Exploring the interplay of late-time dynamical dark energy and new physics before recombination

Abstract

Cosmological models exhibiting crossing of the phantom divide improve the fit to current data, suggesting late-time dark energy (DE) dynamics at $\sim3\sigma$ CL. However, they favor low values of $H_0$, in tension with SH0ES. This may point to the presence of new physics prior to the decoupling era. In this work, we reconstruct the background DE functions using the Weighted Function Regression (WFR) method, introducing three main improvements compared to our previous JCAP 12 (2025) 049. First, we adopt the Frequentist-Bayesian approach for the weights. Second, we combine CMB and BAO with the DES-Dovekie SNIa sample and compare our findings with those derived from Pantheon+, still assuming standard recombination. Third, we study in a model-independent manner the viability of early-time ``solutions'' to the Hubble tension and how they affect the evidence for dynamical DE at late times, under the influence of the SH0ES and the more conservative CCHP calibration of the cosmic ladders, separately. We find that, if the physics prior to decoupling is unmodified, the probability of phantom crossing is $\sim 96.7\text{--}98.5\%$, with $\Lambda$CDM excluded at $\sim 2.5\sigma$ and $\sim 3\sigma$ CL. New physics before recombination can alleviate the Hubble tension, but requires extremely large values of the reduced matter density parameter when the SH0ES calibration is employed, in strong tension with those inferred from full CMB analyses. This raises serious concerns about the actual viability of these models to explain the SH0ES measurement. We find that phantom crossing, while not excluded, is no longer required, with only a very mild preference for quintessence. Nevertheless, given the aforesaid tension in $\omega_m$, it would be rash to draw firm conclusions about how the dynamical DE signal is affected in these scenarios. [abridged]