Is the DESI-era preference for evolving dark energy (a Quintom-B phantom crossing) real physical dynamics or an artifact of the CPL parametrization and supernova systematics?

The DESI DR2 baryon-acoustic-oscillation (BAO) measurements, combined with CMB and Type Ia supernova (SNe Ia) data, favor a dynamical dark-energy equation of state over the cosmological constant at roughly 2.8-4.2 sigma, with best-fit parameters in the 'Quintom-B' quadrant: phantom behavior ($w < -1$) at $z \gtrsim 0.5$ crossing to quintessence ($w > -1$) at low redshift. A December 2025 critical review by Capozziello, Chaudhary, Harko and Mustafa (published in Physics of the Dark Universe, February 2026) consolidated this evidence and found the preference biased by low-redshift supernova calibration, unable to reduce the sound horizon by the ~7% needed to ease the Hubble tension, and accompanied only by a ~1.6-sigma hint of nonzero neutrino mass. The wider 2025-2026 literature examines these claims: recalibrated supernova samples (DES-Dovekie) weaken but do not remove the preference; non-CPL reconstructions give mixed verdicts; independent DES growth probes leave its significance essentially unchanged; and an August 2026 analysis argues background data cannot by themselves decide the physical interpretation. The core question - whether the DESI-era evolving-dark-energy preference is real physical dynamics or an artifact of parametrization, supernova systematics, and survey systematics, and whether it persists with Stage-IV data - remains open.

Background

In the standard cosmological model (ΛCDM, a spatially flat universe dominated by a cosmological constant Λ and cold dark matter), the accelerated expansion of the Universe is driven by a cosmological constant, whose equation-of-state ratio of pressure to energy density, $w = p/\rho$, is exactly -1 at all times. Dark energy is called 'dynamical' if $w$ evolves with redshift $z$; quintessence denotes $w > -1$, phantom behavior denotes $w < -1$, and a 'Quintom-B' trajectory crosses the phantom divide from phantom at higher redshift to quintessence at late times, around $z \approx 0.5$-1 in current fits. The observational anchors are: baryon acoustic oscillations (BAO), the frozen acoustic scale in the matter distribution measured by spectroscopic surveys such as the Dark Energy Spectroscopic Instrument (DESI), whose second data release (DR2, 2025) measured BAO distance and expansion-rate ratios for over 14 million galaxies and quasars; Type Ia supernovae (SNe Ia), standardizable candles compiled in samples such as Pantheon+, DES-SN5Y (and its recalibrated version DES-Dovekie), and Union3; and the cosmic microwave background (CMB), which fixes the early-Universe normalization, in particular the sound horizon at baryon drag, $r_d$, the ruler length underpinning all BAO distances. The CPL (Chevallier-Polarski-Linder) parametrization approximates the dark-energy equation of state as $w(a) = w_0 + w_a(1-a)$ with $a$ the scale factor; because it is only a two-parameter ansatz, a fitted evolution of $w$ may reflect the parametrization's own flexibility rather than physical dynamics. The 'Hubble tension' is the ~4-5 sigma discrepancy between the CMB-inferred and locally measured present expansion rate $H_0$; resolving it within BAO/CMB consistency requires reducing $r_d$ by about 7%, which late-time dynamical dark energy cannot do because dark energy is negligible at recombination. Against this backdrop, DESI DR2 BAO combined with CMB and various SNe Ia compilations prefers CPL parameters $w_0 > -1$, $w_a < 0$ over the ΛCDM point ($-1$, 0) at 2.8-4.2 sigma depending on the supernova compilation (DESI Collaboration, Physical Review D 112, 083515 (2025); arXiv:2503.14738). The critical review by Capozziello, Chaudhary, Harko and Mustafa (arXiv:2512.10585; Physics of the Dark Universe 51 (2026) 102196) reanalyzed DESI DR2 BAO (including Lyman-alpha forest data), CMB compressed likelihoods, Big Bang nucleosynthesis data, cosmic chronographers, and multiple SNe Ia compilations in $w_0 w_a$CDM and extensions with free neutrino mass sum $\Sigma m_\nu$ and effective relativistic species number $N_{\mathrm{eff}}$. Their diagnosis: the Quintom-B preference sits at about 2.8 sigma for most dataset combinations and rises to the quoted higher significance only when the DES-SN5Y supernova sample is included; that preference is biased by the low-redshift ($z < 0.1$) SNe Ia inherited from the CfA/CSP calibration samples, whose exclusion restores consistency with ΛCDM in their pipeline; neither $w_0 w_a$CDM nor its extensions reduces $r_d$ by the required ~7%; and the nonzero-$\Sigma m_\nu$ detection reaches only about 1.6 sigma. 'Stage IV' denotes the next generation of precision surveys - further DESI releases, the Rubin Observatory, Euclid, the Roman Space Telescope, and the Simons Observatory - whose data are expected to decide whether the hints persist.

Problem Statement

Determine whether the preference for evolving dark energy in the DESI era reflects genuine physical dynamics - an actual evolving dark-energy component or an equivalent modification of gravity - or an artifact of (i) the two-parameter CPL parametrization, (ii) residual low-redshift Type Ia supernova calibration systematics, or (iii) tracer-level BAO systematics. Concretely: assess, with parametrization-robust, systematics-controlled inference - and, as they become available, with the independent higher-precision Stage-IV data (further DESI releases, Rubin Observatory, Euclid, Roman Space Telescope, Simons Observatory) that the critical review identifies as decisive - whether the Quintom-B-type phantom crossing indicated by CPL fits to DESI DR2 BAO + CMB + SNe Ia persists, strengthens, or disappears. If a preference survives these observational tests, a further question is whether background-level data can decide its physical interpretation at all, or whether growth and perturbation observables are required to distinguish a fluid dark-energy component from modified-gravity or inhomogeneity explanations of the same background expansion.

References

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