A truncation test of the DESI evolving dark energy signal: three supernova calibrations compared

Abstract

The DESI DR2 collaboration reported ∼ 4σ evidence for evolving dark energy (w₀ gt −1, w_a lt 0) from baryon acoustic oscillations (BAO) combined with Type Ia supernova (SN Ia) distances. A persistent concern is whether low-redshift SN calibration systematics drive this signal. We introduce a redshift truncation scan – progressively excising low-z SNe and re-fitting CPL cosmology – and apply it to three SN data sets with independent absolute calibrations: Pantheon+ (SH0ES Cepheid distance ladder), DESY5 (internal BAO-anchored calibration), and DES-Dovekie (white dwarf SEDs + Gaia spectrophotometry). All chains use the full Planck 2018 TTTEEE_lite likelihood. We show that the signal is not dominated by photometric calibration. DES-Dovekie serves as a positive control: its CPL signal, already weakened by the recalibration, collapses under truncation – the signature of a calibration-driven signal. In contrast, Pantheon+ and DESY5 retain a ∼ 2σ deviation (Δ χ ² ≈ 4) after all z ≤ 0.12 SNe are removed that does not weaken with further truncation. Quantitatively, in χ ² space the signal decomposes into photometric calibration (Δ χ ² = 7.6, 2.8σ) and low-z non-calibration (Δ χ ² = 7.4, 2.7σ), which together with a residual consistent with the CMB+BAO baseline (Δ χ ² ≈ 2, 1.4σ) reproduce the DESY5 total (4.0σ) to within a few per cent. This truncation-immune residual is either genuine dark energy evolution or a shared SN Ia standardization systematic beyond calibration.

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BibTeX RIS

BibTeX

@article{zhang2026truncation,
  title = {{A truncation test of the DESI evolving dark energy signal: three supernova calibrations compared}},
  author = {Tao Zhang},
  journal = {Monthly Notices of the Royal Astronomical Society},
  year = {2026},
  month = sep,
  doi = {10.1093/mnras/stag1868},
  url = {https://academic.oup.com/mnras/article/doi/10.1093/mnras/stag1868/8855714},
  note = {CC BY 4.0},
}

RIS

TY  - JOUR
TI  - A truncation test of the DESI evolving dark energy signal: three supernova calibrations compared
AU  - Tao Zhang
PY  - 2026
DA  - 2026/09/30
JO  - Monthly Notices of the Royal Astronomical Society
DO  - 10.1093/mnras/stag1868
UR  - https://academic.oup.com/mnras/article/doi/10.1093/mnras/stag1868/8855714
AB  - The DESI DR2 collaboration reported ∼ 4σ evidence for evolving dark energy (w_0 gt −1, w_a lt 0) from baryon acoustic oscillations (BAO) combined with Type Ia supernova (SN Ia) distances. A persistent concern is whether low-redshift SN calibration systematics drive this signal. We introduce a redshift truncation scan – progressively excising low-z SNe and re-fitting CPL cosmology – and apply it to three SN data sets with independent absolute calibrations: Pantheon+ (SH0ES Cepheid distance ladder), DESY5 (internal BAO-anchored calibration), and DES-Dovekie (white dwarf SEDs + Gaia spectrophotometry). All chains use the full Planck 2018 TTTEEE_lite likelihood. We show that the signal is not dominated by photometric calibration. DES-Dovekie serves as a positive control: its CPL signal, already weakened by the recalibration, collapses under truncation – the signature of a calibration-driven signal. In contrast, Pantheon+ and DESY5 retain a ∼ 2σ deviation (Δ χ ^2 ≈ 4) after all z ≤ 0.12 SNe are removed that does not weaken with further truncation. Quantitatively, in χ ^2 space the signal decomposes into photometric calibration (Δ χ ^2 = 7.6, 2.8σ) and low-z non-calibration (Δ χ ^2 = 7.4, 2.7σ), which together with a residual consistent with the CMB+BAO baseline (Δ χ ^2 ≈ 2, 1.4σ) reproduce the DESY5 total (4.0σ) to within a few per cent. This truncation-immune residual is either genuine dark energy evolution or a shared SN Ia standardization systematic beyond calibration.
SN  - 0035-8711
SN  - 1365-2966
ER  -

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