Discovery of fast radio burst finds universe’s ‘missing matter’
The location of a fast radio burst has been identified for the first time, allowing the distribution of dark matter to be confirmed
An international team of scientists using a combination of radio and optical telescopes has for the first time managed to identify the location of a fast radio burst, allowing them to confirm the current cosmological model of the distribution of matter in the universe.
On 18 April 2015, a fast radio burst (FRB) was detected by the Commonwealth Scientific and Industrial Research Organisation (CSIRO)’s 64-metre Parkes radio telescope in Australia. An international alert was triggered to follow it up with other telescopes and within a few hours, a number of telescopes around the world were looking for the signal, including CSIRO’s Australia Telescope Compact Array (ATCA).
FRBs are mysterious bright radio flashes generally lasting only a few milliseconds. Their origin is still unknown, with a long list of potential phenomena associated with them. FRBs are very difficult to detect; before this discovery only 16 had been detected.
“In the past FRBs have been found by sifting through data months or even years later. By that time it is too late to do follow up observations.” says Evan Keane, Project Scientist at the Square Kilometre Array Organisation and the lead scientist behind the study. To remedy this, the team developed their own observing system at Swinburne University of Technology in Australia to detect FRBs within seconds, and to immediately alert other telescopes, when there is still time to search for more evidence in the aftermath of the initial flash.
Thanks to the ATCA’s six 22-metre dishes and their combined resolution, the team was able to pinpoint the location of the signal with much greater accuracy than has been possible in the past and detected a radio afterglow that lasted for around 6 days before fading away. This afterglow enabled them to pinpoint the location of the FRB about 1000 times more precisely than for previous events.
The team then used the National Astronomical Observatory of Japan (NAOJ)’s 8.2-m Subaru optical telescope in Hawaii to look at where the signal came from, and identified an elliptical galaxy some 6 billion light-years away. “It’s the first time we’ve been able to identify the host galaxy of an FRB” adds Keane. The optical observation also gave them the redshift measurement (the speed at which the galaxy is moving away from us due to the accelerated expansion of the universe), the first time a distance has been determined for an FRB.
FRBs show a frequency-dependent dispersion, a delay in the radio signal caused by how much material it has gone through. “Until now, the dispersion measure is all we had. By also having a distance we can now measure how dense the material is between the point of origin and Earth, and compare that with the current model of the distribution of matter in the universe,” explains Simon Johnston from CSIRO’s Astronomy and Space Science division. “Essentially this lets us weigh the universe, or at least the normal matter it contains.”
In the current model, the universe is believed to be made of 70 per cent dark energy, 25 per cent dark matter and 5 per cent ‘ordinary’ matter, the matter that makes everything we see. However, through observations of stars, galaxies and hydrogen, astronomers have only been able to account for about half of the ordinary matter, the rest could not be seen directly and so has been referred to as ‘missing.’
“The good news is our observations and the model match, we have found the missing matter,” explains Keane. “It’s the first time a fast radio burst has been used to conduct a cosmological measurement.”
Looking forward, the Square Kilometre Array, with its extreme sensitivity, resolution and wide field of view is expected to be able to detect hundreds of FRBs and to pinpoint their host galaxies. A much larger sample will enable precision measurements of cosmological parameters such as the distribution of matter in the universe, and provide a refined understanding of dark energy.
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