Late-Transition Interacting Thawer Dark Energy: Model Definition and Benchmark Consistency Tests

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 a non-universal dark-sector interaction: baryons and radiation remain minimally coupled, while CDM follows a conformally related metric. This structure ties three observational sectors to one microscopic mechanism--the pre-recombination calibration scale, the low-redshift expansion history, and the coupled growth response. We derive the exact background equations, the exact CDM scaling identity $\rhoc(a)=ρ_{c0}a^{-3}C[ϕ(a)]/C(ϕ_0)$, and the coupled CDM--scalar perturbation equations in synchronous gauge in a form suitable for Einstein--Boltzmann solvers. In the representative benchmarks, $Ω_ϕ(z_*)\sim10^{-9}$, $|Δr_d/r_d|\le3.82\times10^{-3}$, and $\max_{0<z<3}|E/E_{Λ\mathrm{CDM}}-1|\le0.380\%$, while the quasi-static growth indicator ranges from $0.725\%$ to $5.94\%$. LTIT therefore demonstrates how late activation can preserve the early calibration sector while producing a perturbation-sensitive signature, providing a concrete target for Boltzmann-level tests of interacting dark energy.