The Friedmann equation is the heartbeat of modern cosmology. It tells you how fast the universe expands given what it contains — matter, radiation, curvature, and dark energy.
Alexander Friedmann derived it in 1922 from Einstein's field equations applied to a homogeneous, isotropic universe.
Reading the equation
The Hubble parameter squared (ȧ/a)² equals 8πGρ/3 minus curvature term kc²/a² plus Λc²/3. Each term competes to dominate at different cosmic eras.
Why it matters today
Planck satellite data fit this equation to measure Ω_m, Ω_Λ, and the age of the universe (~13.8 Gyr). The transition from decelerating to accelerating expansion occurred roughly 7.7 billion years ago.
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