The COSMOS consortium (dp002) continues to exploit DiRAC HPC Facilities and has made progress towards ambitious sciences in extreme gravity, inflation and the early universe, the study of the cosmic microwave sky and large-scale structure (including the Final Data Release for the ESA Planck satellite). Here, we highlight recent breakthroughs in general relativistic simulations of black holes and inflation using GRChombo, a flexible AMR numerical relativity code publicly released by COSMOS consortium members in early 2018 (see grchombo.org and github.com/GRChombo).
The groundbreaking discovery of a gravitational signal from a binary black hole merger in 2016 by LIGO is now followed by an exciting series of detections, including that of the first neutron star binary merger in 2018. With the current LIGO/VIRGO O3 science run with upgraded detectors detecting multiple events weekly, the new era of gravitational wave astronomy is well underway.
The GRChombo AMR code, originally developed and now anchored by COSMOS consortium members, is well-placed to exploit these new observations of gravitational waves (GWs). We have continued our long-standing industrial collaboration through the COSMOS Intel Parallel Computing Centre (IPCC) to modernize our research codes in cosmology and general relativity, focusing on the optimization of GRChombo for multi/many-core systems; for example, GRChombo is one of the few codes that can take full advantage of the many-core Xeon Phi systems. In addition, GRChombo continues to grow in functional capabilities, with the addition of a powerful MultiGrid initial conditions solver and the pioneering many-core OSPRay realtime ray tracer and volume renderer with in-situ capabilities (supported by the Intel Viz team).
The advent of GW astronomy has opened up a window into the one of the key questions in modern cosmology about the nature of dark matter. COSMOS investigators are taking a leading role in this search. One of the prime candidates for dark matter is an ultralight field which are sometimes called “axion-like particles” or ALPs. In recent papers (notably arXiv:1609.04724 and arXiv:1806.09367), we have investigated the formation of highly compact “axion dark stars” from these light fields, and the potential GW signals created by their collisions with neutron stars and black holes (arXiv:1808.04668, shown in the figure below), and with other axion stars (arXiv:1802.06733). In (arXiv:1904.12783), we showed that these fields can also interact directly with black holes to grow “scalar hair”, leading to a possibly detectable dephasing of the GW signals from BH binary mergers.
Beyond GW astronomy, COSMOS consortium investigators are also leading in the numerical study of fundamental questions in physics. For example, while inflation is now the paradigmatic theory of the early universe, providing a dynamical mechanism to explain why the universe is homogenous over cosmological scales and provide a source of the seeds of large scale structure. Nevertheless, can inflation begin in the first place, if the initial conditions are not homogenous? In recent work (e.g. arXiv:1712.07352), we classified the models of inflation which are not robust to inhomogenous initial conditions. Intriguingly, we found that low scale inflation – preferred by many string theory phenomenological models – is not robust, failing to inflate in the presence of large scale but subdominant inhomogeneities.
In addition, we continue to take the lead in investigating the nature of gravity in higher dimensions. Following our work in showing the failure of weak cosmic censorship in 5D due to black ring instabilities, we continue to study the end states of higher dimensional rotating (Myers-Perry) black holes. We find that, similarly to the black ring systems, these black holes exhibit the Gregory-Laflamme instability, leading to a fractal instability with the ultimate end state likely to exhibit naked singularities in an asymptotically flat space time (shown above). Given these results in extra dimensions, it is clear that, if cosmic censorship is indeed true, then it must be a property specific to the three-dimensional space that we live in; without this essential property, Einstein’s theory of general relativity could lose its predictive power.