The Expanding Universe
In 1929 Edwin Hubble combined Vesto Slipher's galaxy redshift measurements with his own distance estimates to reveal a stunning pattern: galaxies recede from us at velocities proportional to their distance. This simple linear relation, v = H₀d, demonstrated that the universe is expanding — one of the most profound discoveries in the history of science. The Hubble constant H₀ encodes the current expansion rate and, inversely, provides a first estimate of the universe's age.
Redshift as a Cosmic Ruler
Cosmological redshift stretches the wavelength of photons traveling through expanding space. For nearby galaxies the relationship is straightforward: z ≈ v/c. At higher redshifts the full general-relativistic formula is needed, integrating the expansion history through the Friedmann equations. Type Ia supernovae, baryon acoustic oscillations, and the CMB all serve as standard rulers and candles calibrated against Hubble's law to map the expansion history.
The Hubble Tension
Modern cosmology faces an unresolved puzzle: local measurements of H₀ using Cepheid-calibrated supernovae consistently yield ~73 km/s/Mpc, while the Planck satellite's analysis of the CMB gives ~67 km/s/Mpc. This 4–5σ discrepancy, known as the Hubble tension, has survived multiple independent cross-checks and may point to new physics — early dark energy, additional relativistic species, or modified gravity — beyond the standard ΛCDM model.
From Hubble Flow to Cosmic Fate
At small scales peculiar velocities (gravitational motions within clusters) can mask the Hubble flow, but beyond ~100 Mpc the expansion dominates. The ultimate fate of the universe depends on the balance between matter (Ωm) and dark energy (ΩΛ): our observed values suggest eternal accelerated expansion, with galaxies beyond the Local Group eventually disappearing beyond our cosmic horizon.