Some six billion light-years away, one of the brightest explosions in the Universe unleashed a mammoth blast of radio light.
Lasting no longer that a millisecond but containing as much energy as the Sun’s output over 10,000 years, the eruption eventually swept across Earth on 15 April 2015.
Thanks to that radio flash, an international team of astronomers has, for the first time, pinpointed the exact galaxy to produce a mysterious fast radio burst, the remains of an as-yet unknown type of explosion.
And by studying the burst's afterglow, which illuminated the faint and tenuous material lying in the vast regions of near empty intergalactic space, astronomers also confirmed a sobering fact: all atoms in the Universe are but 5% of the contents (or roughly as much alcohol as is in beer).
The work was published in Nature.
So how do astronomers "weigh" the Universe using radio waves?
As the fast radio burst reached the Parkes telescope in New South Wales, Australia, it was picked up by Swinburne University’s "gstar" supercomputer in seconds, triggering the world’s telescopes to identify the cosmic culprit.
The radio afterglow lasted six days. The six radio dishes of the Australia Telescope Compact Array 400 kilometres from Parkes pinpointed a tiny region of space, a thousand times smaller than we've managed to narrow down before, thanks to a new project dedicated to hunting down radio bursts.
Then it was time for optical telescopes such as the 8.2-metre Subaru telescope in Hawaii to shine. Concentrating its efforts to search that small region of space, Subaru identified the galaxy hosting the afterglow.
What seems to be a simple game of "hide and seek" has huge implications for astronomers. Only 16 fast radio bursts have been detected, and their source is a cosmological mystery.
This particular fast radio burst appeared to come from a massive “trainwreck” galaxy six billion light-years away, which formed 100 billion Suns’ worth of stars, but is currently almost dead.
At the moment it's producing less than two Suns a decade (a factor of ten less than our much-smaller Milky Way galaxy).
Without new stars, the source of fast radio bursts seems to be collisions of dense, long-dead stars known as neutron stars, which typically explode as short gamma ray bursts.
There may even be different classes of objects that produced the 16 currently known fast radio bursts.
But even though the cause of this fast radio burst is still a mystery, its light can be used to illuminate the otherwise impossible-to-detect wisps of material that lies between the galaxies in what’s called the “intergalactic medium”.
It’s hard to picture just how empty this space is – barely a handful of atoms per cubic metre. The same volume of air you’re breathing now contains a trillion trillion atoms.
If space was truly empty, radio waves from the explosion would race outwards at the speed of light. But even these few atoms will cause a noticeable delay over billions of light-years.
In particular, they slow down lower frequencies of light, This is a little like how white light shining through a glass prism splits into a rainbow as the redder colours (with lower frequencies) travel slower than blue.
This delay has a precise relation with distance. And with the six-billion-light-year distance to the host galaxy now known, astronomers calculated the average density of the material along our line of sight, and directly weighed the atomic content of the Universe.
The intergalactic medium, together with all atoms in stars, planets and gas in galaxies, makes up just 5% of the contents of the Universe. The rest is an unknown particle holding galaxies together termed Dark Matter, and an even more mysterious Dark Energy driving galaxies apart.
The 5% value is in perfect agreement with measurements made from the afterglow of the Big Bang, the Cosmic Microwave Background.
We are a small component of the Universe, but as far as we know, the only piece to look back and measure itself.
So don’t feel small – feel unique!
Alan Duffy is an astrophysicist at Swinburne University of Technology, Melbourne. He was Lead Scientist of The Royal Institution of Australia from 2017 to 2021. Twitter | @astroduff
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