How a mystery pulse helped weigh the Universe

25 February 2016

This burst has enabled us to conduct a unique census of the Universe's matter content in a new, independent and direct way and in a sense weigh the Universe.

Scientists have pinpointed the source of a rare burst of radio waves from the heavens for the first time, identifying the culprit as an elliptical galaxy six billion light years away.

The fleeting pulse also gave the international scientific team a glimpse of the make-up of the space the radio waves passed through on their journey, enabling them to calculate the Universe's body-mass index.

"This pulse has travelled six billion light years to reach us and it only lasted a millisecond," said team member Manisha Caleb, PhD student at The Australian National University (ANU).

"We've spent almost a decade trying to identify an origin for these elusive fast radio bursts. It's exciting to finally pinpoint a distant galaxy as the source," said Ms Caleb, from the ANU Research School of Astronomy and Astrophysics.

Lead researcher Dr Evan Keane from the Square Kilometre Array (SKA) Organisation conceived the study while working for the ARC Centre of Excellence for All-sky Astrophysics (CAASTRO) at Swinburne University of Technology.

"It's the first time we've been able to identify the host galaxy of a fast radio burst," Dr Keane said.

Fast radio bursts are strong pulses of radio waves that typically last only a few milliseconds, thought to originate from outside our galaxy.

Only 17 fast radio bursts have ever been observed, but until now, astronomers had never identified one so quickly and been able to point telescopes towards the burst to see what had caused it.

The short, sudden nature of the pulse is a gift to astronomers because it gives them insights into what the radio waves have travelled through during their six billion light-year journey.

The team found the pulse had been delayed slightly at lower frequencies, an effect caused by free electrons in the space between galaxies.

Because the scientists had identified the distance to the galaxy the burst came from, they could calculate the density of electrons between galaxies, and extrapolate that to the whole Universe.

"This burst has enabled us to conduct a unique census of the Universe's matter content in a new, independent and direct way and in a sense weigh the Universe," Ms Caleb said.

The burst was captured by the 64-metre Parkes radio telescope, whose data was being scanned by a supercomputer at University of Swinburne. Optical telescopes around the world, including the ANU SkyMapper telescope, helped pinpoint the area.

The CSIRO Australia Telescope Compact Array's six 22-metre dishes monitored the region over the next few weeks and determined the location of the signal with much greater accuracy than had been possible previously.

The team then used the National Astronomical Observatory of Japan's 8.2-metre Subaru optical telescope in Hawaii to identify an elliptical galaxy six billion light years away as the source.

"Fast radio bursts show a frequency-dependent delay in their radio signal caused by how many electrons they have passed through. The host galaxy identification enabled us to place the Universe on the scales," said Dr Simon Johnston from CSIRO's Astronomy and Space Science.

The research is published in Nature.