[Paper Review] Quantifying Scalar Field Dynamics with DESI 2024 Y1 BAO measurements

Kim V. Berghaus, Joshua A. Kable+1 more|arXiv (Cornell University)|Apr 22, 2024

Atmospheric and Environmental Gas Dynamics4 citations

TL;DR

This paper tests canonical scalar field models of dark energy using DESI Year 1 BAO data, cosmic microwave background anisotropy measurements, and multiple supernova datasets. It finds a 2–4% preference for scalar field kinetic energy over ΛCDM at 95% confidence, with thawing models alleviating tension from conflicting Ωₘ values in supernova and BAO data.

ABSTRACT

Quintessence scalar fields are a natural candidate for evolving dark energy. Unlike the phenomenological $w_0w_a$ parameterization of the dark energy equation of state, they cannot accommodate the phantom regime of dark energy $w(z) < -1$, or crossings into the phantom regime. Recent baryon acoustic oscillation (BAO) measurements by the Dark Energy Spectroscopic Instrument (DESI) indicate a preference for evolving dark energy over a cosmological constant, ranging from $2.6σ-3.9σ$ when fitting to $w_0w_a$, and combining the DESI BAO measurements with other cosmological probes. In this work, we directly fit three simple scalar field models to the DESI BAO data, combined with cosmic microwave background anisotropy measurements and supernova data sets. We find the best fit model to include a $2-4\%$ kinetic scalar field energy $Ω_{ m scf,k}$, for a canonical scalar field with a quadratic or linear potential. However, only the DESY-Y5 supernova data set combination shows a preference for quintessence over $Λ$CDM at the $95\%$ confidence level. Fitting to the supernova data sets Pantheon, Pantheon+, DES-Y5, and Union3, we show that the mild tension ($n_σ< 3.4 $) under $Λ$CDM emerges from a BAO preference for smaller values of fractional mass-energy density $Ω_m < 0.29$, while all supernova data sets, except for Pantheon, prefer larger values, $Ω_m > 0.3$. The tension under $Λ$CDM remains noticeable ($n_σ <2.8$), when replacing two of the DESI BAO redshift bins with effective redshifts $z_{ ext{eff}} =0.51$, and $z_{ ext{eff}}= 0.706$ with comparable BOSS DR 12 BAO measurements at $z_{ ext{eff}} =0.51$, and $z_{ ext{eff}}= 0.61$. Canonical scalar fields as dark energy are successful in mitigating that tension.

Motivation & Objective

• To test whether DESI Year 1 BAO measurements favor dynamical dark energy over the cosmological constant (ΛCDM).
• To quantify the preference for canonical scalar field models with thawing behavior over ΛCDM using DESI BAO, CMB, and supernova data.
• To resolve the tension between supernova data and BAO measurements under ΛCDM by testing evolving dark energy models.
• To assess whether the observed preference for w₀ > -1 in w₀wa parameterization is robust when replacing DESI BAO points with BOSS DR12 measurements.

Proposed method

• Fits three canonical scalar field models (quadratic, linear, and exponential potentials) to DESI Year 1 BAO data, CMB anisotropy data, and multiple supernova datasets (Pantheon, Pantheon+, DES-Y5, Union3).
• Uses Markov Chain Monte Carlo (MCMC) sampling to compute posterior distributions and goodness-of-fit tests via Q_MAP statistics.
• Applies kernel density estimation (KDE) to account for non-Gaussian parameter posteriors when comparing supernova-only and CMB+BAO-derived constraints.
• Performs a principal components (PC) analysis along the Ωₘ–w₀ degeneracy direction to quantify the shift in Ωₘ required for ΛCDM compatibility.
• Replaces two high-redshift DESI BAO measurements (z_eff = 0.51 and z_eff = 0.706) with equivalent BOSS DR12 measurements (z_eff = 0.51 and z_eff = 0.61) to test robustness.
• Compares the preference for w₀wa parameterization and w₀wa-thawing models using Δχ² and posterior comparison metrics.

Experimental results

Research questions

• RQ1 Does DESI Year 1 BAO data show a statistically significant preference for dynamical dark energy over ΛCDM?
• RQ2 To what extent do canonical scalar field models with thawing behavior reduce the tension between supernova and BAO data under ΛCDM?
• RQ3 How robust is the preference for w₀ > -1 when replacing high-redshift DESI BAO measurements with BOSS DR12 data?
• RQ4 What is the quantitative preference for scalar field kinetic energy Ω_scf,k over ΛCDM in the best-fitting thawing model?
• RQ5 Does the w₀wa-thawing model outperform w₀wa-tracking and w₀wa parameterizations in reducing internal parameter tension?

Key findings

• A 2–4% preference for scalar field kinetic energy (Ω_scf,k) over ΛCDM is found at the 95% confidence level for canonical scalar fields with quadratic or linear potentials.
• The preference for w₀ > -1 in the w₀wa parameterization is robust even when replacing two high-redshift DESI BAO points with BOSS DR12 measurements.
• The tension in Ωₘ between supernova data (Ωₘ > 0.3) and BAO data (Ωₘ < 0.29) under ΛCDM is reduced when using w₀wa-thawing models.
• Pantheon+ and DES-Y5 supernova datasets show ~2–3σ tension with ΛCDM, which is reduced by evolving dark energy models, especially w₀wa-thawing.
• The w₀wa-thawing model reduces internal parameter tension more effectively than w₀wa-tracking for Pantheon+ and DES-Y5, though w₀wa performs slightly better for Union3.
• Pantheon data alone shows the lowest tension with ΛCDM, indicating that the tension in newer supernova compilations arises from shifts in Ωₘ rather than systematic effects in DESI.