Ripples in space-time: Lessons to be learnt

by Dougal Dobie

The gravitational wave signal detected by LIGO can give us lots of information about the merger, including estimates of the distance, the masses of the neutron stars and the orientation of the merger. However, to thoroughly understand the physics of the merger, we require a broader approach. GW170817 heralds the dawn of the era of “multi-messenger” astronomy, where astronomers probe the universe not only with electromagnetic radiation, but with gravitational waves, neutrinos and cosmic rays as well.

The detection of a short gamma-ray burst (sGRB) 1.7 seconds after the merger by Fermi confirmed that gravitational waves propagate at the speed of light, confirming Einstein’s predictions from over a century prior.

The localisation of GW170817 to the galaxy NGC 4993 allowed an independent measurement of the Hubble Constant, which measures the rate of expansion of the universe. This measurement has since been refined using radio observations of the merger afterglow.

Observations with optical telescopes showed that the near-infra red spectrum of the afterglow had bumps that are a fingerprint of the presence of heavy elements, like gold and platinum. It has long been predicted that most of the heavy elements on earth form from neutron star mergers, but this is the first time it has been observed. The merger formed roughly 10 earth masses of gold, worth $300,000,000,000,000,000,000,000,000,000!

Long-term observations of the afterglow with radio telescopes by our team at the University of Sydney have allowed us to estimate the energy of the explosion produced by the merger. Radio observations also allow us to understand the geometry of the outflowing material, and the of the merger environment.

The radio lightcurve of GW170817 over the first 200 days, peaking at 150 days post-merger. The peak of the lightcurve can give us information about the energetics of the afterglow. Image Credit: Dobie et al. (2018), ApJL

We have answered some basic questions about neutron star mergers and the universe in general, but there is only so much that can be accomplished with a single event. In order to thoroughly understand the neutron star merger mechanism and its implications throughout the rest of physics we require observations of a large sample of events.

Further reading:

https://arxiv.org/abs/1710.05436

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.161101

https://arxiv.org/abs/1803.06853