A hot coffee on the table cools down: heat spreads from the hot center to the edges. The authors suggest that the universe behaves in a similar way, just on scales of billions of light-years. Their model describes not the usual dark energy, but a smooth redistribution of energy within the cosmos, akin to the slow flow of heat from denser regions to rarefied ones.
This energy flow is not just a pretty metaphor. It strictly obeys the second law of thermodynamics: entropy, the measure of disorder, must increase. The scientists derived a relation that satisfies this condition and tested it against two datasets. The first is the flashes of distant supernovae, those cosmic beacons. The second is the sound waves frozen in the distribution of galaxies. The comparison showed that energy could indeed have flowed with a strength inversely proportional to the square root of time since the Big Bang.
The most unexpected consequence is the geometry of space. The model favors a slightly closed universe, like the surface of a giant sphere. A beam of light sent forward in such a world would eventually come back from behind. The curvature is small but noticeable: it helps reconcile two competing ways of measuring the age of the cosmos without sacrificing either. The coffee has cooled, but the cup turned out to be not flat but slightly curved, and that explains everything.
If the universe were closed, as this model suggests, its volume would be finite — about 10 trillion cubic light-years, which is 10 times larger than the part we can see.
In Stanislaw Lem's novel "Solaris," the planet's ocean redistributes energy in a manner similar to the diffusion model — only on a stellar scale.