The Expanding Universe
In 1929, Edwin Hubble made one of the most profound discoveries in the history of science: the universe is expanding. By measuring the distances and recession velocities of nearby galaxies, he found a linear relationship — farther galaxies move away faster. This simple proportionality, v = H₀d, now known as Hubble's Law, implies that space itself is stretching, carrying galaxies apart like dots on an inflating balloon.
The Hubble Constant
The proportionality constant H₀ sets the scale of cosmic expansion. Its value — currently measured around 70 km/s/Mpc — tells us that for every megaparsec (3.26 million light-years) of distance, a galaxy recedes 70 km/s faster. Inverting H₀ gives a rough estimate of the universe's age: about 14 billion years. Precise determination of H₀ has been one of astronomy's great quests, requiring painstaking calibration of cosmic distance ladders.
The Hubble Tension
Modern cosmology faces a genuine crisis: two independent methods of measuring H₀ give different answers. Local measurements using supernovae and Cepheid variables yield ~73 km/s/Mpc, while the cosmic microwave background (observed by the Planck satellite) implies ~67 km/s/Mpc. This ~9% discrepancy — the Hubble tension — has survived years of scrutiny and may point to physics beyond the standard ΛCDM cosmological model.
Cosmological Redshift and the Observable Universe
As light travels through expanding space, its wavelength stretches — producing cosmological redshift. The most distant galaxies observed have redshifts above z = 10, meaning the universe has expanded by a factor of 11 since their light was emitted. The observable universe has a radius of about 46.5 billion light-years — larger than the 13.8 billion light-year age would suggest, because space has been expanding throughout the light's journey.