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DESI constraints on two-field quintessence with exponential potentials

George Alestas, Marienza Caldarola, Indira Ocampo, Savvas Nesseris, Shinji Tsujikawa

TL;DR

The paper addresses the challenge that single-field exponential quintessence potentials are often too steep to drive late-time cosmic acceleration. It analyzes a two-field quintessence model with a double-exponential potential that exhibits assisted quintessence, yielding an effectively shallower slope and sustained acceleration. Using a fully Bayesian analysis against Planck 2018 CMB shift parameters, DESI DR2 BAO, and DESY5 SnIa data, the authors constrain the slopes λ_φ and λ_χ and derive an effective slope λ_eff, finding λ_φ≈1.03, λ_χ≈0.96 and λ_eff≈0.5 with modest uncertainties; the model is moderately favored over ΛCDM with Δln B≈4. The results align with higher-dimensional theory expectations and show dynamical dark energy consistent with data without invoking phantom crossing, highlighting the viability of a theory-informed, multi-field approach to cosmic acceleration. The authors also make their analysis code publicly available, enabling independent verification with future data.

Abstract

We investigate a quintessence model involving two scalar fields with double-exponential potentials. This configuration allows the system as a whole to emulate the dynamics of a single field with a shallower potential, enabling scalar fields that individually cannot drive cosmic acceleration to collectively achieve and sustain it. We assess the viability of this model by performing a fully Bayesian analysis and confronting its predictions with observational data, including the Planck 2018 Cosmic Microwave Background (CMB) shift parameters, the newly released Dark Energy Spectroscopic Instrument (DESI) DR2 Baryon Acoustic Oscillation (BAO) measurements, and the Dark Energy Survey Year 5 (DESY5) Type Ia supernova (SnIa) sample. Our analysis shows that the two-field quintessence model yields a log Bayes factor relative to the flat $Λ$CDM model of $Δ\ln B \sim 4$, indicating moderate evidence against the latter. We also find that the central values of the two slopes of the exponential potentials are both close to 1, whereas the slope of an effective single-field system is constrained to be less than order unity. This property is theoretically desirable from the perspective of higher-dimensional theories. Thus, the two-field quintessence model with exponential potentials provides a physically motivated and compelling mechanism that is consistent with both observational and theoretical requirements.

DESI constraints on two-field quintessence with exponential potentials

TL;DR

The paper addresses the challenge that single-field exponential quintessence potentials are often too steep to drive late-time cosmic acceleration. It analyzes a two-field quintessence model with a double-exponential potential that exhibits assisted quintessence, yielding an effectively shallower slope and sustained acceleration. Using a fully Bayesian analysis against Planck 2018 CMB shift parameters, DESI DR2 BAO, and DESY5 SnIa data, the authors constrain the slopes λ_φ and λ_χ and derive an effective slope λ_eff, finding λ_φ≈1.03, λ_χ≈0.96 and λ_eff≈0.5 with modest uncertainties; the model is moderately favored over ΛCDM with Δln B≈4. The results align with higher-dimensional theory expectations and show dynamical dark energy consistent with data without invoking phantom crossing, highlighting the viability of a theory-informed, multi-field approach to cosmic acceleration. The authors also make their analysis code publicly available, enabling independent verification with future data.

Abstract

We investigate a quintessence model involving two scalar fields with double-exponential potentials. This configuration allows the system as a whole to emulate the dynamics of a single field with a shallower potential, enabling scalar fields that individually cannot drive cosmic acceleration to collectively achieve and sustain it. We assess the viability of this model by performing a fully Bayesian analysis and confronting its predictions with observational data, including the Planck 2018 Cosmic Microwave Background (CMB) shift parameters, the newly released Dark Energy Spectroscopic Instrument (DESI) DR2 Baryon Acoustic Oscillation (BAO) measurements, and the Dark Energy Survey Year 5 (DESY5) Type Ia supernova (SnIa) sample. Our analysis shows that the two-field quintessence model yields a log Bayes factor relative to the flat CDM model of , indicating moderate evidence against the latter. We also find that the central values of the two slopes of the exponential potentials are both close to 1, whereas the slope of an effective single-field system is constrained to be less than order unity. This property is theoretically desirable from the perspective of higher-dimensional theories. Thus, the two-field quintessence model with exponential potentials provides a physically motivated and compelling mechanism that is consistent with both observational and theoretical requirements.
Paper Structure (7 sections, 14 equations, 4 figures, 1 table)

This paper contains 7 sections, 14 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Theoretical formalism
  3. Observational data and methodology
  4. Bayesian Analysis Results
  5. Conclusions
  6. Code availability:
  7. The triangle plots

Figures (4)

  • Figure 1: The theoretically derived present-day fractional DE density, $\rho_{{\rm DE},0}/\rho_\mathrm{cr}$, is shown as a function of $\lambda_{\phi}$ and $\lambda_{\chi}$ for the mean parameter values reported in Table \ref{['tab:MCMC_bf']}, without performing shooting. The initial conditions are chosen such that $\phi$, $\chi$, and their first derivatives are set to 0. We observe that values of $\lambda_\phi \sim \lambda_\chi \sim 1$ can jointly yield $\rho_{{\rm DE},0}/\rho_\mathrm{cr} \sim 0.685$, in agreement with current observational data. Note that $\rho_{{\rm DE},0}/\rho_\mathrm{cr}$ is not necessarily equal to $\Omega_{{\rm DE},0}$ unless a shooting procedure is performed.
  • Figure 2: Confidence regions for the parameters $\lambda_{\phi}$ and $\lambda_{\chi}$ (left panel), and for $\lambda_\mathrm{eff}$ versus today's matter density parameter, $\Omega_{\mathrm{m},0}$ (right panel). The constraints are derived from CMB, DESI DR2, and DESY5 data. The inner and outer contours correspond to the 68.3 % and 95.5 % credible intervals, respectively.
  • Figure 3: Plots of the DE equation of state $w_\mathrm{DE}(z)$ and the total effective equation of state $w_\mathrm{eff}(z)$ are shown for the two-field quintessence model of Eq. (\ref{['action']}). We also show the evolution of $w_{\rm DE,CPL}(z)$ in the Taylor-expanded CPL model of Eq. \ref{['eq:CPL']}. All plots use the mean parameter values listed in Table \ref{['tab:MCMC_bf']}. For the two-field quintessence model, we have $w_\mathrm{eff} = -1/3$ at $z=0.65$.
  • Figure 4: The one-dimensional marginalized posteriors and the inner and outer contours correspond to the 68.3 % and 95.5 % credible intervals for the parameters of the two-field quintessence model, including the effective slope $\lambda_{\mathrm{eff}}$,obtained using CMB, DESI DR2, and DES Y5 data.