Astronomy tricks that make the cosmos click

The Big Bang was NOT an explosion of matter through existing space. Space itself expanded from an infinitely hot dense state. Before the Big Bang: physics breaks down, unknown. Evidence: galaxy redshift, CMB radiation, Big Bang nucleosynthesis (H, He, Li). Age: 13.8 billion years from Planck satellite data. Expansion still ongoing — and accelerating (dark energy).

Hubble (1929): distant galaxies show redshift proportional to distance. v = H₀ × d. Key insight: galaxies aren't moving through space — space itself is expanding. No center of expansion (raisins in rising bread). H₀ ≈ 70 km/s/Mpc — the 'Hubble tension': CMB measurement vs distance ladder give slightly different values. An unresolved discrepancy that may point to new physics.

At 380,000 years: universe cooled enough for electrons + protons → hydrogen (recombination). Photons decoupled → CMB. Discovered by Penzias and Wilson (1965, Nobel 1978). Temperature 2.725K. COBE (Cosmic Background Explorer), WMAP (Wilkinson Microwave Anisotropy Probe) and Planck mapped tiny anisotropies (1 part in 100,000) — quantum fluctuations stretched by inflation → became large-scale structure. CMB is the most direct evidence for the Big Bang.

Alan Guth (1980). Horizon problem: distant regions of CMB were never in causal contact — inflation stretched one connected patch to cosmic scales. Flatness problem: inflation drove curvature toward zero. Monopole problem: inflation diluted exotic particles. Quantum fluctuations during inflation → CMB anisotropies → galaxy seeds. Evidence: primordial gravitational waves in CMB B-modes (not yet confirmed). Inflation is the leading explanation but not yet proven.

Perlmutter, Schmidt, Riess (1998, Nobel 2011): Type Ia supernovae (standard candles) showed universe expanding faster than expected — acceleration. Dark energy: 68% of total energy content. Simplest model: cosmological constant Λ (energy of empty space), w = −1. Alternatives: quintessence (evolving field). Ultimate fate depends on nature: Big Freeze (likely), Big Rip (if w < −1). Biggest mystery in modern cosmology.

Zwicky (1933): Coma cluster too fast — missing mass. Vera Rubin (1970s): galaxy rotation curves flat at edges → dark matter halo. Evidence: gravitational lensing (Bullet Cluster), CMB, large-scale structure. Not: ordinary matter, black holes, neutrinos. Candidates: WIMPs, axions, sterile neutrinos. Direct detection experiments (XENON, LUX): no confirmed detection. Constitutes 27% of universe. Modified gravity (MOND) proposed as alternative — disfavored by Bullet Cluster.

In an infinite, eternal, static universe: every line of sight eventually hits a star → sky uniformly bright as solar surface. Resolution: (1) universe has finite age — light from distant stars hasn't arrived yet. (2) Expansion redshifts distant starlight out of visible range. Named after Olbers (1823) but raised by Kepler and Halley. The dark night sky is direct evidence the universe is not infinitely old. Profound cosmological conclusion from everyday observation.

Einstein (1916): accelerating masses create ripples in spacetime traveling at speed of light. LIGO: arms 4 km long — detects length changes of 10⁻¹⁸ m (1/10,000 proton diameter). First detection (Sept 14, 2015): two merging black holes, 1.3 billion light-years. Nobel 2017. GW170817: neutron star merger detected in GW + gamma rays + optical → multi-messenger astronomy. LISA (space-based): will detect supermassive black hole mergers. New window on the universe.

Parallax: trigonometric, <10,000 ly, Gaia measured 1 billion stars. Cepheid variables: period-luminosity relation (Leavitt) → up to ~100 Mpc. RR Lyrae: similar, older stellar populations. Type Ia supernovae: standard candles (same peak luminosity) → up to 1000+ Mpc. Tully-Fisher (spirals) and Fundamental Plane (ellipticals). Hubble tension: distance ladder gives H₀ ≈ 73 vs CMB gives H₀ ≈ 67 — unexplained discrepancy.

Big Freeze (Heat Death): expansion continues, all stars die (~10¹⁴ yr), black holes evaporate via Hawking radiation (~10¹⁰⁰ yr), maximum entropy → no usable energy. Big Crunch: if dark energy weakens, gravity eventually reverses expansion. Big Rip: if dark energy strengthens (w < −1), expansion tears apart galaxies, stars, then atoms. Current data (w ≈ −1) favors Big Freeze. Poincaré recurrence: infinite time → any configuration repeatable — but timescale ~10^(10^120) years.

< 10⁻⁴³ s: Planck epoch — quantum gravity unknown. 10⁻³⁶ s: inflation. 10⁻¹² s: electroweak phase transition. 10⁻⁶ s: quarks → protons/neutrons. 1 s: neutrinos decouple, antimatter annihilated (slight matter excess). 3 min: Big Bang nucleosynthesis — 75% H, 25% He-4, trace D, He-3, Li. 380,000 yr: recombination, CMB released. ~200 million yr: Population III stars ignite — first light after the cosmic Dark Ages.

Eternal inflation: inflation continues forever in most regions — bubble universes nucleate with different physical constants. String landscape: ~10⁵⁰⁰ vacuum states → anthropic principle explains fine-tuning. Many-worlds interpretation: every quantum measurement spawns parallel branches. Common thread: constants in our universe are compatible with life — perhaps anthropically selected from vast ensemble. Criticism: untestable by current means. CMB bubble-collision signatures searched — not found. Fascinating but controversial.

Planck era (0-10^-43 s): all forces unified. Inflation (10^-36 to 10^-32 s): exponential expansion. Big Bang Nucleosynthesis (first 3 minutes): H, He, and trace Li formed — no heavier elements, universe too hot then too cold. Recombination (380,000 years): electrons join nuclei, universe becomes transparent, CMB released. First stars/Population III (~400 million years). Galaxy formation: ~1 billion years. Today: 13.8 billion years.

Hubble's 1929 observation: all galaxies beyond the Local Group are receding, and recession speed is proportional to distance. This is not galaxies moving through space — space itself is expanding. Redshift (z) measures light stretching: z = (wavelength observed minus wavelength emitted) divided by wavelength emitted. Hubble time (1/H0 approx 14 billion years) gives rough age estimate. The Hubble tension between early-universe and late-universe H0 measurements may indicate new physics.