COSMOS consortium members (dp002) have made good progress over the past year using DiRAC HPC Facilities, particularly implementing adaptive mesh refinement (AMR) techniques for the study of extreme gravity in a cosmological context. This year we highlight high resolution simulations of radiation from cosmic strings which were generated using GRChombo, that is, the pioneering numerical relativity code developed within the COSMOS consortium and now attracting a growing research community (see the public repository github.com/GRChombo). The improving sensitivity of the LIGO/Virgo gravitational wave detectors ultimately motivated this work because of the discovery potential of these experiments for cosmic strings, but the initial applications are actually on the origin of dark matter axions.
Cosmic strings – heavy line-like defects traversing through space – can arise when the rapid cooling of the hot early Universe triggers a phase transition. For this reason, cosmic strings are strongly tied to fundamental physics and so their detection could offer deep insights into the underlying structure of particle physics and spacetime itself. Accurate signal templates are essential for observational searches, but numerical simulations of cosmic strings are computationally challenging because of the wide range of physical scales involved; the ratio between the string width and the scale of the Universe across which the strings stretch is approximately 1030! This is far beyond the capability of current simulations to resolve, so different approximations are made for simulations to be computationally viable, usually either taking a zero thickness limit or a fixed comoving width; the outcome has been quite different predictions in the literature for both gravitational wave signatures and string axion (dark matter) radiation. A recent consortium paper (arxiv.org/1910.01718.pdf) addresses this controversy by using AMR to simulate global (or axion) string configurations and networks, allowing the numerical grid resolution to adapt to the scale of the problem. Figure 1 shows a simulation of an oscillating sinusoidal string, with massless radiation emanating out in a quadrupole pattern. Performing a careful quantitative analysis of this axion radiation led to the conclusion that the evolution of global strings tends towards that predicted in the thin-string limit, with additional radiation damping or backreaction. This provides a significant step towards resolving this issue and making precise predictions (in this case) of the small axion mass, which is a key parameter defining current dark matter axion searches.
This research is also being taken forward with much larger simulations of full cosmic string networks (see Figure 2), using diagnostic tools developed to measure competing massless (axion or GWs) and massive (Higgs) decay channels. This cosmic string work with the flexible GRChombo code has proved to be an important exemplar for testing new HPC capabilities, particularly for the COSMOS Intel Parallel Computing Centre collaboration on in-situ visualization (with the Intel Viz team, TACC and Kitware); this pioneering work using the OSPRAY ray-tracing visualization package has been demonstrated by consortium members at recent international supercomputer conferences and at SIGGRAPH 2019. We have also initiated a collaboration with the Chombo team at Lawrence Berkeley National Laboratory to speed up core libraries.