Picture the universe at roughly one-third of its current age. Galaxies are clustering, matter is drawing together, and dark energy — that mysterious ingredient that supposedly pushes everything apart — is actually doing the opposite. It is pulling. That is the startling suggestion at the heart of a new study published in Physical Review D, co-authored by researchers including Abraão J.

S. Capistrano and Suresh Kumar. For twenty-five years, the standard cosmological model, known as LCDM, has held up remarkably well. It predicted the faint afterglow of the Big Bang, mapped how galaxies cluster, and matched the brightness of exploding stars called Type Ia supernovae.

But one number has refused to cooperate: the Hubble constant, which describes how fast the universe is expanding right now. When scientists measure it using ancient light from the early universe, they get a different answer than when they measure it directly using nearby pulsating stars called Cepheids. The gap between those two answers is between five and seven standard deviations wide — far too large to dismiss as a mistake. The new model, called LsCDM, proposes that the cosmological constant was negative in the universe's youth, behaving almost like ordinary gravity and pulling matter together, before flipping to a positive value.