At the very beginning of the universe, when all of space was compressed into a point, the laws of physics break down. This point is called a singularity — an edge where our equations lose meaning. But what if it's not an end, but a special type of boundary?
The authors of this work propose viewing the singularity as a free boundary. Usually, when solving equations, we rigidly impose conditions at the edges. Here, the boundary is allowed to "breathe"—it finds its own position from the requirement that the action (a key quantity in physics describing the dynamics of a system) be minimal. This approach yields a surprising result: among all possible scenarios for the early universe, only the smoothest and most orderly ones survive.
This means that chaotic, jerky models like Kasner solutions (where space contracts and stretches in fits and starts) are discarded. Instead, conformally regular spacetimes remain, including the Friedmann-Lemaître-Robertson-Walker (FLRW) model familiar to cosmologists — the foundation of our understanding of the expanding universe. Importantly, any small perturbations, the seeds of future galaxies, must reflect off the singularity, like waves off the wall of a pool.
Thus emerges an elegant picture: the singularity is not a dead end but a mirror that sets the rules of the game for everything that came after.
If we apply these conditions to our universe, any "sound" of the Big Bang, gravitational waves, should have been reflected back, leaving no traces in the earliest epoch.