Big Bang Cosmology
The universe began ~13.8 billion years ago in a state of extreme density and temperature. From a region smaller than a proton, it expanded to produce trillions of galaxies, billions of stars, and on at least one rocky planet, beings trying to figure out how that happened.
1. Evidence for the Big Bang
The Big Bang is not a guess — it's supported by three independent pillars of evidence:
- Hubble expansion: Galaxies recede faster the farther they are (Hubble's law, 1929). Running time backward, everything converges to a single point ~13.8 billion years ago. (Hubble's constant H₀ ≈ 67–73 km/s/Mpc.)
- Cosmic Microwave Background: A uniform thermal glow at 2.725 K pervading all of space — the cooled afterglow of the hot early universe, first detected by Penzias & Wilson (1965), mapped in extraordinary detail by COBE, WMAP, and Planck satellites.
- Big Bang Nucleosynthesis: The theory predicts that in the first few minutes, ~75% hydrogen, ~25% helium (by mass), and trace amounts of deuterium and lithium were synthesised. This matches observed primordial element abundances to extraordinary precision.
2. Timeline of the Universe
3. Cosmic Inflation
Before ~10⁻³² seconds, something drove an exponential expansion of space — cosmic inflation (Guth 1980, Linde, Starobinsky). Inflation was proposed to solve three puzzles:
- Horizon problem: The CMB is uniform to 1 part in 100,000 in all directions. But regions more than ~2° apart have never been in causal contact. Inflation stretched a tiny causally connected region to the entire observable universe.
- Flatness problem: The universe is spatially flat to extraordinary precision. Flatness is unstable — any slight curvature at early times grows. Inflation drives spatial curvature toward zero.
- Monopole problem: Grand Unified Theories predict magnetic monopoles; inflation dilutes them to undetectable densities.
Inflation is driven by a hypothetical scalar field (the inflaton) with a potential energy that dominates the stress-energy tensor and acts as a repulsive cosmological constant. When the field "rolls" to its minimum, inflation ends ("reheating") and normal particle physics resumes. No inflaton has been detected; the mechanism is still not confirmed.
4. Big Bang Nucleosynthesis
Between ~1 second and ~3 minutes, the universe was a nuclear reactor. Temperature fell from ~10¹⁰ K to ~10⁹ K — the window for nuclear fusion. The neutron-to-proton ratio (frozen out at ~1:7 when weak interactions decouple) determines the final abundances:
Deuterium abundance is the cleanest probe of the baryon density of the universe: more baryons → more D burned into ⁴He → less D surviving. Observations of D/H in high-redshift absorption systems give Ω_b h² = 0.0224 — matching Planck CMB observations independently.
The lithium problem: standard BBN predicts 3× more ⁷Li than observed in old halo stars. Either the stars have destroyed lithium, or there is new physics. Still unresolved.
5. The Cosmic Microwave Background
At recombination (z ≈ 1100, t ≈ 380,000 yr), the universe cooled enough for electrons and protons to combine into hydrogen. The universe transitioned from opaque plasma to a transparent gas, releasing photons that have travelled freely ever since — the Cosmic Microwave Background.
Today the CMB shows nearly perfect blackbody radiation at T = 2.72548 K. Tiny temperature fluctuations (ΔT/T ~ 10⁻⁵) reflect density fluctuations in the primordial plasma — the seeds of all structure. The angular power spectrum (Cℓ vs ℓ) has peaks at specific angular scales corresponding to acoustic oscillations in the primordial plasma before recombination.
- First peak (~1°): horizon size at recombination. Size constrains total density → flat universe.
- Odd/even peak height ratio: constrains baryon density.
- Peak positions: constrain dark energy and spatial curvature.
The Planck satellite (2009–2018) measured the CMB power spectrum with exquisite precision, pinning down cosmological parameters: H₀ = 67.4, Ω_m = 0.315, Ω_Λ = 0.685.
6. Structure Formation
Density fluctuations imprinted by inflation (and tracked in the CMB) seeded cosmic structure. Dark matter — invisible but gravitating — collapsed first into halos. Ordinary matter fell into the dark matter potential wells, forming galaxies.
The process is hierarchical (bottom-up in cold dark matter): small structures formed first and merged into larger ones. Numerical N-body simulations (Millennium, IllustrisTNG, EAGLE) reproduce the observed cosmic web — filaments, sheets, voids, and clusters — starting from CMB initial conditions.
Acoustic oscillations in the early plasma also appear as "baryon acoustic oscillations" (BAO) in the large-scale galaxy distribution at ~150 Mpc scale — a standard ruler for measuring cosmological distances.
7. The ΛCDM Model
The Λ Cold Dark Matter model is the standard model of cosmology. Its composition today:
- Λ (dark energy, ~68%): A cosmological constant driving accelerated expansion since z ≈ 0.4. w = −1 (equation of state). Origin unknown.
- Cold dark matter (~27%): Non-baryonic, non-luminous, non-relativistic particles. Evidence: rotation curves, lensing, CMB, structure. Particle nature unknown (WIMPs, axions, sterile neutrinos...?).
- Ordinary (baryonic) matter (~5%): Atoms — everything we've ever directly observed.