Cosmology and Galactic Astronomy
Core concepts
Hubble's law v = H₀d describes cosmic expansion; redshift z = (λ_obs − λ_em)/λ_em and v ≈ cz for low z. The Friedmann equations connect expansion rate to density and curvature. The cosmological principle asserts homogeneity and isotropy on large scales. Key epochs: Big Bang, recombination (CMB release at z ≈ 1100, T ≈ 2.7 K today), and structure formation. Dark matter (≈27%) and dark energy (≈68%) dominate the energy budget; baryonic matter is only ≈5%. Galaxy types-spiral, elliptical, irregular-and rotation curves provide evidence for dark matter. Distance ladder: parallax, Cepheids, Type Ia supernovae.
Traps
Redshift is not always a Doppler shift-cosmological redshift arises from expansion of space. Hubble's law applies only on large scales where peculiar velocities average out. The CMB is a relic of recombination, not of the Big Bang itself. Dark energy accelerates expansion; it is not the same as dark matter. Lookback time and comoving distance are distinct concepts.
Worked example
A galaxy at 100 Mpc with H₀ = 70 km/s/Mpc recedes at 7000 km/s, giving z ≈ 7000/300000 ≈ 0.023. Rule of thumb: 1 Mpc ≈ 3.26 million light-years, and H₀ ≈ 70 km/s/Mpc implies a Hubble time of about 14 billion years.