The Mystery of the Intergalactic Radio Bursts

Scientists have discovered a new class of cosmic energy in outer space — nearly 10,000 bursts of radio waves popping off every day — but where they come from and how they occur are still a mystery

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Dr Jon Heras /Science Photo / Corbis

Illustration showing the absorbance of electromagnetic waves by the Earth's atmosphere.

It’s a recurring theme in astronomy: observers see a blast of energy out in the cosmos, scratch their heads in confusion for a while, and finally uncover the existence of something entirely surprising and new. It happened with the quasars (now known to be gigantic burps from black holes swallowing hot gas), the pulsars (fast-spinning neutron stars sending out blips of radio noise hundreds of times every second), and even the Big Bang itself, first seen as a stream of microwaves slamming into Earth from all directions, nearly 14 billion years after the event itself.

Now it may be happening again. Back in 2007, astronomers detected a burst of radio noise, lasting maybe a second or so, the cause of which was totally unclear. There was reason to suspect it came from beyond the Milky Way, and must be extremely powerful to be visible at all. But it never repeated, and neither did a second, similar blast seen in 2011, making it very tough to puzzle out what was going on. Maybe both events were just some sort of rare fluke.

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But a new paper in Science makes that seem very unlikely. Using the giant Parkes radio telescope in Australia, astronomers have recorded four more of these mysterious bursts, and when the scientists extrapolated across the entire sky, they concluded that perhaps 10,000 of these blasts are popping off every day, all over the heavens. “It’s still a mystery what they are,” says lead author Dan Thornton, of the University of Manchester, in the U.K. “But at least it’s not a mystery that they exist.” In fact, Thornton and his co-authors claim that the observations reveal what he calls a “new cosmological population” of energy blasts, whose true nature is unknown.

At the time of the first observations, back in 2007, there was some talk that the original burst might have come from inside the Milky Way. There was a way to test that proposition: radio outbursts generally come in a range of frequencies — channels, essentially, like those on a radio dial. As they speed through the empty spaces in our galaxy, the waves run into loose electrons that linger between the stars. The electrons slow the radio waves down a bit, with the lowest-frequency waves slowing the most. A radio burst that was emitted in a fraction of a second might be received over a longer period, depending on how far the burst had been traveling, and through what part of the Milky Way.

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The first and second bursts were indeed spread out, or dispersed, this way, and the first, especially, seemed to be too dispersed to have originated in our own galaxy. But the second was marginal, leaving astronomers stuck. The four new blasts, however, were unmistakable. “The dispersion is so high,” says Thornton, “that from what we know, they could not have come from the Milky Way.”

Instead, he says, he and his co-authors estimate that whatever is sending out these radio bursts is located between five and10 billion light-years away — a substantial fraction of the way out to the edge of the visible universe.

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So that’s the “where” of the mystery, but nobody has a good idea yet about the “what.” Some of the possibilities of the blasts cause include: evaporating black holes (something predicted by Stephen Hawking), or giant black holes eating neutron stars. Or, writes Cornell astronomer James Cordes, tantalizingly, in a commentary also appearing in Science, “they could represent an entirely new class of source.”

The only way to figure it out is to follow up on the radio bursts with observations by visible-light, X-ray and other telescopes to try and get a glimpse of whatever’s going on from another perspective. That’s how astronomers figured out the inner workings of gamma-ray bursts, which turned out to be a special kind of exploding star — and just as with gamma-ray bursts, it will be important to do those follow-ups quickly. “We discovered these events about a year after they happened,” says Thornton, by which time any lingering visible or X-ray glow from the triggering event would long since have faded. “But now we’re working on real-time alerts.”

If those can be organized — and given the fact that one of these blasts probably goes off about once every second around the clock — it might not be long before pulsars, quasars, gamma-ray bursts and other blasts of cosmic energy welcome a new cousin to the family.

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