🌌 Hubble's Law & Expanding Universe
📖 The Physics
In the 1920s Edwin Hubble made a startling discovery: almost every galaxy is moving away from us, and the farther one is, the faster it recedes. Their light is stretched to longer, redder wavelengths (redshift).
This is the key evidence that the whole universe is expanding — and, run backwards in time, it points to a single hot beginning: the Big Bang.
v = H₀ · d (H₀ ≈ 70 km/s per Mpc)
Age of the universe ≈ 1 / H₀
Watch the graph: every galaxy sits on the same straight line through the origin — its slope is H₀. Farther galaxies (right) always move faster (up).
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