Detectors pick up the “ripples” of gravitational waves from mergers of black holes or neutron stars — like an echo from the kitchen where a cosmic dish was being prepared. Astronomers are trying to reconstruct the recipe: how two massive stars turned into an exotic pair and collided. At the heart of the debate is whether the “common envelope” stage is needed, when one star swells into a giant gas ball and engulfs its neighbor, forcing them to rapidly spiral together.

It turned out that for duets of two black holes or a black hole with a neutron star, many models — with and without a common envelope — yield roughly the same number of dishes at the end. As if the pie came out the same whether the dough was kneaded by hand or with a mixer. The contribution of the common envelope phase floats from zero to one hundred percent, but the final merger rate is almost unchanged. In other words, a taster wouldn’t be able to tell the difference.

For pairs of neutron stars, it’s a different story: almost always (>90%) they go through this gas phase. The reason lies in the complex interplay of processes: from how stars exchange matter to supernova explosions. These “ingredients” act together, so changing them one by one is unreliable. To decipher the recipe, we need not only counts but also detailed data on masses and spins. Gravitational waves are just the first sip in the tasting.