Pinpointing the Origin of Noisy Compact Binary Mergers

Gravitational waves are ripples in the fabric of spacetime, which carry information away from the source that created them, at the speed of light. Any non-spherically symmetric accelerated masses will produce gravitational waves, just at differing strengths. Compact object mergers, such as the collisions of neutron stars and black holes, are some of the strongest sources of detectable gravitational waves.

Around 1.3 billion years ago, two black holes roughly 30 and 35 times the mass of the Sun collided with one another, and sent gravitational waves rippling throughout our Universe. In 2015, these gravitational waves swept through the Earth, and were detected by the LIGO interferometers. Since this groundbreaking initial discovery, nine more signals from the merger of black holes have been observed with the LIGO and the Virgo detectors. We estimate the masses of the black holes which collided to create these ten signals to be between 5 and 67 times the mass of the Sun. The black hole mass distribution is not thought to be continuous however. Simulations of metal-poor massive stars, around 130-250 the mass of the Sun, predict they end their lives in a pair instability supernova. In these stars, electron and positron pairs are created in the core, which cause the star to become unstable and collapse. In these supernovae no remnant is created. This means there should be a gap in the black hole mass distribution; there should be no black holes with masses between 50 - 130 times the mass of the Sun. Gravitational waves provide a unique way of probing this gap.

The aims of this proposal are twofold. Our first aim is to map out the gap in the black hole mass distribution using gravitational-wave observations. The techniques we need to develop to be capable of probing this mass gap will also lead us toward our second goal - facilitating future joint gravitational-wave and electromagnetic observations.

The first and only gravitational-wave signal from a neutron star coalescence, was almost simultaneously detected with gamma rays. In subsequent hours and days, the counterpart was observed across the electromagnetic spectrum. With these spectacular observations, we have been able to determine the nature of short gamma ray bursts and understand where much of the heavy elements are made. This proposal will maximise the chances of making further multi-messenger observations, which will be vital for probing many areas of physics, including estimating the expansion of the Universe.

Although these two goals are very distinct, the techniques we need to develop to achieve these objectives are very similar. Both aims rely on investigating and improving the output of gravitational-wave detectors. This proposal will therefore develop novel methods of overcoming and understanding the variation of gravitational-wave detector noise. In addition to achieving the goals of this proposal, this research will have far reaching consequences in other areas of gravitational-wave astronomy. For example, our work will allow for more rigorous tests of General Relativity to be performed and ensure confident detections of gravitational-wave signals that are different to those that have currently been observed.

The Institute of Cosmology and Gravitation (ICG) at the University of Portsmouth is committed to obtaining impact from our research. Nuttall and her team are fully engaged in this commitment. In fact, Nuttall is the ICGs Champion of Outreach and Public Engagement. The ICG is part of the South-East Physics Network (SEPnet) and as such has access to a regional network of outreach and employer engagement officers, as well as the SEPnet Outreach and Employability Directors. There are three full-time members of staff at the ICG, who deliver and provide support for impact: Nic Bonne (Isaac Physics Widening Participation Fellow and Tactile Universe Outreach Officer), Jen Gupta (SEPnet/Ogden Trust Public Engagement and Outreach Manager) and Gill Prosser (SEPnet IPS Fellow).

The impact strategy of the ICG is:

Engagement through Citizen Science: The ICG has over a decade of experience with online citizen science projects, such as GalaxyZoo and Zooniverse. Millions of people around the world engage with Zooniverse across 70 active projects; we are one of only two UK universities with institutional membership. We provide dedicated development and support of the Zooniverse and plan to continue our involvement.

Engagement through Large Events: The ICG runs an annual event in the Portsmouth Dockyard called Stargazing Live. Last years event (2018) was attended by 800 people. We hold at least one large-scale public event each year, and support ICG members to create new and innovative activities of their research for use at such events. We also support Entropy, an immersive art performance; since 2017 it has reached over 2000 people. Members of the ICG also provide numerous public talks in public venues (Astronomy on Tap) and to interested groups (i.e. astronomy societies).

Engagement with School Children: Our new schools outreach programme partners with specific schools to engage with the same school pupils throughout their school career (9-18). Through this programme we will deliver workshops and other activities, using our research to inspire pupils and encourage them to study physics at a higher level. Nuttall is already engaging in this effort and is a STEM ambassador.

Engagement with Visually Impaired Children: Tactile Universe enables those with visual impairments to engage with our astrophysics research. We will demonstrate that astrophysics is accessible to primary and secondary school children by working with them. Over this grant period we will nationally launch this project; we also have international connections with South America (funded by the IAU Office of Astronomy and Development). SEPnet recognises the importance of this project, who awarded the Tactile Universe its 2017 Public Engagement Innovation Project Award.

Student Industrial Engagement: A key goal of the governments Industrial Strategy is to reverse the national shortage of STEM skilled graduates. To address this, the ICG will enhance the training of our PhD students and some PDRAs through industrial placements. These will be sourced through an STFC-funded doctoral training centre called Data Intensive Science Centre in SEPnet (DISCnet). We recently received GCRF funding (from STFC) to extend DISCnet training to South Africa (SA-DISCnet), which includes industrial placements for both UK and African students. The ICG will host regular Deep Data Dives (D3) events where PhD students and PDRAs can apply their knowledge and expertise to an organisations data intensive problems. The first D3 was held with Portsmouth City Council in 2018.

Staff Industrial Engagement: The ICG is developing a portfolio of strategic innovation activities with key external sectors. Data-Data-intensive skills will be used to address the Grand Challenges of Growing AI and Data-Driven Economy, identified in the governments Industrial Strategy. Initial projects are in cardiovascular data, risk analysis of environmental hazards and smart meters.