The expansion of the Universe. Hubble’s law
The spectral lines of distant galaxies are shifted to the red: z = Δλ / λ (for small z, v ≈ c · z). In 1929 Hubble (and Lemaître in 1927) found that galaxies recede from us with speeds proportional to their distance: v = H₀ · d, where H₀ ≈ 70 km/s/Mpc (different measurements range from 67 to 73 – the “Hubble tension”). This does not mean galaxies fly through space; it is space itself that expands, while systems bound by gravity (the Galaxy, the Local Group) do not. 1/H₀ ≈ 14 billion years – close to the age of the Universe; modern measurements give ≈ 13.8 billion years. Evidence for the Big Bang theory: the expansion, the cosmic microwave background discovered in 1965 (≈ 2.725 K, Penzias and Wilson), and the observed ratio of hydrogen to helium. The composition of the Universe: ≈ 5 % ordinary matter, ≈ 27 % dark matter, ≈ 68 % dark energy; in 1998 the expansion was found to be accelerating.
Expansion model: with an adult, mark dots on a balloon with a marker and inflate it – all dots move apart, the farther ones faster. This models Hubble’s law.