Relaxation Dark Energy: Compositional Dynamics and the Future of Cosmic Acceleration
Abstract
If cosmic acceleration is driven by a positive cosmological constant, the universe approaches an eternally accelerating de Sitter state, whose interpretation in quantum gravity remains elusive. We develop a framework in which acceleration is instead a transient stage of dark-sector relaxation towards compositional equilibrium: matter and dark energy approach a stationary density ratio, eventually diluting like pressureless matter. Guided by linear response theory and Onsager's near-equilibrium regression hypothesis, we describe this evolution through a composition variable measuring departure from equilibrium. Extending constant fractional relaxation to the full background evolution yields the dark-energy equation of state w=w0a−n, where n is the relaxation rate per e-fold of expansion. This history connects early dark-energy suppression, an upward phantom crossing and a density maximum to a pressureless scaling future. Using DESI DR2 BAO and Planck distance priors, with and without three alternative supernova compilations, we find a nominal $2.6−−3.6σ$ preference for nonzero relaxation over constant w. The composition framework also motivates a constitutive prescription for linear perturbations that remains regular at phantom crossing. Near equilibrium, regular long-wavelength composition disturbances relax at the background rate to leading order, while small-scale fluctuations propagate as damped sound waves. We prove, however, that for separately conserved matter and dark energy, the pointwise constitutive relation cannot sustain a regular, gradient-stable negative-pressure branch throughout its full past evolution. We explore modifications of the pressure response and their consistency with neighbouring homogeneous histories, identifying requirements for extending the near-equilibrium description to the full cosmological evolution.
Related
- Testing cosmic acceleration from thermogravity without vacuum energy
- A new window on dynamical dark energy: combining DESI-DR2 BAO with future gravitational wave observations
- Where a Dark-Energy-Tied Galaxy Law Holds and Where It Breaks: Low-Acceleration Tests, a Two-Footing Shear Pass, and the Satellite-Dwarf Excess (Working Note)
- Cusps, cores, and one acceleration: dark-matter haloes versus modified dynamics across the full SPARC sample
- Formation dynamics of a rotating solar spicule-like event and associated energy-mass transport
BibTeX
@misc{tasinato2026relaxation,
title = {{Relaxation Dark Energy: Compositional Dynamics and the Future of Cosmic Acceleration}},
author = {Gianmassimo Tasinato},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.13739},
eprint = {2610.13739},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.13739},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Relaxation Dark Energy: Compositional Dynamics and the Future of Cosmic Acceleration
AU - Gianmassimo Tasinato
PY - 2026
DA - 2026/10/09
PB - arXiv
DO - 10.48550/arxiv.2610.13739
UR - https://arxiv.org/abs/2610.13739
AB - If cosmic acceleration is driven by a positive cosmological constant, the universe approaches an eternally accelerating de Sitter state, whose interpretation in quantum gravity remains elusive. We develop a framework in which acceleration is instead a transient stage of dark-sector relaxation towards compositional equilibrium: matter and dark energy approach a stationary density ratio, eventually diluting like pressureless matter. Guided by linear response theory and Onsager's near-equilibrium regression hypothesis, we describe this evolution through a composition variable measuring departure from equilibrium. Extending constant fractional relaxation to the full background evolution yields the dark-energy equation of state w=w_0a^−n, where n is the relaxation rate per e-fold of expansion. This history connects early dark-energy suppression, an upward phantom crossing and a density maximum to a pressureless scaling future. Using DESI DR2 BAO and Planck distance priors, with and without three alternative supernova compilations, we find a nominal $2.6−−3.6σ$ preference for nonzero relaxation over constant w. The composition framework also motivates a constitutive prescription for linear perturbations that remains regular at phantom crossing. Near equilibrium, regular long-wavelength composition disturbances relax at the background rate to leading order, while small-scale fluctuations propagate as damped sound waves. We prove, however, that for separately conserved matter and dark energy, the pointwise constitutive relation cannot sustain a regular, gradient-stable negative-pressure branch throughout its full past evolution. We explore modifications of the pressure response and their consistency with neighbouring homogeneous histories, identifying requirements for extending the near-equilibrium description to the full cosmological evolution.
ER -