Gravitational Waves at the University of Portsmouth

In this project we will ensure that researchers at the University of Portsmouth continue to play crucial roles within the LIGO Scientific Collaboration. We will continue to lead efforts in observing new compact binary mergers in the data recorded by LIGO, Virgo and KAGRA and in the "detector characterization" effort identifying and mitigating the causes of non-Gaussian, and non-stationary behaviour in the LIGO instruments.

One of our main objectives in this project is to ensure that the gravitational-wave signals emitted by compact binary mergers are reliably detected. We will play a leading role in real-time searches, where the goal is to analyse the data being recorded by LIGO, Virgo and KAGRA as it is being taken. In doing so we identify new compact binary mergers seconds after the data is taken and produce the information needed to determine possible locations on the sky where the observed signals could have originated from. Such low-latency searches are crucial to produce the information needed to enable follow up observations of gravitational-wave signals, such as the binary neutron star merger GW170817. In addition we perform archival searches, where we use recalibrated data, detailed knowledge of the instrument behaviour and search techniques that cannot be applied in real-time to perform "deep" searches for compact binary mergers, identify any signals not identified in real time, and confirm previous observations. Results of these searches form the core of the LIGO-Virgo catalogues of compact binary mergers, which in turn are used to measure the distribution of the masses of such objects, infer the mechanisms by which such systems form and evolve, perform precision tests of general relativity, independently measure the Hubble constant and allow us to probe the behaviour of matter inside neutron stars.

We will also develop search techniques to target the first observations of new types of compact binary mergers, including processing neutron-star black-hole mergers and intermediate-mass binary black-hole mergers. Such sources are rarer than the "vanilla" observations made to date, but even a single observation of either source would revolutionise our understanding of such objects and the environments in which they form. To date, searches of LIGO and Virgo data do not search explicitly for these sources and so as we enter the era of gravitational-wave astronomy it is crucial to ensure that we are able to observe any type of compact binary merger that might be present in our data.

We will also work towards better characterizing the data produced by the LIGO observatories, drawing on our world-leading expertise in this field. We will identify the problems that cause artefacts to be present in the data and work with instrumentalists on-site at the observatories to fix the problem thereby improving the quality of future data. Additionally, by understanding the character of the data we can quantify the effect that non-Gaussianities and non-stationarities in the LIGO data and ensure that this information is used when estimating the parameters of the systems observed in the LIGO data, and avoiding any biases that can occur when one does not consider these effects. We will also explore and develop innovative new techniques for detector characterization, including the use of sensor arrays and modelling of instrumental couplings. Currently, both the ability to observe compact binary mergers, and our ability to accurately measure the source parameters, are limited by non-Gaussianities and non-stationarity in the LIGO data. By addressing and resolving these problems we can maximise the number of sources observed, and mitigate the bias in parameter estimates.

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!