Both branches are flat matter + dark-energy (ΛCDM) universes with the same closed-form solution to the Friedmann equation (radiation ignored — negligible after the first ~50,000 years):
a(t) = (Ωm/ΩΛ)^(1/3) · sinh^(2/3)( (3/2)√ΩΛ H₀ t )
Age t₀ (at a=1):
t₀ = 2 / (3 H₀ √ΩΛ) · asinh( √(ΩΛ/Ωm) ) with ΩΛ = 1 − Ωm
The two branches share Ωm but use different H₀: the "early" value comes from fitting the acoustic-peak pattern in the cosmic microwave background (Planck satellite, H₀≈67.4), the "late" value from the local distance ladder — Cepheid variables calibrating Type Ia supernovae (SH0ES, H₀≈73.0). A larger H₀ means the universe is expanding faster today, which — for the same expansion history — means it needed less time to reach a=1. That is exactly why the local-ladder branch is younger.
- H₀early / H₀late sliders — move either measurement within its real published range; the two flat 2D galaxy grids rescale live using the exact a(t) formula above.
- Ωm slider — shared matter density; raising it slows the transition into dark-energy-dominated (accelerating) expansion and shifts both ages.
- Play expansion — animates cosmic time from a≈0 to today for both universes at once, so the growing age gap between the two grids is visible directly, not just as a number.
- Drag / scroll on the view — pan and zoom the flat top-down view; it is a plain 2D projection, so panning never changes perspective, only which part of each grid you see.
This ~5σ mismatch between early- and late-universe H₀ measurements is a real, unresolved problem in modern cosmology called the Hubble tension — it is either new physics beyond ΛCDM, or an unidentified systematic error in one of the two measurement chains.