Gravitational Waves at the University of Portsmouth

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 twelve 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 localisation 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, reliably estimate their source parameters and to do it quickly enough that we can alert 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 localised on the sky and that this information is rapidly communicated to astronomers. 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 work to better understand the data that is produced by the LIGO observatories. These gravitational-wave observatories are highly precise and complex machines and producing a "clean" data stream free of instrumental noise is a significant challenge. We will work in collaboration with the instrument scientists at the LIGO sites to "characterise" the data being recorded by these instruments. This will allow us to identify the causes of any "imperfections" in the data stream. These imperfections, which often show up as bangs and whistles in the data, harm our ability to observe genuine astrophysical signals. Additionally, if we do not include the effects of noisy data when assessing the parameters of sources that we observe, we run the risk of quoting incorrect, or biased, parameters for our observations. By identifying the causes of such signals we can fix the instrument to stop them happening. We can also understand the effect that these bangs and whistles will have on our ability to understand our new observations. This problem is illustrated in the online project Gravity Spy. If interested, you can visit the Gravity Spy website and help us in this effort!