cosmological-observation

# Cosmological Observation Modern cosmology is the shortest path from "what can I measure?" to "how old is the universe, what is it made of, and how does it end?" This skill covers the observational pillars: Hubble's law and redshift, the cosmic microwave background (CMB), Big Bang nucleosynthesis (BBN), large-scale structure, galaxy rotation curves as evidence for dark matter, Type Ia supernovae as evidence for dark energy, and the composition and parameters of the current concordance model (Lambda-CDM). It also covers the active frontier — the Hubble tension, the S8 tension, and the tests that could break or confirm Lambda-CDM in the next decade. **Agent affinity:** hubble (expansion and distance), rubin (dark matter), burbidge (nucleosynthesis context) **Concept IDs:** astro-big-bang, astro-dark-matter, astro-dark-energy, astro-cmb, astro-hubbles-law ## The Four Pillars of Big Bang Cosmology 1. **Hubble expansion** — galaxies recede with velocity proportional to distance 2. **Cosmic microwave background** — blackbody radiation at 2.725 K filling all space 3. **Big Bang nucleosynthesis** — observed primordial abundances of H, D, He-3, He-4, Li-7 match hot-early-universe predictions 4. **Large-scale structure** — galaxy distribution traces initial density fluctuations amplified by gravity Any cosmological model that breaks one of these pillars is in serious trouble. Lambda-CDM — the current concordance model — was built to fit all four and has passed increasingly sharp...