Addressing the DESI DR2 Phantom-Crossing Anomaly and Enhanced $H_0$ Tension with Reconstructed Scalar-Tensor Gravity

TL;DR

This paper demonstrates that scalar-tensor modified gravity models can resolve the DESI DR2 phantom-crossing anomaly and increase the Hubble constant estimate, alleviating tensions within the cosmological data.

Contribution

The authors reconstruct a scalar-tensor gravity model that naturally allows phantom crossing and raises the H_0 value, addressing key cosmological tensions.

Findings

Scalar-tensor theories can reproduce phantom crossing in dark energy equation of state.

Modified gravity models increase the inferred H_0 to 70.6 km/s/Mpc.

Reconstructed Lagrangian functions fit observational data well.

Abstract

Recent cosmological data, including DESI DR2, highlight significant tensions within the $\Lambda$CDM paradigm. When analyzed in the context of General Relativity (GR), the latest DESI data favor a dynamical dark energy (DDE) equation of state, $w(z)$, that crosses the phantom divide line $w=-1$. However, this framework prefers a lower Hubble constant, $H_0$, than Planck 2018, thereby worsening the tension with local measurements. This phantom crossing is a key feature that cannot be achieved by minimally coupled scalar fields (quintessence) within GR. This suggests the need for a new degree of freedom that can simultaneously: (A) increase the best-fit value of $H_0$ in the context of the DESI DR2 data, and (B) allow the crossing of the $w=-1$ line within a new theoretical approach. We argue that both of these goals may be achieved in the context of Modified Gravity (MG), and in particular, Scalar-Tensor (ST) theories, where phantom crossing is a natural and viable feature. We demonstrate these facts by analyzing a joint dataset including DESI DR2, Pantheon+, CMB, and growth-rate (RSD) data in the context of simple parametrizations for the effective gravitational constant, $\mu_G(z) \equiv G_{eff}/G_N$, and the DDE equation of state, $w(z)$. This MG framework significantly alleviates the tension, leading to a higher inferred value of $H_0 = 70.6 \pm 1.7 \, \text{km s}^{-1} \text{Mpc}^{-1}$. We also present a systematic, data-driven reconstruction of the required underlying ST Lagrangian and provide simple, generic analytical expressions for both the non-minimal coupling $F(\Phi) = 1+\xi\Phi^{2}e^{n\Phi}$ and the scalar potential $U(\Phi) = U_{0}+ae^{b\Phi^{2}}$, which well-describe the reconstructed functions.