Dark energy at the Compton threshold: a self-gravitating frozen vacuum confronted with DESI DR2

Baryon acoustic oscillations from DESI DR2, combined with the cosmic microwave background and type Ia supernovae, prefer dark energy that is evolving away from w = −1. The simplest physical candidate is a thawing scalar field. We organise its physics around one dimensionless number, the expansion per tick of the field’s Compton clock, ε = ℏH/mc2 = m2 P /2MHm, where MH is the energy inside the Hubble sphere. Three results follow. (i) Exact solutions show that a coherent field turns from vacuum (w = −1) into matter (⟨w⟩ = 0) across the band 0.3 ≲ ε ≲ 2.2. (ii) We propose that the frozen vacuum sets its own clock, (mc2/ℏ) 2 = λ (8πG/3) ρfrozen, equivalently ε 2Ωfrozen = 1/λ: the field thaws when it dominates, so the questions “why does dark energy dominate now?” and “why does it evolve now?” become one. For a quadratic potential the principle fixes the initial displacement, ϕi = p 6/λ Mred, and the model has the same number of free parameters as ΛCDM. (iii) We fit DESI DR2 BAO, Planck 2018 distance priors and, in turn, Pantheon+, Union3, DES-Y5 and DES-Dovekie. The threshold-centre value λ = 1 (m = 0.90 H0 ≃ 1.3 × 10−33 eV, w0 ≃ −0.89, wa ≃ −0.18) gives ∆χ 2 = −0.4, −3.8, −9.0 and −2.2 relative to ΛCDM, with the same number of free parameters. The free-fall value λ = 3/2, which we had singled out before fitting, is preferred by DES-Y5 (−5.0) but disfavoured by Pantheon+ (+8.0) and by the recalibrated DES-Dovekie (+6.4). Thawing fields with m ≳ 1.3 H0 are disfavoured by every sample, and the CPL “phantom crossing” is an artefact of the parametrization. Combined with the stochastic equilibrium of a light field during inflation, the principle fixes the inflationary Hubble rate at Hinf ≃ 6 meV (an energy scale of about 5 TeV), recovering the known numerical relation ρ DE 1/4 ∼ MEW2 /MP as a consequence. The model predicts that w never crosses −1 and that cosmic acceleration ends in about 30 Gyr.