📄 Showing the "Simple" version — "Popular" is not ready yet. Add it to favorites to help prioritize it. Ask for the full read

Baking bread starts with nearly homogeneous dough, in which tiny gas bubbles are already hidden. They grow, and the rate of their growth determines the future structure of the crumb. In cosmology, the role of such bubbles is played by the seeds of galaxies — barely noticeable density enhancements in the hot 'soup' of the early universe. Their distribution is imprinted in the cosmic microwave background (the faint glow left over from the Big Bang).

The ACT telescope, perched high in Chile's Atacama Desert, recently measured this radiation with unprecedented precision. The result was surprising: it seems that inhomogeneities grew not uniformly but with acceleration, as if the bubbles in the dough suddenly started to inflate faster and faster. Astrophysicists call this a 'positive running of the spectral index.' The essence is simple: the rate of growth of inhomogeneities increased over time.

The authors of the new work tested which inflationary models can produce such accelerated growth. The simplest versions, with a single scalar field or with modified gravity of the $F(R)$ type, fail. They predict an almost constant growth rate, which agrees with earlier data from the Planck satellite but diverges from the hints from ACT. However, more sophisticated scenarios (for example, with multiple fields or with the addition of a Gauss-Bonnet term to the gravitational equations) easily produce the required acceleration. Moreover, they do not contradict other observations, in particular the detection of gravitational waves from the merger of neutron stars (event GW170817). That event placed stringent constraints on theories of gravity.

So the cosmic 'recipe' is getting more complicated. Perhaps the early universe was not as simple as it seemed just a couple of years ago. Describing it will require richer ingredients — additional fields or quantum corrections to Einsteinian gravity.