Big Bang ~13.8 billion years ago — space itself expanded from a hot dense state. Not an explosion in space.
The Big Bang
The origin and age of the universe — and a very common misconception
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).
Redshift
Galaxies moving apart — Hubble 1929
CMB
Afterglow radiation — discovered 1965
Nucleosynthesis
H and He forged in first 3 minutes
Age
13.8 billion years — Planck data
Hubble's Law
Hubble's Law: v = H₀ × d — farther galaxies recede faster. H₀ ≈ 70 km/s/Mpc.
Hubble's Law
The expanding universe — recession velocity proportional to distance
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.
Cosmic Microwave Background
CMB: 2.7K afterglow of the Big Bang at 380,000 years old. Tiny fluctuations are seeds of all galaxies.
Cosmic Microwave Background
The oldest light in the universe — a snapshot of the infant cosmos
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.
Cosmic Inflation
Inflation: exponential expansion at 10⁻³⁶ to 10⁻³² seconds — solves horizon, flatness, and monopole problems.
Cosmic Inflation
Why the universe looks the same in all directions — and why it's so geometrically flat
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.
Dark Energy
Dark energy: ~68% of universe — causes accelerating expansion. Nature unknown. Discovered 1998.
Dark Energy
The mysterious energy causing the universe to expand faster and faster
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.
Dark Matter
Dark matter: ~27% of universe — detected only by gravity. Never directly seen. Not ordinary matter.
Dark Matter
The invisible mass holding galaxies and galaxy clusters together
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.
Olbers' Paradox
Olbers' paradox: infinite eternal universe → bright night sky. Sky is dark because universe is finite in age.
Olbers' Paradox
A simple observation about the night sky that reveals profound cosmological truth
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.
Gravitational Waves
Gravitational waves: ripples in spacetime from accelerating masses. LIGO detected first in 2015 (Nobel 2017).
Gravitational Waves
Einstein's 1916 prediction — finally confirmed 100 years later
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.
Cosmic Distance Ladder
Distance ladder: parallax → Cepheid variables → Type Ia supernovae. Each rung extends range further.
Cosmic Distance Ladder
How astronomers measure distances across billions of light-years — step by step
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.
Parallax
< 10,000 ly — direct geometry, Gaia
Cepheids
Up to ~100 Mpc — pulsation period
Type Ia SN
Up to ~1000 Mpc — standard candle
Hubble tension
CMB vs ladder H₀ values disagree
Fate of the Universe
Universe fates: Big Freeze (most likely), Big Crunch, Big Rip. Depends on dark energy equation of state w.
Fate of the Universe
Three possible endings for the cosmos — all incomprehensibly far in the future
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.
Early Universe Timeline
Planck epoch → inflation → quarks → nucleosynthesis → CMB → first stars. First 380,000 years mapped.
Early Universe Timeline
The first moments of the universe — from incomprehensible energy to hydrogen and helium
< 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.
Three independent theoretical reasons why our universe may not be unique
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.
Big Bang Timeline
PLAN BIG — Planck era, Lepton era, Atom nuclei, Neutral atoms, Background radiation (CMB), Inflation, Galaxies
10 MINUS 43 SECONDS AND 3 MINUTES AND 380,000 YEARS AND 400 MILLION YEARS AND 13.8 BILLION YEARS
The CMB snapshot at 380,000 years is our oldest observable light — before this the universe was opaque
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.
Inflation
10^-36 to 10^-32 s — exponential expansion solving flatness and horizon problems
Nucleosynthesis
First 3 minutes — H and He and trace Li formed, nothing heavier
Recombination
380,000 years — CMB released, universe becomes transparent
Population III
First stars at 400 million years — pure H and He, no metals
Hubble's Law
v = H0 × d — recession velocity equals Hubble constant times distance — more distant means faster recession
HUBBLE CONSTANT H0 APPROXIMATELY 70 km/s/Mpc — each megaparsec of distance adds 70 km/s recession
Hubble tension: CMB gives H0=67.4, local distance ladder gives H0=73 — an unsolved cosmological crisis
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.
v = H0 × d
Velocity proportional to distance — Hubble's Law
H0 approx 70
Hubble constant — universe expansion rate today (km/s per Mpc)
Redshift z
Light stretched by expansion — z=1 means universe was half current size
Hubble tension
CMB (67.4) vs distance ladder (73) — unsolved problem in cosmology
Mnemonic
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🎓 Common Exam Questions
Q: What is Hubble's Law and what does it tell us about the universe?
A: Hubble's Law (v = H0 × d) states that the recession velocity of a galaxy is proportional to its distance. H0 approximately 70 km/s/Mpc means a galaxy 1 Mpc away recedes at ~70 km/s, one 2 Mpc away at ~140 km/s. This tells us the universe is expanding — space itself is stretching, not galaxies flying through it. Extrapolating backward implies a finite age (~13.8 billion years) and a hot dense beginning — the Big Bang.
Q: What is the Cosmic Microwave Background (CMB) and why is it important?
A: The CMB is thermal radiation released ~380,000 years after the Big Bang when the universe cooled enough for electrons to combine with protons (recombination), making the universe transparent. It has since redshifted to microwave wavelengths at 2.725 K. Its importance: (1) Confirms the Big Bang. (2) Its near-perfect uniformity supports inflation. (3) Its tiny fluctuations are seeds of today's large-scale structure. (4) Precise measurements give cosmological parameters including H0 and dark matter density.
Q: What is dark energy and what is the evidence for it?
A: Dark energy is an unknown form of energy causing the universe's expansion to accelerate. Evidence: in 1998, two independent teams studying Type Ia supernovae found distant supernovae were dimmer than expected — farther away than a decelerating expansion would predict — implying accelerating expansion driven by dark energy. Dark energy comprises ~68% of the universe's total energy content. Its nature is unknown — it may be Einstein's cosmological constant (vacuum energy) or a dynamical field.
Q: What problems does cosmic inflation solve?
A: Inflation (exponential expansion at ~10^-36 seconds) solves: (1) Horizon problem — CMB regions that couldn't have communicated have the same temperature; inflation put them in causal contact first. (2) Flatness problem — the universe is nearly perfectly flat; inflation stretched any initial curvature to negligible levels. (3) Magnetic monopole problem — GUTs predict exotic particles not observed; inflation diluted their density. Inflation also produced quantum fluctuations seeding the CMB and large-scale structure.
Q: What is Big Bang Nucleosynthesis and what elements did it produce?
A: Big Bang Nucleosynthesis occurred in the first 3 minutes when temperatures allowed nuclear fusion. It produced ~75% hydrogen and ~25% helium-4 by mass, plus trace amounts of deuterium, helium-3, and lithium-7. No heavier elements — the universe expanded and cooled too quickly for further fusion. This is why all elements heavier than lithium were produced in stars and supernova explosions. Predicted BBN abundances precisely match observed primordial abundances — strong evidence for the Big Bang.