The Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo interferometer have been turning up more and more binary black hole mergers in their observing runs. Do the black holes involved in these mergers have anything in common or are they paired purely by chance?
Binary Black Holes and Where They Come From
The question of how binary black holes (BBHs) form is still wide open, further complicated by the fact that the masses of the black holes involved are higher than expected. Some astronomers have suggested that BBHs are the result of massive stars that were already in binaries, while others have proposed scenarios where black holes in dense stellar populations encounter each other and pair off. Another possibility is that the black holes in BBHs formed as they are in the early universe — skipping existence as a star — and ended up in binaries.
Artist’s illustration of the merger of two black holes in space. [LIGO/T Pyle]BBH mergers are a good way to study BBHs themselves; properties of the merger components (like mass) are imprinted into the resulting gravitational waves. In their first two observing runs, LIGO and Virgo spotted ten BBH mergers, and the black holes involved ...