Enabling rapid observation of compact binary mergers with a network of gravitational-wave observatories

In this project our main objectives are to ensure that the gravitational-wave signals emitted by compact binary mergers are detected within seconds of those signals passing through the Earth and to exploit the observations that we will make to better understand the physics of compact binary mergers. Regularly observing the mergers of neutron stars and black holes in the Universe will allow us to measure the distribution of the masses of such objects and infer the mechanisms by which such systems form and evolve. These observations will allow precision tests of general relativity, an independent measure of the Hubble's constant and allow us to probe the behaviour of matter inside neutron stars.

Observing these signals promptly is also important to enable multi-messenger followup of the mergers. The observation of the binary neutron star merger GW170817, simultaneously observed by the Fermi gamma ray observatory, demonstrated the importance of multi-messenger astronomy and the wealth of scientific knowledge such observations can produce. It is vital that we lay the groundwork to ensure that as many systems as possible are observed in low latency and appropriate sky localization regions produced and made public to facilitate external followup observations.

To be ready to promptly observe as many compact binary mergers as possible, we will ensure that information from the four observatories expected to be operational from 2021-Advanced LIGO, Advanced Virgo and KAGRA-can be utilized effectively when identifying compact binary merger signals in low latency and when identifying plausible regions on the sky that such signals could have originated from. We will also improve the sensitivity of these low latency searches by improving the methods by which the code distinguishes between instrumental artefacts and astrophysical signals.

Finally we will use the observations that we will make to measure the distribution of the angular momentum of compact binary mergers and to probe the physical processes at the cores of neutron stars.

Gravitational waves are one of the most remarkable predictions of Einstein's General Theory of Relativity. These can be thought of as ripples in the fabric of spacetime propagating at the speed of light. Gravitational waves are emitted by non-spherically symmetric accelerated masses, such as two black holes or neutron stars orbiting each other. Gravitational waves are incredibly difficult to detect, but in the last years large-scale observatories, including Advanced LIGO, Advanced Virgo and KAGRA, have reached the necessary sensitivity to observe gravitational waves.

The first gravitational-wave signal observed in September 2015 was produced by two black holes roughly 35 times the mass of our Sun colliding approximately one billion light years away. Since then nine additional binary black hole mergers have been observed. The crowning achievement of gravitational-wave astronomy to date was the observation of two merging neutron stars in August 2017. This signal was special because it was observed simultaneously as a gamma-ray burst by the Fermi observatory and then, following the release of the gravitational-wave sky localization region to astronomers, was observed across the electromagnetic spectrum.

The potential of "multi-messenger" astronomy-observing sources with multiple "messengers", such as gravitational-waves, photons, neutrinos or cosmic rays-is remarkable. We can explore the validity of Einstein's theory in one of the most extreme environments possible. We can make an independent measurement of the rate at which the Universe is accelerating. We can probe the nature of matter deep within a neutron star, where it is so dense that 1 teaspoon of material weighs as much as a mountain on the Earth.

However, all of this requires us to actually observe these gravitational-wave signals, and to do it quickly enough that we can alert external astronomers to search for a coincident signal. In this grant we will develop methods to promptly search data from Advanced LIGO, Advanced Virgo and KAGRA to observe the gravitational-wave signature of merging compact objects. We will ensure that such observations are rapidly localized on the sky and that this information is rapidly communicated to external observers. We will also develop techniques to further improve the sensitivity of these searches, allowing us to dig deeper into the noise, and to observe new types of compact binary mergers that have not been observed to date.

We will also exploit the astrophysical potential of our observations. We will investigate if double black hole systems form from binary star systems, where both stars have gone through a supernova resulting in a pair of orbiting black holes, or if the two black holes formed in isolation and were later came together as a result of interactions in some dense environment. We will also look to probe the behaviour of matter at the core of neutron stars, one of the most extreme environments in the Universe.