Early against Late: A contrast on dark energy in the light of DESI DR2
Abstract
Recent findings from Dark Energy Spectroscopic Instrument (DESI) Baryon Acoustic Oscillation (BAO) measurements, combined with Type Ia supernovae and Cosmic Microwave Background (CMB) data, suggest potential parameter-level deviations from ΛCDM. However, parameter exclusions omit prior-volume penalties, whereas Bayesian evidence uncovers a stark dichotomy between early- and late-time dynamics. To quantify this effect, we perform a Bayesian model comparison that explicitly accounts for the cosmological epoch of dark energy dynamics, contrasting two schemes acting in opposite epochs of cosmic history: pre-recombination Early Dark Energy (EDE) and late-time Chevallier--Polarski--Linder (CPL), both measured against a baseline ΛCDM. We validate the learned harmonic-mean estimator against UltraNest (at background level) before applying it to the CMB analysis. Background-only data already disfavor both extensions, yielding {ln BΛCDM,EDE = 1.48 ± 0.18 and ln BΛCDM,CPL = 2.36 ± 0.24}. Including CMB sharpens this result: {EDE is very strongly rejected (ln BΛCDM,EDE ≈ 12.61 ± 0.17)} with its early-time fraction constrained to 103 ΩeEDE = 2.0+0.6−1.0, whereas CPL remains only mildly disfavored (ln BΛCDM,CPL ≈ 2.34 ± 0.18), even though 2D parameter posteriors for both models display a multi-σ deviation from ΛCDM. Thus, Bayesian evidence weakens reported preferences for dark energy dynamics, with the impact depending on the cosmic epoch involved, confirmed by our functional reconstructions of wde(z) and Ωde(z) from posterior samples.
Related
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- Conformally Coupled Scalar- Field Dark Energy: Cosmological Dynamics with Early- and Late- Measurements
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- Response to "Late Time Dynamical Dark Energy and the CMB-Distance Ladder Tension"
BibTeX
@misc{zapata2026early,
title = {{Early against Late: A contrast on dark energy in the light of DESI DR2}},
author = {Miguel A. Zapata and Karim Carrion and Gabriela Garcia-Arroyo},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.05410},
eprint = {2609.05410},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.05410},
}
RIS
TY - UNPB
TI - Early against Late: A contrast on dark energy in the light of DESI DR2
AU - Miguel A. Zapata
AU - Karim Carrion
AU - Gabriela Garcia-Arroyo
PY - 2026
DA - 2026/09/24
PB - arXiv
DO - 10.48550/arxiv.2609.05410
UR - https://arxiv.org/abs/2609.05410
AB - Recent findings from Dark Energy Spectroscopic Instrument (DESI) Baryon Acoustic Oscillation (BAO) measurements, combined with Type Ia supernovae and Cosmic Microwave Background (CMB) data, suggest potential parameter-level deviations from ΛCDM. However, parameter exclusions omit prior-volume penalties, whereas Bayesian evidence uncovers a stark dichotomy between early- and late-time dynamics. To quantify this effect, we perform a Bayesian model comparison that explicitly accounts for the cosmological epoch of dark energy dynamics, contrasting two schemes acting in opposite epochs of cosmic history: pre-recombination Early Dark Energy (EDE) and late-time Chevallier--Polarski--Linder (CPL), both measured against a baseline ΛCDM. We validate the learned harmonic-mean estimator against UltraNest (at background level) before applying it to the CMB analysis. Background-only data already disfavor both extensions, yielding {ln B_ΛCDM,EDE = 1.48 ± 0.18 and ln B_ΛCDM,CPL = 2.36 ± 0.24}. Including CMB sharpens this result: {EDE is very strongly rejected (ln B_ΛCDM,EDE ≈ 12.61 ± 0.17)} with its early-time fraction constrained to 10^3 Ω_e^EDE = 2.0^+0.6_−1.0, whereas CPL remains only mildly disfavored (ln B_ΛCDM,CPL ≈ 2.34 ± 0.18), even though 2D parameter posteriors for both models display a multi-σ deviation from ΛCDM. Thus, Bayesian evidence weakens reported preferences for dark energy dynamics, with the impact depending on the cosmic epoch involved, confirmed by our functional reconstructions of w_de(z) and Ω_de(z) from posterior samples.
ER -