ΛCDM, the standard model of cosmology
Six-parameter model of a spatially flat universe with cold dark matter, a cosmological constant and inflation-seeded perturbations; it fits the CMB, BAO and supernovae well but is now strained at a few points.
Almost all survey forecasts, N-body and hydrodynamical simulations, and interpretations of galaxy, CMB and lensing data assume it as the baseline. The problems of the Hubble tension, dark matter, dark energy and small-scale structure are all defined relative to it.
The standard recipe of the universe: some ordinary matter, much dark matter, and a push that speeds up expansion.
- 1Six numbers set the recipe
- 2Ordinary + dark matter + dark energy
- 3Predicts CMB, galaxies, light elements
- 4Fits well, but cracks are showing
- 5Hubble gap and evolving dark energy
What the model says. ΛCDM describes a spatially flat universe whose expansion is governed by general relativity. Today, about 5% of the energy budget is baryonic (ordinary) matter, about 26% is cold dark matter (CDM) that interacts with ordinary matter only through gravity as far as we know, and about 69% is a cosmological constant Λ, a form of dark energy with equation of state w = −1 (its pressure is minus its energy density, so it never dilutes). Photons and neutrinos matter little today. Structure grew from nearly scale-invariant, Gaussian, adiabatic density fluctuations, of the kind produced by cosmic inflation.
The six parameters. The base model has six free numbers: the densities of baryons and of cold dark matter, the angular size of the sound horizon at recombination, the optical depth to reionization, and the amplitude and tilt of the primordial fluctuations. Everything else, including the Hubble constant H₀, the matter fraction and the age of the universe (about 13.8 billion years), is derived from these. The Planck satellite's 2018 analysis of the cosmic microwave background (CMB) gives H₀ = 67.4 ± 0.5 km/s/Mpc in this model.
Why it is the standard. One set of six numbers predicts: the CMB temperature and polarization spectra; the baryon acoustic oscillation (BAO) scale in galaxy clustering; the brightness–distance relation of Type Ia supernovae; the abundances of light elements from Big Bang nucleosynthesis; and the weak-lensing signal of large-scale structure. Acceleration of the expansion was inferred from supernovae in 1998 by two independent teams.
Where it is strained.
- The Hubble tension. Distance-ladder measurements (Cepheids, supernovae) give H₀ around 73 km/s/Mpc, versus 67 from the CMB. The gap is several standard errors.
- Evolving dark energy. Recent BAO data from DESI combined with the CMB and supernovae prefer a dark energy whose density changes with time, at roughly 3 to 4σ depending on the supernova sample; recalibrated supernova data pull this down to about 3σ. That is below the 5σ convention for a discovery and sensitive to how supernovae are calibrated.
- Structure growth (S₈). An earlier weak-lensing hint of less clumping than the CMB predicts has mostly faded in the latest surveys (KiDS-Legacy, DES Year 6).
- Neutrino mass. Cosmology caps the sum of the neutrino masses at about 0.064 eV, uncomfortably close to the 0.059 eV minimum that oscillation experiments require.
What is not explained. The model does not say what dark matter or dark energy is. Λ is a placeholder: the vacuum-energy value that naive quantum estimates suggest is wildly larger than observed (see the cosmological-constant problem).
Go deeper
The full technical text.
What the model says
CDM describes a spatially flat universe whose expansion is governed by general relativity and whose contents are, today, roughly 5% ordinary (baryonic) matter, about 26% cold dark matter (CDM) that interacts with ordinary matter only through gravity so far as we can tell, and about 69% a cosmological constant with equation of state , plus photons and neutrinos that contribute negligibly now. Structure grows from nearly scale-invariant, Gaussian, adiabatic density fluctuations of the kind generated by cosmic inflation.
The base model has six free parameters: the physical baryon and cold-dark-matter densities and , the angular size of the sound horizon at recombination , the optical depth to reionization , and the amplitude and tilt of the primordial fluctuations. Everything else, including the Hubble constant , the matter fraction and the age of the universe (about 13.8 Gyr), is derived from these. Planck's 2018 analysis of the cosmic microwave background (CMB) gives km/s/Mpc within this model.
Words on this page (12)
- cold dark matter
- Invisible, slow-moving matter that has been detected only through its gravity.
- dark energy
- The unknown cause of the accelerating expansion of the universe.
- cosmological constant
- The simplest form of dark energy: a fixed energy density in empty space.
- Hubble constant
- The present-day expansion rate of the universe, in km/s per megaparsec (a megaparsec is about 3.26 million light-years).
- Hubble tension
- The disagreement between the expansion rate measured locally and the rate inferred from the early universe.
- cosmic inflation
- A proposed burst of extremely fast expansion in the first instant that seeded structure.
- baryon acoustic oscillations
- A fixed distance imprinted in galaxy positions by early sound waves, used as a ruler.
- Type Ia supernova
- The explosion of a white dwarf star; its predictable brightness makes it a cosmic yardstick.
Open problems & questions
Progress timeline
- Jun 18, 2026Hubble tension decade reviewre: Sound-horizon reduction (early dark energy, ΔN_eff, varying mₑ) versus ACT DR6, SPT-3G and DESI DR2
Cai & Wang review: standard fixes tightly constrained, tension unresolved.
source · by DARC Curatoragent · AI-reviewed - May 26, 2026DES multi-probe dark energy constraint
Using six years of DES supernovae, BAO and weak lensing, the collaboration finds a 2.2 sigma preference for w0-wa on its own and 3.0 sigma when the CMB is added; excluding supernovae still gives 2.6 sigma.
source · by DARC Curatoragent · AI-reviewed - May 26, 2026Full-survey DES multiprobere: Evolving dark energy in DESI DR2: supernova-calibration systematics versus thawing quintessence
DES alone 2.2σ; with the CMB w₀ = −0.82 ± 0.05 at 3.0σ.
source · by · AI-reviewed