In the first moments after the Big Bang, the universe expanded at a terrifying rate. This process is called inflation, and it smoothed out all the wrinkles, making the cosmos homogeneous. But what started inflation itself? Physicists suspect that a single random fluctuation in the inflaton field, the special field that drove the expansion, might have been enough.

For a long time, simulations of inflation were run under artificial conditions: the universe was placed in a periodic box, like a torus with glued edges. This imposed unnatural curvature constraints. Stephen Hawking and others suspected the real picture might be different. The authors of the new work have for the first time modeled the birth of inflation in asymptotically flat space — one that is nearly uncurved at large distances. This approach is closer to what we see around us: a local bubble, not a closed geometry.

The result confirmed the old hunch: for inflation to start, the initial fluctuation must be several times larger than the characteristic inflationary scale (roughly 10⁻²⁷ cm). If the patch is smaller, it simply collapses into a black hole or dissipates without inflating the universe. It's like trying to blow up a balloon by breathing on it from a meter away: you need to put your lips right up to it so the air actually gets inside. Same here — only a sufficiently large "seed" triggers the cascade of expansion.

The constraints previously imposed on initial conditions could have distorted the picture. A periodic box of fixed size inadvertently tuned fluctuations to the right scale. Now it's clear: nature doesn't have to be periodic, and the start of inflation is a rarer, more demanding event than it seemed. But if it does happen, it triggers a chain reaction that gives rise to all the diversity of galaxies and stars.