Cosmology
Cosmic Microwave Background
Afterglow of the Big Bang — universe's oldest light, observed at 2.725 K
The cosmic microwave background (CMB) is the radiation left over from the Big Bang. Released ~380,000 years after BB during recombination — when atoms first formed and the universe became transparent. Today, photons stretched by 1100× redshift to microwave wavelengths, observed at 2.725 K. Almost perfectly uniform sky-wide (anisotropies ~10⁻⁵). Detailed measurements (COBE 1989, WMAP 2003, Planck 2013) reveal universe's age, geometry, content (4.9% baryons, 26.8% dark matter, 68.3% dark energy). Most precisely measured cosmological observable.
- Temperature today2.725 K (microwave; peak ~1.9 mm)
- DiscoveryPenzias & Wilson, 1964 (Nobel 1978)
- Origin~380,000 years after Big Bang
- Anisotropies~10⁻⁵ (very uniform)
- Probed byCOBE (1989), WMAP (2003), Planck (2013)
- Cosmological parametersH₀, Ω_m, Ω_Λ, Ω_b — all from CMB
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Why CMB matters
- Big Bang confirmation. Strongest evidence.
- Cosmology. Determines all key parameters.
- Universe age. 13.8 Gyr from CMB analysis.
- Inflation. Tests inflationary predictions.
- Dark matter / DE. Required to fit data.
- Initial conditions. Seeds for structure.
- Geometry. Universe is flat to <1%.
Common misconceptions
- CMB is from BB itself. From recombination, ~380,000 yr later.
- CMB is uniform completely. 10⁻⁵ anisotropies are crucial.
- CMB is at one temperature. Average 2.725 K; varies slightly.
- CMB is human-made. Natural; left over from BB.
- CMB doesn't constrain cosmology. Most precise constraints come from CMB.
- CMB will fade. Continues forever; just stretches further.
Frequently asked questions
How was CMB discovered?
Arno Penzias and Robert Wilson at Bell Labs (1964), trying to measure radio sources. Found unexplained noise — uniform across sky. Initially attributed to pigeon droppings. Cleared antenna; noise persisted. Robert Dicke at Princeton recognized: this is the predicted CMB (Gamow had predicted 1948). Penzias and Wilson Nobel Prize 1978.
Why is it uniform?
Universe at recombination was nearly homogeneous. Recombination released photons everywhere simultaneously (within fluctuations). Today, photons reach us from all directions — uniform background. Tiny anisotropies (10⁻⁵): seeds of all structure (galaxies, clusters).
What's recombination?
~380,000 yr after BB. Universe cooled to ~3000 K — protons captured electrons → neutral hydrogen. Before: opaque plasma (photons scatter off electrons). After: transparent (photons free-stream). Surface of last scattering — boundary. CMB is photons emitted at this time.
Why 2.725 K today?
Universe expanded ~1100× since recombination. Photon wavelengths stretched by same factor. Original ~3000 K → 2.725 K today (1/1100). Will keep cooling as universe expands. CMB temperature precisely measured.
What do anisotropies tell us?
(1) Density fluctuations at recombination — seeds of structure. (2) Geometry of universe — flat (precise to <1%). (3) Composition — matter, baryons, dark matter, dark energy ratios. (4) Hubble constant. (5) Age of universe. (6) Inflation properties. CMB power spectrum is treasure trove of cosmology.
What about Planck mission?
ESA spacecraft, launched 2009, completed 2013. Most precise CMB measurement to date. Mapped CMB temperature to 5 µK precision. Polarization measurements added. Results: H₀ = 67.4 km/s/Mpc, age 13.8 Gyr, baryons 4.9%, DM 26.8%, DE 68.3%, Universe flat.
How does CMB connect to Big Bang?
Observation of CMB confirms hot, dense early universe. Spectrum is blackbody (matches BB prediction perfectly). Temperature consistent with Hubble expansion. Anisotropies match inflation predictions. CMB is single most important cosmological observation — without it, BB much harder to confirm.