General Relativistic Astrophysics

The last year has seen a string of outstanding successes in gravity and relativistic astrophysics. The breakthrough detection of gravitational waves from merging black holes provided a clear demonstration of the discovery potential of this new area of astronomy. As the sensitivity of gravitational-wave instruments improves, and a wider network of detectors come online, a broader range of sources is expected to be detected. Observations of the late stages of binary neutron star inspiral and merger are anticipated with particular excitement, especially since such events may have counterpart electromagnetic emission (e.g. short gamma-ray bursts).

As we enter the era of gravitational-wave astronomy in earnest, there are many reasons for enthusiasm. The LISA Pathfinder demonstration of technology readiness of the drag-free interferometry required for space-based instruments, followed by the ESA selection of the LISA project (due for launch in the 2030s), ensures that gravitational physics will continue to develop for (at least) the next two decades.

The main emphasis of gravitational-wave astronomy is on problems involving neutron stars and black holes. These fascinating and enigmatic objects involve truly inspirational science and represent unique laboratories for the exploration of the extremes of physics. Black-hole astrophysics impacts on a range of fundamental issues, from the nature of gravity to problems in cosmology, e.g., associated with structure formation in the early Universe. Meanwhile, neutron star observations allow us to probe the state of matter under extreme conditions, providing us with information which complements that gleaned from colliders like the LHC at CERN. The modelling of these highly relativistic systems involves a broad range of physics that is not accessible in the laboratory. As our observational capabilities improve, we are reaching the point where precise modelling is required both to interpret data and to facilitate the observations in the first place.

The proposed research represents a coherent programme aimed at exploring the astrophysics of black holes and neutron stars in order to improve our understanding of the fundamental laws of physics of the Universe and reveal how nature operates on scales where our current understanding breaks down, a theme that remains central to the STFC mission.

Neutron star modelling involves much complex physics and relates to a range of astrophysical phenomena, primarily probed by radio timing and X-ray timing and spectra. Neutron stars may also radiate detectable gravitational waves (through a variety of scenarios ranging from the supernova core-collapse in which they are born to the merger of binary systems). The challenge is to decode observed signals to "constrain" current theories, including the elusive equation of state for supranuclear matter. This proposal aims to improve our understanding of neutron stars, including their evolution and dynamics and how they interact with their environment.

Black holes interact with their environment in a complex fashion. The modelling of this interaction provides a serious challenge. In particular, we need a precise description of the gravitational radiation-reaction-driven inspiral and eventual coalescence of binary systems. This problem is central for ongoing and future gravitational-wave searches. The gravitational capture of compact objects by massive black holes in galactic nuclei is particularly relevant for space-borne instruments like LISA, as the signal encodes information that allows high-precision tests of general relativity and precision studies of massive black-hole physics. A central objective for the proposed research is to model the inspiral dynamics in binary sources detectable by current ground-based and future space-based observatories.

This summary identifies some of the routes by which our astrophysics research programme will impact upon the wider world, including the general public, other scientific disciplines, and the technology sector.

COMMUNICATIONS AND ENGAGEMENT: The Southampton Gravity group has a consistent track record of engaging with the public to communicate the latest and most exciting aspects of its research. These have included public talks, lectures to school students, coordination of a Royal Society Summer Exhibition and contact with Members of Parliament. The Group plans to enhance this activity, with STFC-funded researchers playing a leading role in exploring new dissemination outlets, including the construction of interview/video clips for electronic circulation. The most exciting research advances will be promoted via the University Press office, whose help in publicising recent breakthroughs played a role in several articles on Southampton research appearing in the press, including in National Geographic, New Scientist and national newspapers.

COLLABORATION: The richness of the physics needed to model compact objects naturally leads to the possibility of developing collaborations with traditionally disparate scientific disciplines. In particular, there is scope for collaboration with experts in low-temperature physics, whose knowledge of condensed matter many prove invaluable in understanding neutron star interiors. Equally exciting is the possibility of exploiting links between the black hole inspiral problem and the problem of nonlinear electromagnetic pulse propagation in an optical fibre. We intend to explore these exciting overlaps, by organising focused study groups to explore the key issues.

EXPLOITATION AND APPLICATION: The theoretical work of the group is intimately linked to several large experimental efforts, whose innovative technological development impacts on industry, with spin-offs including satellite stabilisation systems, seismic isolation, the construction of large vacuum cavities, and laser stabilisation. As well as providing motivation for these efforts, the theoretical modelling of the group is crucial in making informed decisions as to how changes in expenditure and project duration impact on science capabilities.

CAPABILITY AND RESOURCE: We are rapidly approaching an era of precision astronomy, with new electromagnetic and gravitational experimental projects under rapid development. The young researchers that the group plans to recruit will receive training in exploiting these opportunities that few other groups could provide. Several previous group researchers have already secured prestigious academic appointments (e.g. in Cambridge, Amsterdam and Warsaw). Beyond the world of academia, the skill sets that the researchers would acquire would also equip them to play important roles in the wider community, as is reflected in the success of previous members in gaining attractive jobs in industry. The University's Doctoral College helps systematize this training, and enables the group to continue to produce well-rounded researchers with skills well suited to demands of the 21st Century economy and academia.