Unifying Early and Late Dark Energy: Dynamical Requirements and Obstructions
Abstract
We investigate whether early- and late-time dark energy could arise from a single scalar field. Adopting a bottom-up perspective, we first identify the sequence of dynamical regimes that any unified scenario must traverse to account for both an early dark energy phase relevant for pre-recombination solutions of the Hubble tension and the late-time acceleration of the Universe. We derive the corresponding requirements on the scalar energy density and equation of state. We then adopt a complementary top-down perspective and translate these requirements into constraints on the phase-space structure of minimally coupled scalar fields with tracking-like dynamics. We show that satisfying all requirements necessitates a potential with three distinct slopes, arranged in a steep-steeper-shallow hierarchy. This conclusion remains unchanged in the presence of conformal couplings to dark matter. These results place strong constraints on attempts to construct unified models of early- and late-time dark energy. We discuss implications for model-building.
Versions
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BibTeX
@misc{giare2026unifying,
title = {{Unifying Early and Late Dark Energy: Dynamical Requirements and Obstructions}},
author = {William Giarè and Jeremy Sakstein},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2605.26116},
eprint = {2605.26116},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2605.26116},
}
RIS
TY - UNPB
TI - Unifying Early and Late Dark Energy: Dynamical Requirements and Obstructions
AU - William Giarè
AU - Jeremy Sakstein
PY - 2026
DA - 2026/09/23
PB - arXiv
DO - 10.48550/arxiv.2605.26116
UR - https://arxiv.org/abs/2605.26116
AB - We investigate whether early- and late-time dark energy could arise from a single scalar field. Adopting a bottom-up perspective, we first identify the sequence of dynamical regimes that any unified scenario must traverse to account for both an early dark energy phase relevant for pre-recombination solutions of the Hubble tension and the late-time acceleration of the Universe. We derive the corresponding requirements on the scalar energy density and equation of state. We then adopt a complementary top-down perspective and translate these requirements into constraints on the phase-space structure of minimally coupled scalar fields with tracking-like dynamics. We show that satisfying all requirements necessitates a potential with three distinct slopes, arranged in a steep-steeper-shallow hierarchy. This conclusion remains unchanged in the presence of conformal couplings to dark matter. These results place strong constraints on attempts to construct unified models of early- and late-time dark energy. We discuss implications for model-building.
ER -