A Golden (Binary) Future for Gravitational Wave Astronomy

Prof Nicholas Lockerbie writes:

The very first direct detection of gravitational waves, these coming from the coalescence of a pair of Black Holes, occurred from a scientific perspective only relatively recently—on 14 September, 2015. This first detection was made by the two LIGO (Laser Interferometer Gravitational wave Observatories) detectors, located at Hanford and Livingston in the USA.

Prior to this detection, the most compelling—if indirect—evidence for the existence of gravitational waves came from the measured, if rather slow, orbital decay of binary neutron star systems, such as PSR B1913 +16, in which one of the neutron stars is also a radio pulsar. Russell Hulse and Joseph Taylor were awarded the 1993 Nobel Prize for Physics, for their work on PSR B1913 +16.

Consequently, the two LIGO interferometers in the USA, together with the Virgo interferometer (their counterpart in Europe), were designed so as to be particularly sensitive to gravitational waves emitted during mergers of pairs of neutron stars, insofar as the final inspiral and coalescence of such corporeal bodies were understood, theoretically.

In the event, the last two years have seen four major detections (and one minor one) of gravitational waves, these being made firstly by LIGO, and more recently by LIGO operating together with Virgo, and yet all of these were the outcomes of collisions between very distant pairs of Black Holes.

However, on 17 August, 2017, all that changed.

The two LIGO detectors, together with Virgo, 'heard' the inspiral and merger of a pair of neutron stars for the very first time—this merger having taken place 'just' 40 million parsecs (~130 million light years) away, near the galaxy NGC 4993.  Moreover, this event was captured also in γ-rays, X-rays, UV and visible light, and infrared light. In addition, the gravitational waves were recorded over a significant period before the eventual collision of the two neutron stars, the eventual gravitational wave merger signal arriving less than 2 seconds before the initial γ-ray flash from that merger—after a passage through the Cosmos of 130 million years!

A great wealth of information (and a slew of research papers) has followed on from this discovery, and we now know that

• The mathematical (computer) modelling of gravitational waveforms arising from these merger events has been extraordinarily precise. Indeed, an experimentalist might say, eerily prescient.
• Gravitational Wave detectors are able to identify and to locate on the sky (by triangulation, using 3 detectors) binary neutron star inspirals before the stars collide—in the case of this binary neutron star merger approximately 100 seconds before. In the future, automated telescopes might receive alerts sufficiently in advance of a merger to pick up the very first light signals from such events.
• The speed of gravitational waves (which are ripples in the fabric of space-time) has been found to be the same as that for electromagnetic waves—to within 2 seconds in 130 million years (i.e. to 5 parts in 10¹⁶). This is a direct validation of a prediction made by Albert Einstein in his theory of General Relativity.
• It has been possible to calibrate the distance of this binary neutron star merger (approximately 130 million light-years away) using the polarization and strength of the gravitational waves; and so it has been feasible to 'lay a ruler on the Cosmos' out to this event.
• The link to the electromagnetic (EM) emissions, given knowledge of the distance to the source from the gravitational wave signal, has allowed emitted EM powers/energies to be found, so that the ensuing 'kilonova' can be modelled.
• Characteristic emissions from heavy elements such as gold have been found in the EM spectra. Perhaps such binary neutron star collisions are a significant (the primary?) origin of the heavy (trans-iron) elements found on Earth.

Links to articles:

Multi-messenger Observations of a Binary Neutron Star Merger

GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral

University news - Crashing neutron stars unlock secrets of the Universe

Congratulations to the winners of the 2017 Nobel Prize in Physics for Gravitational Wave Research

October 2017