CMB angular power spectrum and sky map — the fingerprint of the early universe
COSMOLOGICAL PARAMETERS
0.020.0460.08
0.10.2330.5
556780
0.900.961.05
ACOUSTIC PEAKS
1st peak ℓ≈220 (horizon at decoupling)
2nd peak ℓ≈540 (even: compression)
3rd peak ℓ≈800 (odd: rarefaction)
4th peak ℓ≈1100
Peak heights depend on baryon-to-photon ratio. Higher Ωb enhances odd peaks over even. Ωc shifts peak positions.
About: The CMB is thermal radiation from 380,000 years after the Big Bang (redshift z≈1100), when photons decoupled from matter. Temperature fluctuations δT/T≈10⁻⁵ encode acoustic oscillations of the primordial plasma. The angular power spectrum C_ℓ = ⟨|a_ℓm|²⟩ shows peaks at ℓ≈n×220 corresponding to modes that completed n half-oscillations at decoupling. The first peak constrains geometry (universe is flat), the ratio of odd to even peaks gives baryon density, and the overall damping tail (Silk damping, ℓ>1000) reveals diffusion length. The 2018 Planck satellite measured these parameters to <1% precision.