On November 23, 2023, LIGO’s instruments captured gravitational waves — a signal from a black hole merger. According to calculations, the black holes turned out to be unprecedentedly heavy—137 and 101 times the mass of the Sun, and spinning unusually fast. But the signal’s shape was strange: like a reflection in a funhouse mirror.

Such distortion can arise from spacetime curvature around a massive body, like a heavy ball sagging a stretched sheet. Light or a gravitational wave passing by changes its path. This is called gravitational lensing. If the lens is small, it not only creates a copy but alters the signal’s very shape — an effect known as microlensing, or the funhouse mirror effect. Perhaps GW231123 wasn’t a record merger, but a lighter one, amplified and distorted by a compact mass — say, a rogue black hole or a clump of dark matter.

To sort this out, scientists compared two hypotheses using Bayesian inference — a statistical method that accounts for event rarity. They ran numerical simulation of different lens types and processed data with signal processing algorithms. The key was estimating how often suitable lenses occur in the Universe. For this, they used data on redshift, the stretching of light from distant objects due to cosmic expansion, and on the large-scale structure of galaxies. It turned out that the probability of randomly obtaining such distortion, given all factors, is less than one in a hundred thousand. Even the most “fortunate” scenario, a lens made of self-interacting dark matter, gives a chance below 0.1%.

So the cosmic mirror didn’t deceive — nature really did create such a massive black hole duo. And the verification method will help future observatories distinguish real records from gravitational mirages.