In a bakery, raisin dough, as it rises quickly, leaves voids and an uneven pattern on the crust from the berries. Something similar may have happened in the early Universe: the ultra-fast expansion — inflation (predicted by Alan Guth) — stretched space, imprinting onto it the traces of massive particles born from the energy of expansion. That 'bread' is the cosmic microwave background — a thermal imprint of the Universe at 380,000 years old, discovered by Arno Penzias following a prediction by George Gamow. It appears uniform but holds a pattern of tiny temperature variations.

By analyzing sky maps from radio telescopes Planck and ACT, scientists used computer simulations to study the spectrum — the distribution of brightness — and searched for a pattern resembling dents from raisins on a baked crust. This could be the imprint of giant particles, billions of times more massive than protons. Only a faint hint was found — at the two-sigma level (roughly like guessing heads in two out of three coin tosses; a major discovery requires five or six). Future instruments, such as the Simons Observatory, will test the signal more precisely.

If the imprint is confirmed, it will open a view into an era when the entire observable Universe fit within the volume of an atom, and the forces that governed that world will become clearer. This will connect expansion with gravitational lensing and the reddening of distant galaxies.