Dark Energy: From Brake to Engine express
The universe is expanding faster and faster thanks to dark energy. But measurements of the rate give contradictory results. A new explanation: in the past, dark energy acted as a brake, slowing the galaxies' recession, and then switched to acceleration. Like a car that first rolls backward and then lunges forward. This transition is described by the Ph-ΛsCDM model, based on the theory of gravity by Einstein. It uses a phantom field — a substance capable of smoothly changing the sign of energy from attraction to repulsion. This avoids the 'Big Rip' that previous theories threatened. This is a development of ideas that emerged after the discovery of acceleration by Perlmutter. Most unexpectedly, the field starts accelerating even while the energy is still attracting — as if the engine started working before the clutch engaged. The model promises a stable future despite the grim predictions of standard cosmology.
Phantom energy is so named for its ability to violate physical laws, like a ghost passing through walls.