Why Does the Hubble Constant Have Two Values?
The Hubble constant H₀ currently has two discordant values because astronomers measure it at opposite ends of cosmic history using different physics. Measuring the early universe (z ≈ 1,090) via the Cosmic Microwave Background and BAO sound horizon and evolving forward with standard ΛCDM gives H₀ = 67.4 ± 0.5 km/s/Mpc, whereas measuring the local universe (z < 0.15) directly with Cepheid-calibrated Type Ia supernovae gives H₀ = 73.04 ± 1.04 km/s/Mpc.
Physical Mechanism & Dynamics
Importantly, the CMB does not measure today's expansion rate directly: it measures the angular size θ_* = r_s / D_M(z_*) of the acoustic sound horizon at 380,000 years, then relies on the six-parameter flat ΛCDM Friedmann equation to compute how fast the universe must be expanding today. If new early-universe physics—such as an 'Early Dark Energy' scalar field contributing ~10% of energy density around z ~ 3,000—briefly boosted expansion before recombination, the physical sound horizon r_s would be ~7% smaller, shifting the CMB-inferred H₀ up toward ~72–73 km/s/Mpc.
Conversely, on the local distance ladder side, while JWST has ruled out Cepheid crowding systematics, alternative stellar calibrators in the same galaxies—specifically Tip of the Red Giant Branch (TRGB) and carbon-rich JAGB stars studied by the CCHP collaboration—yield H₀ ≈ 69.8–70.4 ± 1.7 km/s/Mpc, sitting mid-way between Planck and SH0ES.
Quantitatively, this mechanism is governed by Sound Horizon Dependence of the Inverse Distance Ladder (θ_BAO(z) = r_d / D_M(z) ∝ r_d · H₀ ⇒ H₀,inferred ∝ 1 / r_d). Because BAO and CMB fix the product r_d H₀, increasing the early-universe inferred H₀ by 8% requires shrinking the sound horizon r_d from 147 Mpc to ~136 Mpc.
Sound Horizon Dependence of the Inverse Distance Ladder
θ_BAO(z) = r_d / D_M(z) ∝ r_d · H₀ ⇒ H₀,inferred ∝ 1 / r_dBecause BAO and CMB fix the product r_d H₀, increasing the early-universe inferred H₀ by 8% requires shrinking the sound horizon r_d from 147 Mpc to ~136 Mpc.
Quantitative Benchmarks & Parameters
| Parameter / Observable | Measured Value / Scale | Method & Context |
|---|---|---|
| Early-universe CMB (Planck) | 67.36 ± 0.54 km/s/Mpc | Flat ΛCDM fit to TT,TE,EE+lowE+lensing |
| Local Cepheid ladder (SH0ES) | 73.04 ± 1.04 km/s/Mpc | Gaia/NGC 4258/LMC + HST/JWST Cepheids + SNe Ia |
| Local TRGB/JAGB ladder (CCHP) | 69.9 ± 1.7 km/s/Mpc | JWST Tip of the Red Giant Branch + JAGB stars |
| Statistical discrepancy | ~5.0σ (Planck vs SH0ES) | p-value < 1 in 1.7 million under Gaussian errors |
Observational Evidence & How Scientists Know
Independent arbiters that require neither Cepheids nor the CMB—such as LIGO-Virgo-KAGRA gravitational-wave standard sirens, strong-lensing time delays (TDCOSMO + JWST stellar kinematics), and surface brightness fluctuations—are rapidly narrowing their error bars to adjudicate the tension.
Across reference datasets (Planck 2018 Cosmological Parameters (PR3), SH0ES Local Distance Ladder Hubble Constant, JWST / MAST Science Archive & Mission Documentation, ESA Gaia Archive — Data Release 3), quantitative benchmarks for why does the hubble constant have two values? are anchored by: Early-universe CMB (Planck) (67.36 ± 0.54 km/s/Mpc, via Flat ΛCDM fit to TT,TE,EE+lowE+lensing); Local Cepheid ladder (SH0ES) (73.04 ± 1.04 km/s/Mpc, via Gaia/NGC 4258/LMC + HST/JWST Cepheids + SNe Ia); Local TRGB/JAGB ladder (CCHP) (69.9 ± 1.7 km/s/Mpc, via JWST Tip of the Red Giant Branch + JAGB stars); Statistical discrepancy (~5.0σ (Planck vs SH0ES), via p-value < 1 in 1.7 million under Gaussian errors).
Uncertainties, Systematics & Open Questions
Whether the tension resolves via subtle multi-calibrator systematics in Type Ia supernova hosts, new pre-recombination physics (Early Dark Energy), or late-time evolving dark energy (w(z)) is the premier open question in observational cosmology.
Real Astronomical Benchmarks & Cross-Domain Links
Hubble Tension Comparison Explorer
Interactive calculator comparing early- vs late-universe H₀ measurements
The Hubble Tension
Full technical breakdown of the H₀ discrepancy
Cosmic Distance Ladder
How the three-rung local calibration works
Guided Learning Path
Related Concepts, Methods & Explorers
Scientific Sources & Provenance
- Planck 2018 Cosmological Parameters (PR3) (PR3 (2020 A&A 641, A6)) — Planck Collaboration, Aghanim, N., et al. 2020, A&A, 641, A6. Archive / Reference ↗
- SH0ES Local Distance Ladder Hubble Constant (2022 Baseline Cepheid + SN Ia) — Riess, A. G., et al. 2022, ApJL, 934, L7 (A Comprehensive Measurement of the Local Value of the Hubble Constant). Archive / Reference ↗
- JWST / MAST Science Archive & Mission Documentation (Cycle 1–4 Public Releases (2022–2026)) — Rigby, J., et al. 2023, PASP, 135, 048001 (The Science Performance of JWST as Characterized in Commissioning). Archive / Reference ↗
- ESA Gaia Archive — Data Release 3 (DR3 (2022-06-13)) — Gaia Collaboration, Vallenari, A., et al. 2023, A&A, 674, A1 (Gaia Data Release 3). Archive / Reference ↗
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