Compare · The Dark Universe

Plain ΛCDM vs w0waCDM, evolving dark energy

Standard Cosmological Model· within family

	Plain ΛCDM Consensus	w0waCDM, evolving dark energy Consensus
Proposed	1998 / 2020	2001 / 2003
Key figures	Planck Collaboration, Adam Riess, Saul Perlmutter, David Spergel	Michel Chevallier, David Polarski, Eric Linder, DESI Collaboration
In one sentence	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.	Lets dark energy evolve in time instead of being a fixed constant. 2024 DESI BAO results give the strongest hint yet that this evolution may be real.
Predictions	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	Distance-redshift relation departs from ΛCDM by a measurable amount at z ≈ 0.5 to 1 The BAO acoustic feature traces out a distance-redshift curve that bends measurably away from the ΛCDM prediction, detectable by Stage-IV surveys (LSST, Euclid) mapping the feature across many redshifts Stage-IV surveys (LSST, Euclid, Roman) should crisply confirm or refute the trend within the next decade If (w0 > -1, wa < 0) is real, dark energy crosses the phantom barrier (w = -1) toward more negative w in the past, then trends less negative toward today
Where it breaks	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	Efstathiou (2025) argues the signal is a low-redshift systematic in the SN Ia compilations, not real dark energy evolution Significance is sensitive to the choice of SN compilation; the headline 3.9σ uses DES-SN5YR, a single recent sample The CPL parameterization is just a simple linear approximation of how w changes near the present day (w0 + wa·(1-a)), not a fundamental theory. A real signal would still need a physical origin Posteriors can cross the phantom divide (w < -1), which is theoretically awkward for most quintessence models
Key unresolved problem	The Hubble tension: CMB-inferred and locally measured values of H0 disagree at ~5σ, and no proposed solution has survived scrutiny.	The interpretation problem: if the DESI hint of changing dark energy is real, no underlying theory predicts that exact pattern, the equation of state w starting above -1 and dropping, without awkward fine-tuning.
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Plain ΛCDM

1998 / 2020 · Consensus

w0waCDM, evolving dark energy

2001 / 2003 · Consensus

Proposed

1998 / 2020

2001 / 2003

Key figures

Planck Collaboration, Adam Riess, Saul Perlmutter, David Spergel

Michel Chevallier, David Polarski, Eric Linder, DESI Collaboration

In one sentence

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.

Lets dark energy evolve in time instead of being a fixed constant. 2024 DESI BAO results give the strongest hint yet that this evolution may be real.

Predictions

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

Distance-redshift relation departs from ΛCDM by a measurable amount at z ≈ 0.5 to 1
The BAO acoustic feature traces out a distance-redshift curve that bends measurably away from the ΛCDM prediction, detectable by Stage-IV surveys (LSST, Euclid) mapping the feature across many redshifts
Stage-IV surveys (LSST, Euclid, Roman) should crisply confirm or refute the trend within the next decade
If (w0 > -1, wa < 0) is real, dark energy crosses the phantom barrier (w = -1) toward more negative w in the past, then trends less negative toward today

Where it breaks

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

Efstathiou (2025) argues the signal is a low-redshift systematic in the SN Ia compilations, not real dark energy evolution
Significance is sensitive to the choice of SN compilation; the headline 3.9σ uses DES-SN5YR, a single recent sample
The CPL parameterization is just a simple linear approximation of how w changes near the present day (w0 + wa·(1-a)), not a fundamental theory. A real signal would still need a physical origin
Posteriors can cross the phantom divide (w < -1), which is theoretically awkward for most quintessence models

Key unresolved problem

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

The interpretation problem: if the DESI hint of changing dark energy is real, no underlying theory predicts that exact pattern, the equation of state w starting above -1 and dropping, without awkward fine-tuning.

Reader vote

100% · 1 vote

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