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wCDM, constant equation of state vs Plain ΛCDM

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	wCDM, constant equation of state Consensus	Plain ΛCDM Consensus
Proposed	1997	1998 / 2020
Key figures	Michael Turner, Martin White, DESI Collaboration	Planck Collaboration, Adam Riess, Saul Perlmutter, David Spergel
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.	Six numbers, set by data, describe nearly everything we observe in cosmology, from the cosmic microwave background through galaxy surveys to today's expansion rate.
Predictions	If w is not exactly -1, dark energy density changes over cosmic time rather than staying constant, growing denser in the past if w is more negative than -1 and thinning out if w is less negative (formally ρ_de ∝ a^{-3(1+w)}) 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	Cosmic microwave background temperature anisotropies have a specific acoustic peak structure, peaking at angular scales near 1 degree, with secondary peaks at well-defined harmonics A baryon acoustic oscillation feature at ~150 Mpc (comoving) in galaxy clustering at all redshifts where galaxies form [[Big Bang Nucleosynthesis]] primordial abundances of ~75% hydrogen, ~25% helium-4, and well-defined trace abundances of deuterium and lithium How fast cosmic structure grows and clumps, captured by the growth-rate measure fσ8(z), is predicted exactly from the matter density Ωm, the dark energy density ΩΛ, and the primordial spectrum H0 ≈ 67 to 68 km/s/Mpc inferred from CMB + BAO + the standard inverse-distance ladder
Where it breaks	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	The Hubble tension: CMB + BAO predict H0 ≈ 67.4, the local distance ladder (SH0ES) measures H0 = 73.04 ± 1.04, a ~5σ disagreement that has resisted >100 proposed solutions The S8 / σ8 tension: large-scale-structure surveys (KiDS, DES) measure ~2-3σ lower clustering amplitude than CMB-fixed ΛCDM predicts DESI 2024 BAO combined with supernovae gives 2.5 to 3.9σ preference for an evolving equation of state (w0waCDM) over Λ, with the strongest preference using the DES-SN5YR sample Λ itself: no satisfactory theory explains why the vacuum energy is the observed value rather than zero or 10¹²⁰ times larger Dark matter has never been directly detected; all evidence is gravitational
Key unresolved problem	The no-signal problem: current data show no real preference for this constant-w version over plain ΛCDM, so there is nothing yet that would tell the two models apart.	The Hubble tension: CMB-inferred and locally measured values of H0 disagree at ~5σ, and no proposed solution has survived scrutiny.
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wCDM, constant equation of state

1997 · Consensus

Plain ΛCDM

1998 / 2020 · Consensus

Proposed

1997

1998 / 2020

Key figures

Michael Turner, Martin White, DESI Collaboration

Planck Collaboration, Adam Riess, Saul Perlmutter, David Spergel

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.

Six numbers, set by data, describe nearly everything we observe in cosmology, from the cosmic microwave background through galaxy surveys to today's expansion rate.

Predictions

If w is not exactly -1, dark energy density changes over cosmic time rather than staying constant, growing denser in the past if w is more negative than -1 and thinning out if w is less negative (formally ρ_de ∝ a^{-3(1+w)})
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

Cosmic microwave background temperature anisotropies have a specific acoustic peak structure, peaking at angular scales near 1 degree, with secondary peaks at well-defined harmonics
A baryon acoustic oscillation feature at ~150 Mpc (comoving) in galaxy clustering at all redshifts where galaxies form
[[Big Bang Nucleosynthesis]] primordial abundances of ~75% hydrogen, ~25% helium-4, and well-defined trace abundances of deuterium and lithium
How fast cosmic structure grows and clumps, captured by the growth-rate measure fσ8(z), is predicted exactly from the matter density Ωm, the dark energy density ΩΛ, and the primordial spectrum
H0 ≈ 67 to 68 km/s/Mpc inferred from CMB + BAO + the standard inverse-distance ladder

Where it breaks

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

The Hubble tension: CMB + BAO predict H0 ≈ 67.4, the local distance ladder (SH0ES) measures H0 = 73.04 ± 1.04, a ~5σ disagreement that has resisted >100 proposed solutions
The S8 / σ8 tension: large-scale-structure surveys (KiDS, DES) measure ~2-3σ lower clustering amplitude than CMB-fixed ΛCDM predicts
DESI 2024 BAO combined with supernovae gives 2.5 to 3.9σ preference for an evolving equation of state (w0waCDM) over Λ, with the strongest preference using the DES-SN5YR sample
Λ itself: no satisfactory theory explains why the vacuum energy is the observed value rather than zero or 10¹²⁰ times larger
Dark matter has never been directly detected; all evidence is gravitational

Key unresolved problem

The no-signal problem: current data show no real preference for this constant-w version over plain ΛCDM, so there is nothing yet that would tell the two models apart.

The Hubble tension: CMB-inferred and locally measured values of H0 disagree at ~5σ, and no proposed solution has survived scrutiny.

Reader vote

0% · 0 votes

100% · 1 vote