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Artist's impression of GW170817. (ESO/L. Calçada/M. Kornmesser)

Neutron Star Smash-Up Just Provided a New Measure of a Fundamental Cosmic Feature

8 JUL 2019

When astronomers around the world watched two neutron stars collide in August 2017, it taught us a whole lot of cool things about the Universe. Now there's one more cool thing: Astronomers have used it to refine one of the most fundamental characteristics of the entire Universe - the Hubble Constant.




That's one method to measure it.

Another is by studying the nebulae left behind by Type Ia supernovae. Way back when, Edwin Hubble observed their Doppler shift - that is, the changes in the wavelength of light as the nebula moves farther away. This method recently returned a result of 72.78 kilometres per second per megaparsec.

Yet another, more recent method uses standard candles such as Cepheid variable stars, whose known luminosity allows for accurate distance calculations. And this is where trouble strikes - because these measurements turn in a faster Hubble Constant.



"Either one of them is incorrect, or the models of the physics which underpin them are wrong. We'd like to know what is really happening in the Universe, so we need a[n] ... independent check," said astrophysicist Adam Deller of Swinburne University of Technology.

"The merger of two neutron stars is such a third way."

Cue GW170817, the gravitational wave event that allowed, for the first time, astronomers around the world to observe a collision between two neutron stars in multiple ways - including gravitational wave astronomy, optical astronomy, and radio astronomy.


"This burst of gravitational waves can be used as a 'standard siren': based on the shape of the gravitational wave signal, we can tell how 'bright' the event should have been in gravitational waves. We can then take how bright the event was actually seen to be, and work out what the distance must have been."




By comparing tiny changes in the location and shape of this plasma, the team were able to calculate the orientation of the neutron stars - which, in turn, allowed them to calculate its precise distance.



As you can see, between the Planck measurement and the standard candle measurements, we're in the right ballpark at least - even if the new measurement isn't quite certain enough to tell us whether Planck, supernovae, or Cepheid variables are more accurate.

"But," Deller said, "we showed that in the near future, observations of more merging neutron stars will be able to make that discrimination."

The paper has been published in Nature Astronomy.