Late-Transition Interacting Thawer Dark Energy: Physics and Validation

TL;DR

This paper introduces the late-transition interacting thawer (LTIT), a novel dark energy model with late activation and variable coupling, providing exact equations and analyzing its observational implications.

Contribution

The paper formulates the LTIT model with exact background and perturbation equations, distinguishing it from flexible $w(z)$ models and emphasizing its constrained nature.

Findings

Low-redshift dark energy density $ ho_(z_*)$ is negligible (~10^{-9})

Relative change in sound horizon $| riangle r_d/r_d|$ is less than 0.4%

Background shifts are sub-percent, with growth response varying from sub-percent to several percent.

Abstract

We formulate the late-transition interacting thawer (LTIT) as a late-activating, variable-coupling realization of coupled quintessence in which a canonical scalar field couples conformally only to cold dark matter. The construction is designed to separate two issues that are often mixed in late-time dark-energy analyses: a genuine low-redshift deformation of the expansion history and a shift of the pre-recombination calibration sector. We derive the exact background equations, the exact CDM scaling identity $\rhoc(a)=\rho_{c0}a^{-3}C[\phi(a)]/C(\phi_0)$, and the linear scalar perturbation equations in synchronous gauge in a form suitable for Einstein--Boltzmann solvers. For representative benchmarks we find $\Omega_\phi(z_*)\sim10^{-9}$ and $|\Delta r_d/r_d|<4\times10^{-3}$, while the low-redshift background shift remains sub-percent and the approximate growth response ranges from sub-percent to several percent, depending on the asymptotic coupling amplitude. LTIT is therefore not a flexible $w(z)$ ansatz but a constrained interacting-dark-energy framework in which any apparent background success must survive perturbation-level closure tests.