Grabby Aliens

The universe is 13.8 billion years old, but the longest-lived stars will outlive it a thousandfold — many will still burn five trillion years from now. If advanced life needs a series of ‘hard steps’ — near-impossible evolutionary lottery wins — then the odds of appearing grow steeply with time, and almost all civilizations should arise on long-lived stars in the deep future. By that math, humanity is bizarrely, suspiciously early.

Hanson, Martin, McCarter and Paulson (2021) offer a deadline. Suppose some civilizations become grabby: they expand outward at a good fraction of lightspeed, fill galaxies, and visibly transform whatever they touch — and no new civilizations arise inside the volumes they take. Then the quiet era of the universe ends early, and anyone who exists at all must have appeared before the wave arrived. Our earliness stops being a fluke; it becomes a clue.

The model has only three parameters, each estimable: n, the number of hard steps (from Earth's evolutionary record, roughly 3–9); s, the expansion speed (high — likely over half of lightspeed — because slow, vast alien volumes would be visible in our sky, and none are); and a timing constant k, set by assuming our arrival date is a typical sample of all such civilizations. The silence is itself evidence: if expansion is fast, a civilization you can see is almost already upon you.

This simulation samples births from the model's (t/k)ⁿ law in a cube of comoving space and expands each domain at speed s. Toggle between what truly exists and what Earth's telescopes could see. Under typical parameters, grabby civilizations already control roughly 40–50% of the universe's volume — and if humanity endures, we should expect to meet one in roughly 0.2–2 billion years — assuming our descendants expand too; staying home pushes the median wait to about 2.5 billion.

Model note: we implement the paper's model definition (Sec. 8/10) but with the exact flat ΛCDM scale factor instead of the paper's a(t) ∝ t^2/3 power-law approximation — the paper itself flags dark-energy expansion as an unmodeled refinement. Because conformal time converges to a finite ceiling η_∞ = 63.6 Gly (the timeline’s right edge — even t = ∞ lands there), every expansion front stalls at a final comoving radius s × (η_∞ − η_birth) — its share of the dark-energy event horizon; nothing born too late, or too far away, can ever reach us. We also rank humanity among all grabby birthdates and apply the empty-sky observation as a filter on the realization and forecast; the paper’s headline tables instead build their clock from only those civilizations that, like us, see an empty sky at birth.