Cosmology

Cosmology asks: how did the universe begin, evolve, and what is its fate? The framework rests on the cosmological principle: on large scales (> ~300 Mpc) the universe is homogeneous (the same everywhere) and isotropic (the same in every direction). No place or direction is special.

Olbers's paradox asks why the sky is dark at night if the universe is infinite, eternal, and full of stars. The resolution: the universe is finite in age, and the speed of light is finite, so we only see light from a finite volume. Distant light is also redshifted out of the visible band.

Hubble's law run backwards implies all galaxies were once at a single point — the Big Bang. The simplest estimate of the universe's age is the Hubble time 1/H₀; with H₀ = 70 km/s/Mpc this gives ~14 billion years (the precise modern value is 13.8 Gyr). The Big Bang did not happen somewhere — it happened everywhere.

The universe could be open, flat, or closed depending on its density relative to the critical density. Observations of distant Type Ia supernovae in the late 1990s revealed that expansion is actually accelerating, not decelerating. The driving force is attributed to dark energy — possibly a cosmological constant or vacuum pressure — making up ~68 % of the universe's energy density (with ~27 % dark matter and only ~5 % ordinary baryonic matter). The 2011 Nobel Prize honored this discovery.

The direct fossil of the Big Bang is the cosmic microwave background (CMB): nearly uniform 2.7 K microwave radiation from every direction, emitted ~380,000 years after the Big Bang (recombination), when the universe cooled enough for electrons to combine with protons. Tiny temperature ripples in the CMB trace primordial density fluctuations (seeded by dark-matter clumping) that later grew into galaxies and large-scale structure.