In its first moments, the universe resembled dough beginning to rise. If the yeast is unevenly distributed, the rolls come out in different sizes. Astronomers are hunting for such 'clumps' in the oldest light and in the arrangement of galaxies to understand what the recipe was.

A team of researchers took a catalog of 1201 galaxy clusters — the biggest 'clumps' in the cosmos. These clusters were found by how they distort the cosmic microwave background (the light left over from the Big Bang). The scientists compared how many clusters of different masses lie at different distances from us with calculations for various 'recipe' options for the universe. It turned out that the data best match the simplest option: the initial inhomogeneities were random, like bubbles in well-mixed dough.

This result matters because scientists are testing the hypothesis that in the first fractions of a second, the universe expanded not quite uniformly. If that were the case, noticeable deviations from randomness would appear in the distribution of clusters. So far, none are visible: the measured parameter describing such deviations came out close to zero (55 ± 125, i.e., within the error bars, consistent with zero).

Now, as next-generation telescopes begin building even more detailed sky maps, this method will allow a more precise test of how 'smooth' the birth of our world was.