would mimic those characteristics,” says astrophysicist Maura McLaughlin, also of West Virginia University, who was part of the discovery team. The researchers also ruled out some of the usual cosmic suspects: The burst was too bright to be a spasmodic eruption from a pulsar and too high-frequency to be the radio counterpart to a gamma-ray burst. Magnetars, highly magnetized neutron stars that sizzle with X-rays and gamma-rays, remained a strong possibility. “I tend to go with the least exotic things,” McLaughlin says, citing Occam’s razor: “The simplest thing is always the best. But I wouldn’t be surprised if it was something really strange and exotic, too.”
Such observational puzzles are candy for theorists, and fast radio bursts, or FRBs as they are called, present a particularly sweet mystery: Their extreme properties hint that they might be able to reveal phenomena that push the boundaries of known physics, perhaps probing the properties of dark matter or quantum gravity theories beyond the Standard Model. So while observational astronomers kept searching for more FRBs, theorists began speculating about what they might be.
Imploding Neutron Stars
There were three clues: The burst was short, powerful, and distant. To astrophysicists, a short signal points to a small source—in this case, one so small that a light beam could cross it in the duration of the burst, just a few milliseconds. That means that FRB “progenitors,” whatever they are, probably measure less than one thousandth the width of the sun. What could pack such a huge amount of energy into that tiny package? “The only things that can produce that much energy are neutron stars and black holes,” says Jim Fuller, a theorist at Caltech.
Fuller started thinking seriously about fast radio bursts in 2014, just as they were enjoying a scientific comeback. Studies of the Lorimer burst had languished for years after a group led by Sarah Burke-Spolaor, then a postdoc at the Commonwealth Scientific and Industrial Research Organisation in Australia, detected 16 similar bursts and was able to unambiguously chalk them up to earthly interference. But then, in 2013, Burke-Spolaor found a Lorimer burst of her own. A handful more followed. FRBs were back from the dead.
Meanwhile, Fuller had a different astronomical mystery on his mind: the apparent scarcity of pulsars near the center of the Milky Way. There should be plenty of pulsars within a few light years of the galactic center, Fuller says, but despite years of searching, astronomers have found just one. What happened to the rest of them? Astrophysicists call this the “missing pulsar problem.”
Last year, a pair of astronomers proposed an unconventional answer: those missing pulsars might have “imploded” under the weight of dark matter, which is abundant in the center of the galaxy. Though dark matter passes easily through planets and stars, it could get trapped in the dense meat of a neutron star, they argued. Once there, it would slowly sink down to the star’s center. Over time, dark matter would pile up in the core, eventually collapsing into a tiny black hole that would eat away at the neutron star from the inside out. The star would gradually erode over thousands or millions of years until, in one great slurp, the black hole would devour nearly the whole mass of the neutron star in a matter of milliseconds.
“Probably, it will be a very violent event, where the magnetic field is totally expelled from the black hole and reconnects with itself,” Fuller says. Some of the energy of the ravaged magnetic field would be turned into electromagnetic radiation: a blast of radio waves that might look a lot like an FRB.
“It’s a pretty crazy idea,” Fuller admits. But it does make some predictions that we can observe. If Fuller’s model is right, neutron star implosions should have left behind lots of small black holes near the center of the galaxy, each holding about one-and-a-half times the mass of our sun. Though astronomers can’t see a black hole directly, if the black hole happens to be drawing matter from a companion star, as is relatively common, it will give off characteristic bursts of X-rays. A different kind of X-ray burst, on the other hand, could signal the presence of a neutron star, not a black hole. If there are lots of neutron stars hanging out around the galactic center, that would challenge Fuller’s scenario. (Some recent X-ray observations point toward the existence of those neutron stars, though the evidence is not yet definitive, Fuller says.)
Fuller’s argument also predicts that FRBs should be coming from very close to the center of other galaxies. So far, astronomers haven’t pinpointed the location of a single FRB, and localizing one within a galaxy is an added challenge.
If Fuller’s predictions hold up, they will yield fresh insight into the nature of dark matter, which is still almost totally a blank. First, it will mean that dark matter particles don’t annihilate each other , as some recent observations have hinted. It would also reveal dark matter’s “cross section”—that is, the likelihood that a particle of dark matter will interact with normal matter, as opposed to passing straight through it. For the neutron star implosion scenario to hold up, dark matter’s cross section must be just somewhere in a very narrow range of possibilities, Fuller says.
Bouncing Black Holes
Another possibility for what’s causing FRBs comes from the leading edge of black hole physics, where theorists are puzzling over the difficult answer to an apparently simple question: What happens to the stuff that falls into a black hole? Physicists once thought that it was inevitably compressed into an infinitely small, infinitely dense point called a singularity. But because the known laws of physics break down at this point, the singularity has always been a raw nerve for physicists.
Many physicists would like to find a way to sidestep the singularity, and theorists working on a theory called loop quantum gravity think they have found a way to do so. Loop quantum gravity proposes that the fabric of spacetime is woven of tiny—you guessed it—loops. These loops can’t be compressed indefinitely—push them too far, and they push back. In the universe of loop quantum gravity, a would-be black hole can collapse only until gravity is overcome by the outward pressure generated by the loops, which then hurtles the black hole’s innards back out into space, transforming it into its mathematical opposite, a white hole .
The singularity has always been a raw nerve for physicists.
Abruptly, the contents of the black hole would be converted into a tremendous blast of energy concentrated at a wavelength of a few millimeters, according to Carlo Rovelli, a theorist at Aix-Marseille University, and his colleagues in France and the Netherlands. We might be able to pick up the first of these cosmic kabooms today, coming from some of the universe’s earliest black holes, Rovelli says, and they might look a lot like fast radio bursts. It’s not a perfect match: fast radio bursts emit at a lower frequency, corresponding to a wavelength of about 20 centimeters, and they don’t give off as much energy as the theorists predict for a “quantum bounce.” But, Rovelli says, the model’s predictions are still very crude and don’t account for the black hole’s motion, interactions between the matter it contains, or even the fact that the black hole has mass.
Rovelli says the model does make one clear, testable prediction: a peculiar correlation between the wavelength at which the signal is received and the distance to the black hole. That’s because the wavelength of the emitted energy depends on two things: the size of the black hole and its distance from Earth. The most distant explosions should be coming from the youngest, and therefore smallest, black holes, meaning that their energy will be skewed toward shorter wavelengths. But as the radiation travels across the expanding universe, it will be stretched out, or “redshifted,” so that the signals we pick up on Earth register at a longer wavelength than they were emitted. Add up the effects and you should see the specific curve that Rovelli and his colleagues predict. As astronomers find more fast radio bursts, they will be able to test whether they match the predicted curve.
It may sound like a long shot. But, if it’s right, the payoff would be huge: “If the observed Fast Radio Bursts are connected to this phenomenon, they represent the first known direct observation of a quantum gravity effect,” wrote Rovelli and his colleagues.
It could also get physicists out of a theoretical jam called the black hole information paradox, which pits two unshakable tenets of physics against each other. On one side, the principle of unitarity holds that information can never be lost; on the other, according to the rules of black hole thermodynamics, the only thing that ever escapes from a black hole, Hawking radiation, is randomly scrambled and preserves no information. To solve the paradox, some physicists have proposed that the entanglement between incoming particles and those radiated out as Hawking radiation could be spontaneously broken, putting up a “firewall” of energy at the black hole’s horizon. But the concept is still controversial: plenty of ideas in modern physics violate our intuition about how the world is supposed to work, but a sizzling wall of energy floating around a black hole? Really?
The quantum bounce effect could resolve the information paradox and neutralize the need for a firewall, argue Rovelli and his colleagues. The information inside the black hole isn’t lost: it just comes out later.
Superconducting Cosmic Strings
Fast radio bursts could also be a modern manifestation of something that happened 13.7 billion years ago, just after the Big Bang, when the baby universe was roiling with so much energy that all the fundamental physical forces acted as one. At this moment, the Higgs field had not yet switched on and nothing in the universe had mass. Then, on came the Higgs field, unfurling through space and pinging every particle it encountered with its magic wand, bestowing the gift of mass.
Some theorists think that the field associated with the Higgs boson, discovered in 2012 at the Large Hadron Collider, is just one of many similar fields, each of which plays a role in giving particles mass. But many models predict that these fields would not diffuse perfectly through all of space. Instead, they would miss a few spots. These gaps, the thinking goes, would become defects called cosmic strings, skinny tubes of space that, like springy rubber bands, are tense with stored energy. Extending over millions of light years and traveling close to the speed of light, these hypothetical strings would be so massive that a single centimeter-long snippet would contain a mountain’s-worth of mass, says Tanmay Vachaspati, a physicist at Arizona State University who, along with Alexander Vilenkin at Tufts University, did early work on the formation and evolution of cosmic strings.
Invisible to most telescopes, cosmic strings could be detected via the gravitational waves they emit as they shimmy through space and crash into other cosmic strings. So far, astronomers haven’t made any affirmative detection of these gravitational waves, though the fact that they haven’t shown up yet allows physicists to put some limits on the maximum mass of the strings.
A still-more-exotic breed of cosmic strings called superconducting cosmic strings, which carry an electrical current, could turn out to be easier for astronomers to observe. First proposed by theorist Edward Witten, these electrified strings should give off detectable electromagnetic radiation as they move through space, Vachaspati says. The emission would look like a constant hum of very-low-frequency radio waves, occasionally spiked with brief, higher-frequency bursts from dramatic events called kinks and cusps. Kinks happen when two strings meet and reconnect at their point of intersection, Vachaspati says. Cusps are like the end of a whip, lashing out into space at close to the speed of light. What, exactly, their radio emission might look like depends on many still-unknown parameters of the strings, Vachaspati says. But it is possible that they would look very much like fast radio bursts.
There is one problem, though. Vachaspati and his colleagues predict that the radio emission from superconducting cosmic strings should be linearly polarized: that is, it should oscillate in a plane. So far, polarization has only been measured for one fast radio burst, but that was circularly polarized, meaning that its electric field draws out a spiral around the direction its traveling.
Some theorists, including Vilenkin, think it might be possible for a superconducting cosmic string to produce a circularly polarized signal under certain conditions. And with polarization measured for just one FRB so far, it’s too soon to discount the hypothesis entirely.
Future Observations
Today, astronomers have detected about a dozen fast radio bursts. (A group of apparently similar signals, curiously clustered around lunchtime, were recently traced to a more mundane source: the Parkes observatory microwave oven .) But observers and theorists in every camp agree on this: to figure out what is causing FRBs, they need to find more of them.
“Right now, there are far more theories about what’s causing FRBs than FRBs themselves,” says Burke-Spolaor, who is now leading up a search for FRBs with the Very Large Array (VLA), a network of radio telescopes in New Mexico. With more bursts in their catalog, astronomers will be able to draw more meaningful conclusions about how common FRBs are and how they are distributed across the sky. They will also be able to answer two critical questions: where the bursts are coming from, and what they look like in other parts of the electromagnetic spectrum.
So far, astronomers have localized each Parkes burst to a disc of sky that’s about a half-degree across—about the size of the full moon. To astronomers, that’s an enormous region: extend your vision out to the distance at which FRBs are expecting to be going off, and that little patch of sky could contain hundreds of galaxies. Using the VLA, Burke-Spolaor should be able to pin down a burst’s location to a single galaxy. But first, she has to find one. Based on the number of FRBs that have been seen so far, she estimates that it will take about 600 hours of skywatching to have a solid chance of observing one. So far, she has a little under 200 hours down.
“Right now, there are far more theories about what’s causing FRBs than FRBs themselves.”
Unlike the archival search that turned up the first FRB, Burke-Spolaor’s search campaign is attempting to catch FRBs in the act. That will give astronomers a chance to quickly swivel other telescopes to the same spot and potentially see the bursts giving off energy at other wavelengths. So far, only three FRBs have been caught in real time, including a May 14, 2014, burst observed at Parkes by a team of astronomers including Emily Petroff, a PhD student in astrophysics at Swinburne University of Technology in Melbourne, Australia. Within a few hours, a dozen other telescopes were watching the source of the burst at wavelengths ranging from X-rays to radio waves. But not one of them saw anything unusual. Papers on two more bursts, observed in February and April of this year, are currently being prepared for publication; astronomers followed up on those bursts with observations at multiple wavelengths, too, but haven’t yet announced the result of those studies.
Meanwhile, Jayanth Chennamangalam, a former student of Lorimer’s who is now a post-doc at Oxford, is putting the finishes touches on a system that will scan every 100 microseconds of incoming radio data at the Arecibo dish in Puerto Rico for sudden, short pulses. The system, called ALFABURST, will piggyback on the latest iteration of SERENDIP, a spectrometer that’s been tapping the Arecibo’s feed for years, listening for signals from extraterrestrial civilizations. Ultimately, it will be able to alert astronomers to unusual bursts within seconds—fast enough for rapid follow-up at other wavelengths.
Will fast radio bursts turn out to be a window into new physics or just a new perspective on something more familiar? It’s too early to say. But for now, researchers can relish the moment of being maybe, just possibly, on the verge of finding something genuinely new to science.
Photo credits: CSIRO (CC BY), Chandra X-ray Observatory/Smithsonian Astrophysical Observatory/NASA
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