11 March 2022

Johan Samsing and NBIA Gravity Group publishes in Nature

NBIA Assistant Professor Johan Samsing is lead author on a paper now published in NATURE together with NBIA members Bin Liu, Dan D’orazio, Martin Pessah, and a small team of international collaborators. The paper provides key insight to the fundamental question: How do black holes form and merge in the Universe?

The work centers around a spectacular gravitational wave observation by LIGO/Virgo in 2019 (GW190521) that revealed a merger between two black holes which was unexpected in many ways. Johan Samsing explains: “The black hole masses were heavier than thought physical possible, light was possibly associated with the merger, and the orbits appeared highly eccentric moments before coalescence. Nothing of this was expected but it all happened in one event. How?”. A possible answer should be found in the heart of galaxies. A significant fraction of the galaxies in our Universe have a central black hole, weighing up to 100 million times the mass of the sun that is surrounded by a massive rotating disk of gas. Having black holes to grow and merge in such disks could explain the larger masses and the possible light associated with the source, but what about the eccentricity? That has until now been a major outstanding question.

In the NATURE paper Johan Samsing and his collaborators consider scatterings of not only two, but three black holes in such planar gas disk environments. The study suggests that the gas disk likely captures smaller black holes which over time not only move closer to the center but also to each other. This creates black hole binaries that interact with the remaining single black holes in the disk. The breakthrough comes when post-Newtonian corrections to the 3-body interactions are included. They then find that when interactions take place in a near two-dimensional plane representing the gas disk environment, the probability for eccentric mergers can be up to 100 times larger compared to the three-dimensional case representing most standard stellar systems. The finding concludes the first possible solution to the most spectacular event observed by LIGO/Virgo to date.