If the horizon of a black hole is obstructed by something like a firewall, then the horizon could potentially reflect gravitational waves. If that was so, then LIGO should see evidence for these modifications. In particular, a collision between two black holes should produce an echo.
That’s the basic idea put forward by two independent groups of physicists, one led by
Vítor Cardoso and the other by
Niayesh Afshordi. Using simple models for a horizon with substance, the researchers showed that some of the gravitational waves emitted by a black hole collision should reflect back toward the black hole’s center. The waves would then reflect outwards again, where some would then once again be reflected at the horizon. The black hole would act like a resonant cavity with a semitransparent mirror at one end. It would emit periodic signals with decreasing amplitude. It would echo.
So much for the theory. What about the data? In late 2016, Afshordi and collaborators applied a custom-designed analysis to the publicly available LIGO data and
looked for evidence of echoes. Amazingly enough, they found echoes just where they sought them. The signal was not highly significant — the researchers estimated a 1 in 100 chance that the signal was just noise — but a signal nevertheless.
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