SXS- Simulating eXtreme Spacetimes -is an ongoing scientific collaboration that has been generating simulations of dramatic events in space, particularly mergers of binary black hole systems, for several decades. Recently, SXS published a paper describing version 3 of its catalog of binary black hole simulations, six years after releasing version 2.
In 2015, LIGO (the Laser Interferometer Gravitational-wave Observatory) first successfully observed gravitational waves -ripples in space-time caused by dramatic celestial events-but the theoretical astrophysicists of the SXS collaboration had already been hard at work over the prior two decades calculating what these waves might look like as they reached Earth.
Gravitational waves are created by a variety of cosmological events, including mergers of neutron stars and black holes, and these waves pass through space and indeed through the Earth. Very sensitive detectors like LIGO record the perturbations made by these gravitational waves as they pass by and then try to determine what sort of celestial event gave rise to them. But it is equally important to take on this challenge from the opposite direction, by calculating what gravitational waveforms would result from different types of celestial events before detections are available. This is what the SXS collaboration does.
Solving Einstein's equations for merging black holes is extremely challenging. "You can take Einstein's equations and write them in a form that's described as hyperbolic, that suits analysis of wave-like phenomena," Keefe Mitman (PhD'24), now a NASA fellow at Cornell University, explains. "This means that if you give these equations some kind of initial data, there is a unique solution for how those data will evolve over time. And as we move to higher-resolution output, we can expect convergence: simulations that come closer and closer to the exact solution you would expect from Einstein's equations."
Today the scientists of SXS and LIGO bounce their data and equations back and forth between one another, pairing predictions to observations and back again. At this point, says Mitman, "we're doing enough and LIGO is just catching up. Up until now, when LIGO has detected something, its astrophysicists can go to the SXS catalog and find a simulation that suggests what is going on with their observations. If they don't find what they need, they can ask SXS for a new simulation with different parameters that might better match their data."
