Milky weigh? Astronomers find new method for weighing the universe

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A mysterious deep space radio burst reveals where half the atoms in the universe have been hiding: in clouds between the galaxies

 

The Australian Telescope Compact Array pinpointed the location of the burst of radio waves.

 

You know how when you buy a new set of scales you always hope that they will read lighter than old ones? When is comes to weighing the universe, astronomers are the complete opposite.

 

Theoretical calculations have often suggested that there should be double the amount of atoms than can be seen in stars. So the question has always been: where are they hiding?

 

Now thanks to a mysterious burst of radio waves from deep space, astronomers have their answer. They’re in vast clouds that lie between the galaxies.

In recent years, astronomers have been detecting bursts of radio waves from deep space. Named the Fast Radio Bursts (FRB) because they last for only a millisecond each, only 16 had previously been registered.

 

On 18 April 2015, another burst was caught by the 64-metre Parkes radio telescope in Australia. An alert went out to astronomers around the world and within a few hours a number of telescopes were also looking for the signal.

 

The Australian Telescope Compact Array pinpointed the location of the burst, and the National Astronomical Observatory of Japan’s 8.2-metre Subaru optical telescope found a galaxy there, some 6 billion light years away.

 

This is the first time that the host galaxy, and therefore the distance of an FRB, has been measured. This was crucial to the effort to weigh the universe because as the radio waves move through space the amount of matter they encounter en route separates their different frequencies. It is a bit like light being split into colours by a prism.

 

With FRBs, the higher frequencies arrives before the lower ones. The length of the delay allows astronomers to calculate the amount of matter the radio waves have passed through to reach us.

 

When Evan Keane of the Jodrell Bank Observatory did this calculation, he found that the amount of matter matched perfectly with the high theoretical calculations, not the smaller estimate based on the number of stars in the universe. The missing matter must therefore be floating in intergalactic space in vast tenuous gas clouds.

 

“This is a solid result. We thought the matter had to be there but it’s nice to have actually made a measurement,” says Keane.

The result, which is published in Nature, also contains hints as to the nature of the so-far mysterious FRBs.

 

The Australian Telescope Compact Array, which pinpointed the FRB, also detected a radio afterglow that took six days to fade. Such behaviour is expected from a collision between pairs of stellar corpses called neutron stars.

 

To know whether this interpretation is correct or not would require the detection of gravitational waves from the collision, similar to the black hole collision seen by the LIGO observatory earlier this month.

 

Although at six billion light years away, this particular collision was too far for LIGO to see, FRBs will be happening all over the universe and some will be much closer. This should bring them within range of LIGO and the gravitational wave observatories on Earth.

 

There have been hints that FRBs could be dying stars, leading Keane and colleagues to suggest that there may be different triggering mechanisms at work.

Another boon to the study of FRBs will be the arrival of the Square Kilometre Array (SKA), a giant field of radio telescopes that is currently gearing up for construction in Australia and South Africa.

 

Whereas, astronomers currently find one or two FRBs per year, the SKA will be sensitive enough to see one almost every day. “With SKA we will be able to turn the detection of fast radio bursts into an industry,” says Keane.

 

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