A gravity model that lets dark energy change sign could help resolve a persistent disagreement over how fast the universe is expanding.
In an existing theory of gravity, the same present-day matter density can fit two very different histories of the universe. Researchers had largely pursued the one that kept dark energy’s density positive. Exploring the other has now revealed a possible history in which that density began below zero and turned positive billions of years later.
Dark energy is the mysterious ingredient believed to drive the universe’s accelerating expansion. The simplest explanation treats it as unchanged throughout cosmic history. In the new analysis, however, the effects attributed to dark energy arise from a modified description of gravity. Its density is an effective quantity derived from the gravitational equations, rather than a property of a newly introduced substance.









