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.
Original abstract
as written by the authors arXiv:2606.11062
We present Ph-$$\Lambda_{\rm s}$$CDM, a phantom-scalar realization within General Relativity of the sign-switching cosmological-constant idea, $$\Lambda_{\rm s}$$CDM, in which a phantom scalar evolving on a bounded hyperbolic-tangent potential induces a smooth mirror AdS-to-dS transition in the late-time dark-energy density. The wrong-sign kinetic term, usually viewed as pathological, becomes the mechanism lifting the field from a negative- to a positive-energy vacuum-like regime. The construction also shows that the field can become repulsive while its energy density is still negative. The cosmology nevertheless remains controlled: total energy stays positive, the late-time attractor is de Sitter rather than a Big Rip, and the dynamics remain safely infrared. Ph-$$\Lambda_{\rm s}$$CDM thus offers a concrete late-time mechanism with the potential to address multiple cosmological tensions.
Phantom energy is so named for its ability to violate physical laws, like a ghost passing through walls.