[Paper Review] Hints of Nonminimally Coupled Gravity in DESI 2024 Baryon Acoustic Oscillation Measurements

Gen Ye, M. Martinelli+2 more|arXiv (Cornell University)|Jul 22, 2024

Geophysics and Gravity Measurements14 citations

TL;DR

The paper shows that DESI 2024 BAO data suggest phantom crossing in the dark energy equation of state and identifies non-minimal coupling in Horndeski gravity as key to a stable crossing, proposing the Thawing Gravity model.

ABSTRACT

The cosmic microwave background (CMB) and baryon acoustic oscillations (BAO) are two of the most robust observations in cosmology. The recent BAO measurements from the DESI collaboration have presented, for the first time, inconsistency between BAO and CMB within the standard cosmological model $Λ$CDM, indicating a preference for dynamical dark energy over a cosmological constant. We analyze the theoretical implication of the DESI BAO observation for dark energy and gravity employing a nonparametric reconstruction approach for both the dark energy equation of state $w_{ m DE}(a)$ and the effective field theory coefficients. We find that the DESI data can rule out quintessence dark energy by indicating a crossing of the phantom divide at $z\lesssim1$. Furthermore, when analyzed within the broad context of Horndeski gravity which includes general relativity and many known modified gravity theories such as generalized Galileons, $f(R)$ and Brans-Dicke, our result implies that gravity should be nonminimally coupled to explain the observations, establishing the DESI result as the first hint of modified gravity. Based on these insights, we propose the extit{thawing gravity} model to explain the nonminimal coupling and phantom crossing indicated by observation, which also fits better to DESI BAO, CMB and type Ia Supernovae data than $Λ$CDM.

Motivation & Objective

• Motivate and test whether DESI BAO data truly indicate phantom crossing rather than an artifact of the w0-wa parameterization.
• Survey Horndeski gravity models within the EFT of DE to identify EFT functions that can safely realize phantom crossing.
• Develop a covariant non-minimally coupled Horndeski model, Thawing Gravity, that fits DESI, CMB, and SNIa with the same parameter count as w0-wa CDM.
• Assess the impact of stability conditions (no-ghost, no-gradient) on the viability of phantom crossing in the theory space.
• Explore implications for growth of structure and the fσ8 observable under the proposed model.

Proposed method

• Non-parametric reconstruction of the dark energy equation of state w_DE(a) from DESI alone and from DESI+CMB+SNIa to test phantom crossing independent of parameterization.
• Use EFTCAMB to scan Horndeski theory space by binning EFT functions Ω, γ1, γ2, γ3 in time and fitting to data with stability criteria.
• Adopt a designer approach fixing the background to w0-wa CDM while varying EFT functions one at a time to isolate operators enabling crossing.
• Investigate a covariant non-minimally coupled Horndeski model with L = (M_p^2/2)[1−ξ(φ/M_p)^2]R + X − V0 e^{−λφ/M_p} and study its thawing dynamics and impact on w_DE(z).
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Experimental results

Research questions

• RQ1 Does DESI BAO data require phantom crossing, independent of the CPL-like w0-wa parametrization?
• RQ2 Which EFT/Horndeski operators allow a stable phantom crossing when embedded in a consistent background expansion?
• RQ3 Can a non-minimally coupled Horndeski model reproduce the reconstructed phantom crossing and improve fit to DESI+CMB+SNIa?
• RQ4 What are the implications for growth of structure in such models as encoded in fσ8(z)?

Key findings

• Non-parametric reconstructions show w_DE < −1 at z > 0.6 and w_DE > −1 at z = 0, implying phantom crossing supported by DESI data when including CMB and SNIa.
• Among EFT functions, non-minimal coupling Ω is the key to enabling a stable phantom crossing within Horndeski gravity.
• Turning on γ1, γ2, γ3 alone does not reliably stabilize crossing; Ω is required to reach the phantom-crossing region favored by DESI.
• The Thawing Gravity model, a covariant non-minimally coupled Horndeski theory, yields improved fits to DESI, CMB, and SNIa with the same parameter count as w0-wa CDM.
• Thawing Gravity predicts a thawing scalar field leading to a decreasing effective Planck mass, with w_DE(z) exhibiting phantom crossing around 0.5 < z < 1.
• The model yields Δχ^2 improvements relative to ΛCDM: DESI (−2.1), CMB (−1.8), and SNIa (−1.9).