In space, merging black hole pairs are common, and often one is noticeably more massive than the other. This mass difference isn't just a number: it's a key to their shared history.
Astrophysicists studied the stable mass transfer channel — a process where matter flows smoothly from one star to another without disrupting the system. They built a mathematical model linking two key factors: how stable the transfer is and how efficiently the companion star captures the material. The model naturally split the pairs into two groups. In one, black holes retain their original mass inequality. In the other, the lighter hole manages to 'catch up' to the heavier one through accretion (accumulating matter).
Applying the model to data from the GWTC-4 catalog (collected by the LIGO and Virgo detectors), the scientists focused on the mass peak around 10 solar masses. It turned out that in this group there are almost no 'catching-up' pairs — black holes retain their original inequality. This allowed them to estimate the parameters of the mass transfer itself: it must be fairly stable, and accretion not too efficient. In other words, matter flows calmly, but the second hole barely gains weight.
This approach turns dry statistics into a tool for studying the physics of stellar evolution. Instead of guessing how a particular pair formed, scientists can directly measure conditions in distant binary systems, just from the mass distribution.
In some black hole pairs, the mass difference can be tenfold, as if the Sun were waltzing with a giant planet.
In Carl Sagan's novel 'Contact,' aliens use gravitational waves to send messages — today we receive such 'messages' from nature itself.