wCDM, constant equation of state
The simplest extension of plain ΛCDM. Lets the dark energy equation of state w be a measurable constant, instead of fixing it at -1.
Placeholder for a 3D visualisation of Standard Cosmological Model. The interactive scene will land in Phase 3. The Standard Cosmological Model, ΛCDM, is the working framework of modern cosmology. The universe is flat, expanding, filled with ordinary matter, cold dark matter, photons, neutrinos, and a cosmological term Λ that drives accelerating expansion. The whole framework is fixed by six numbers, plus a handful of nuisance parameters, and fits cosmic microwave background, baryon acoustic oscillations, supernova distances, light-element abundances, and large-scale structure with extraordinary precision.
In one sentence
The simplest extension of plain ΛCDM, treats dark energy's equation of state as one free constant instead of fixing it at exactly -1. Current data say the answer is consistent with a constant.
The claim
wCDM is the minimal one-parameter step beyond plain ΛCDM. Where ΛCDM fixes the dark energy equation of state w at exactly -1, wCDM treats w as a single free parameter constant in time. ΛCDM is the special case w = -1.
Turner and White introduced this framework in 1997 under the name 'CDM with a smooth component', the general dark sector with one fluid characterized by its equation of state. The choice of name matters: a smooth component to distinguish from dark matter, which clusters, and 'CDM' to flag that everything else, structure formation, the cold dark matter component, and the standard cosmological parameters, is held fixed.
Conceptually, wCDM is the stepping stone between ΛCDM and w0waCDM. The hierarchy is ΛCDM ⊂ wCDM ⊂ w0waCDM. If dark energy is just slightly different from a cosmological constant but not evolving, wCDM is enough. If it's evolving, wCDM is not enough and you need w0waCDM. DESI 2024 BAO alone gives w = -0.99 +0.15/-0.13, consistent with -1, so the case for constant-w dark energy is weak. The interesting tension lives in the evolution.
The family stance
Five percent ordinary baryonic matter, twenty-six percent cold dark matter, sixty-nine percent dark energy modelled as a cosmological constant Λ, with essentially zero spatial curvature. The exact percentages depend on the data combination but all serious cosmological measurements converge here.
Predictions
- For w ≠ -1, dark energy density evolves with the scale factor as ρ_de ∝ a^{-3(1+w)}, not constant
- If w < -1 (phantom dark energy), dark energy density grows with cosmic expansion and the universe ends in a finite-time 'big rip'
- Distance moduli to high-z supernovae differ from ΛCDM by a few hundredths of a magnitude, within reach of LSST and Roman
Evidence
- DESI 2024 BAO alone: w = -0.99 +0.15/-0.13
- Planck 2018 + BAO + SN combinations also find w consistent with -1 within ~5%
- Provides a clean one-parameter null test of plain ΛCDM
Counterpoints
- Current data don't show meaningful preference for wCDM over plain ΛCDM. The interesting signal is in the evolution (w0waCDM), not in a constant offset
- wCDM is a parameterization, not a model. No fundamental physics predicts a fluid with constant w ≠ -1
- Phantom dark energy (w < -1) violates the dominant energy condition, theoretically awkward but not ruled out by data alone
Variants in this family
▸Go deeperTechnical detail with proper terminology
Energy density evolution: a perfect fluid with constant w satisfies ρ_de(a) ∝ a^{-3(1+w)}. ΛCDM (w = -1) gives ρ constant. For w = -0.9, dark energy was about 26% denser at z = 1 than today; for w = -1.1 (phantom), it was about 26% less dense. These are small effects but accessible to BAO and SN at z ~ 0.5 to 2.
Quintessence connection: any minimally coupled scalar field with a flat-enough potential approximates wCDM with w slightly greater than -1. Realistic quintessence almost always shows some evolution in w, which is why w0waCDM gets more attention.
Phantom regime: models with w < -1 require either a non-canonical kinetic term (ghost field) or modified gravity. The dominant energy condition (ρ + p ≥ 0) is violated. Future-eternal phantom dark energy with w < -1 leads to a 'big rip' singularity where the scale factor diverges in finite time.
References
- EstablishedTurner & White (1997). Phys. Rev. D 56, 4439, CDM with a smooth component
- EstablishedRiess et al. (1998). Astron. J. 116, 1009, original SN Ia evidence for dark energy
- EstablishedPlanck Collaboration (2020). Planck 2018 results VI: cosmological parameters. Astron. Astrophys. 641, A6
- EstablishedDESI Collaboration (2025). JCAP 02, 021, DESI 2024 VI (reports w = -0.99 +0.15/-0.13)
Last reviewed May 17, 2026
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