Simulating realistic neutron-star mergers

The emergence of gravitational-wave astronomy is rapidly changing the astrophysics landscape. From the breakthrough detection of merging black holes in 2015 to the astonishing neutron-star binary event from August 2017 and the regular alerts sent out by the LIGO-Virgo Scientific collaboration during the latest observing run, it is clear that the gravitational-wave universe is richer than one might have expected. The impact and discovery potential of this new area of astronomy is considerable. As the sensitivity of the gravitational-wave instruments improves, a broader range of sources will come within reach. Further unique events (like GW170817) associated with electromagnetic counterparts will help resolve longstanding mysteries ranging from the formation and evolution of compact binary systems to the central engine of short gamma-ray bursts, from the details of matter under the extreme pressures of a neutron star core to the dynamics of black holes and (through kilonova signatures) the formation of heavy elements in the Universe. In order to realise this potential, we need to improve our models of the violent dynamics associated with neutron-star mergers. This inevitably involves expensive large-scale numerical simulations. The main aim of the proposed project is to add a level of physics realism to the current generation of such simulations.

The need for robust numerical simulations of neutron-star mergers is particularly pressing as we prepare for the next-generation of ground-based gravitational-wave interferometers (the Einstein Telescope in Europe and Cosmic Explorer in the USA). Reliable models of neutron-star dynamics are essential in order to inform the design of both the instruments themselves and future data analysis strategies.

This proposal builds on the Southampton Gravity Group's expertise in neutron-star and gravitational-wave astrophysics, and is aimed at developing a deeper understanding of the dynamics neutron stars, the observational signatures from binary mergers and how these signals can be used to provide information about the involved physics (in particular, the equation of state of extreme density matter at very high temperatures). The programme involves key technology development (e.g. nonlinear numerical simulations with a dynamical spacetime) drawing on complex physics input (e.g. multifluid aspects and electromagnetic fields). The overall aim is to develop significantly improved models that can be tested against future high-precision observations.

Neutron stars are fascinating and enigmatic objects, involving inspirational science and representing unique laboratories for the exploration of the extremes of physics. 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 Large Hadron Collider at CERN and RHIC at Brookhaven. 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 programme represents a focussed effort to explore the astrophysics 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.