Every now and then things go "bump!" in the cosmic night, releasing torrents of energy that astronomers can't easily explain. Not that they mind: most times an energetic riddle flares up in their view of the sky, major epoch-setting discoveries are sure to follow. This was the pattern for pulsars—rapidly spinning city-size stellar remnants that steadily chirp in radio. It was also the pattern for gamma-ray bursts—extreme explosions at the outskirts of the observable universe thought to be caused by stellar mergers and collapsing massive stars. Now the pattern is playing out again, with last week's announcement that an international team of researchers has detected brief, bright bursts of radio waves washing over Earth from mysterious sources that may be billions of light-years away. The findings, reported in the July 5 Science, could open an entirely new window on the universe by allowing scientists to measure the composition and dynamics of the intergalactic medium—the cold, diffuse plasma that lies between galaxies.
Using a year's worth of data gathered from some 10 percent of the sky by the 64-meter Parkes radio telescope in Australia, the team detected four bursts from far outside the galactic plane, each occurring only once and lasting a few thousandths of a second. According to Dan Thornton, a PhD candidate at the University of Manchester in England who led the study, the results suggest that these "fast radio bursts," or FRBs, probably occur as often as every 10 seconds or so, nearly 10,000 times a day. "If we had radio telescopes watching the entire sky, that's how many we think we'd see each day," Thornton says. "We haven't seen more of these until now only because we've been looking at small regions of the sky for small amounts of time."
"The discovery of fast radio bursts at the Parkes Observatory, if confirmed at other observatories, would be a monumental discovery, comparable to that of cosmological gamma-ray bursts and even pulsars," says Shrinivas Kulkarni, a Caltech astrophysicist who was not involved with the recent study. "Great discoveries need the greatest proof, however, and I eagerly look forward to confirmation of these events at other radio bands and other observatories."
One flash, followed by years of uncertainty
In 2007 a different team discovered the first FRB entirely by chance while analyzing archival data from the Parkes telescope. The astronomers’ interest was piqued by an event from the night of August 24, 2001, when a five-millisecond burst whispered into the telescope from a seemingly blank region of sky near the Small Magellanic Cloud, a dwarf galaxy thought to orbit our Milky Way.
Examining the data in more detail, the team's leader, West Virginia University astrophysicist Duncan Lorimer, found something curious in the dispersion of the burst's wavelengths. Its short-wavelength components had arrived at the telescope a fraction of a second before longer wavelengths, an effect that can be caused by longer-wavelength light moving ever so slightly slower through electrons in clouds of cold plasma that suffuse the space between stars and galaxies. The longer the delay between the arrival of a burst's short and long wavelengths, the more intervening electrons it had passed through and the greater the distance it traveled. Lorimer and his colleagues were shocked by the results of their calculations, which suggested the burst had come from as much as a few billion light-years away.
Lorimer looked for the burst's repetition in some 90 hours of additional Parkes observations but came up empty. Because of the burst's extreme brightness, short duration and singular occurrence, Lorimer suspected it might represent an entirely new, previously undetected astronomical radio source that astronomers might somehow use as a plumb line to investigate the ionized contents of the intergalactic medium. Year after year his team's proposals to search for more such bursts were rejected, and separate searches found only one tentative candidate. Other astronomers began suggesting the "Lorimer burst" might have been only some errant terrestrial signal caused by man-made interference or natural sources such as lightning. Perhaps it had all been too good to be true. "That kept us awake at night," Lorimer recalls. "We wondered whether what we had seen was just a fluke, an artifact."