Astronomers have discovered four mysterious radio bursts from beyond the Milky Way, bursts unlike any in the catalog of emissions from well-known radio sources in space or on Earth.
E.T. is an unlikely source, since each burst seems to be a one-off event. Instead, what little evidence astronomers have in hand suggests that the bursts come from astrophysical sources billions of light-years away. The bursts are energetic enough to suggest that they are triggered by extremely powerful astrophysical events.
These fast radio bursts last a few thousandths of a second and slide ever lower in frequency as they fade.
Based on the number of events that the researchers detected in their hunt for the bursts, roughly 10,000 of these should appear across the sky each day – or one every nine to 10 seconds, they say.
"If we could view the sky with 'radio eyes' there would be flashes going off all over the sky every day," said Michael Kramer, director of the Max Planck Institute for Radio Astronomy in Bonn, Germany, and a member of the team reporting its results in Friday's issue of the journal Science.
So far, no one has been able to associate any of these with a particular galaxy, presuming that they have galactic sources. Indeed, once one tries to go beyond the generalization of "exotic sources" for these radio bursts, speculation varies widely on their cosmic transmitters, according to James Cordes, a radio astronomer at Cornell University in Ithaca, N.Y.
Evaporating black holes, supernovae, merging neutron stars, or neutron stars with unusually strong magnetic fields compared with other neutron stars all could be possible sources, Dr. Cordes writes in a commentary in Science tied to the new discovery.
But he also cautions patience, noting it took 20 years for astronomers to uncover the sources of gamma-ray bursts, first detected not by astronomers but by satellites designed to spot above-ground nuclear explosions. Only after astronomers began hunting for the gamma-ray bursts and performing near-instant follow-ups with telescopes operating at other wavelengths were they able to uncover the exotic events that triggered the bursts.
The same is likely to hold true for fast radio bursts, he writes.
This is actually the second reported detection of these unique cosmic signals.
In 2007, a team led by Duncan Lorimer, astrophysicist at West Virginia University in Morgantown, reported the first such burst detected, based on a review of 6-year-old data captured by the 210-foot-diameter radio telescope at the Parkes Observatory in Australia. The team put the source's location in the vicinity of the Large Magellanic Cloud, one of the Milky Way's satellite galaxies.
But the burst appeared far enough away from the satellite galaxy to suggest an origin beyond the Milky Way's neighborhood. Indeed, the team estimated, the source of the burst was some 3 billion light-years away.
The burst "was incredibly bright, and it looked like it was obeying all the properties that we would expect from an astrophysical burst of emissions," recalls Sarah Spolaor, a researcher at NASA's Jet Propulsion Laboratory and a member of the team reporting the results Friday.
That first detection inspired her to hunt for more.
"I found quite a lot of bursts," she says. Indeed, the group she led on that project found 16 more. But the team determined that while they looked like the bursts from deep space, they really came from local sources near the telescope. Still, she and her colleagues, who published their results two years ago, noted that the bursts were unlike those of any known radio source on Earth.
Since then, she says, researchers have reported finding eight more of these. Either lightning or perhaps satellites are the consensus suspects for these false alarms, she says.
"That was a real letdown," she says, adding that people began losing heart to continue the search.
These four new examples should remove any doubt that extragalactic pulses are real, noted Dan Thornton, a PhD student at the University of Manchester in Britain and the new study's lead author, in a prepared statement.
Mr. Thornton's team, which included Dr. Spolaor, also used the 210-foot radio telescope at Parkes in a dedicated hunt for high-speed, fleeting radio bursts.
The most distant of the four bursts was some 11 billion light-years away, the researchers report. All four "were so excellent, we were totally stunned," Spolaor says.
The confirmation that fast radio bursts are for real turns the phenomenon into what Cordes of Cornell calls "a disruptive discovery" that will alter the design of radio telescopes in ways that will allow them to capture fast bursts in enough detail throughout their millisecond lifetimes to allow scientists to study the details of their evolution.
That becomes important as astrophysicists hunt for the universe's missing "baryonic" matter – matter made up of quarks, the fundamental building blocks for protons and neutrons.
Currently, the universe appears to hold less of this "normal matter" than the big-bang theory of the universe's formation predicts. That's uncomfortable news when normal matter already is a distinct minority of the matter and energy that the universe contains. Some 70 percent of the universe consists of "dark energy," which is accelerating the universe's expansion. Another 25 percent is so-called dark matter. The rest is baryonic matter.
Researchers, however, have detected less than half of the baryonic matter that theory says should be there. The balance is thought to exist as ionized gas spread between galaxies.
Cordes and Thornton's team note that because this ionized gas, or plasma, affects the travel of radio waves that pass through it, the fast radio bursts can serve as probes that will allow astrophysicists to better estimate how much baryonic matter is floating between galaxies. And given the all-sky nature of the bursts, the potential holds for estimating this amount with increasing accuracy.
In fact, one of the four new bursts displayed the expected evidence of having passed through turbulent intergalactic gas, Spolaor says.
Even the three fainter ones held good cosmic news. If the sources of the bursts are distributed fairly evenly throughout the universe, one would expect to see some brighter than others because they are closer.
"The fact we do see these three faint ones is a good sign" that they are outside the Milky Way, she says.