The cosmos is like dough rising in an oven. To measure its rise, astronomers stick in 'toothpicks'—explosions of supernovae, whose true brightness is known. From the reddening of their light (redshift), they determine their recession speed, and thus the rate of the expansion of the universe. This relationship was established almost a century ago by Edwin Hubble, and today Adam Riess and Saul Perlmutter refine it with unprecedented precision. Supernovae have already helped uncover the invisible levers of this growth — dark matter and dark energy.

Testing homogeneity works like this: if you stick the toothpicks in unevenly, it will seem like the dough rose higher on one side. Observing 1,701 supernovae, scientists seemed to notice such an asymmetry in nearby galaxies. But it turned out: it's not the cosmos, but the habit of Earth-based telescopes to look more often at favorite patches of sky. Once the data from one major survey (SNLS) was removed, the apparent unevenness vanished.

Look where you may, the rise is the same everywhere. Even the cosmic microwave background, the afterglow of the Big Bang, confirms: the universe has been homogeneous from the very beginning. Future telescopes, like the Vera Rubin Observatory, will 'stick toothpicks' more evenly and finally settle the question of directional expansion.