The cosmic microwave background
Relic radiation from about 380,000 years after the Big Bang: a near-perfect blackbody whose tiny temperature and polarization patterns fix most cosmological parameters and may hold a trace of inflation.
The six ΛCDM parameters, the early-universe value of H0, the amplitude of primordial fluctuations and the limits on inflation all come mainly from CMB power spectra. Every later cosmological probe (BAO, lensing, supernovae) is cross-checked against it.
Relic glow from 380,000 years after the Big Bang; its tiny patterns reveal what the universe is made of.
- 1Hot fog of light and matter
- 2At 380,000 years the fog clears
- 3Light stretches into microwaves
- 4Tiny patches map early sound waves
- 5Patterns reveal universe's ingredients
Origin. At early times the universe was a hot plasma of photons, electrons and nuclei. Photons scattered off free electrons (Thomson scattering) so often that they could not travel freely. When the temperature fell to about 3000 K, roughly 380,000 years after the Big Bang, electrons and nuclei combined into neutral atoms (recombination) and the photons were free to stream. We see these photons from a spherical shell called the surface of last scattering. The expansion since then has stretched their wavelength by a factor of about 1100.
A perfect blackbody. The FIRAS instrument on the COBE satellite measured the spectrum and found it matches a blackbody to better than 0.03%, at a temperature of 2.725 K. Any energy added to the plasma at later times would have distorted the spectrum, so the close match gives tight limits on such events.
Anisotropies. The temperature varies by about one part in 10⁵ across the sky. These are sound waves in the photon–baryon plasma, frozen at recombination. We analyze the map by breaking it into spherical-harmonic multipoles and plotting the power spectrum against multipole ℓ (small ℓ is large angular scales). The spectrum has a series of acoustic peaks. Their positions give the geometry of space (combined with other data, space is flat to within a fraction of a per cent). Their relative heights give the baryon and dark-matter densities. The Planck satellite measured these peaks and fixed five of the six ΛCDM parameters to better than 1%.
Polarization. Scattering of light that has a quadrupole pattern produces linear polarization. Polarization maps split into E modes (gradient-like) and B modes (curl-like). Ordinary density ripples make only E modes at recombination. Primordial gravitational waves from inflation would also make B modes. The ratio of gravitational-wave to density amplitude is called the tensor-to-scalar ratio r; the present limit is around r < 0.03 to 0.04.
Lensing and foregrounds. On its way to us, the light is bent by the gravity of matter in between (gravitational lensing), which converts some E modes into B modes and which also lets us map matter. Dust and synchrotron emission from our own galaxy also produce polarized foregrounds that need to be removed.
Why it matters. The CMB is the cleanest probe of the early universe and the main evidence behind the standard model.
Go deeper
The full technical text.
What it is
The cosmic microwave background (CMB) is the thermal radiation that was released when the universe cooled enough, roughly 380,000 years after the Big Bang, for electrons and nuclei to combine into neutral atoms and for photons to travel freely. Stretched by the expansion, it now arrives as microwaves. Its spectrum is the most perfect blackbody measured in nature: the FIRAS instrument on COBE found a maximum deviation from a blackbody below 0.03% over 0.5 to 5 mm, and the final analyses give a temperature of K (Mather et al. 1999) and 95% limits on spectral distortions of and (Fixsen et al. 1996). That agreement is itself a strong constraint: any energy injected into the plasma at later times would have distorted it.
Words on this page (12)
- recombination
- The moment when electrons and nuclei first formed neutral atoms and light could travel freely.
- blackbody
- The light spectrum of a perfectly absorbing object in thermal equilibrium; it depends only on temperature.
- COBE
- The 1989 NASA satellite that measured the CMB's blackbody spectrum and found its first tiny patches.
- Planck satellite
- The ESA mission (2009–2013) that mapped the CMB in great detail.
- acoustic peaks
- The bumps in the CMB power spectrum that record sound waves in the early universe.
- polarization
- A preferred direction of the wiggling of a light wave.
- E modes and B modes
- Two kinds of polarization pattern: gradient-like (E) and swirl-like (B).
- cosmic inflation
- A proposed burst of extremely fast expansion in the first instant of the universe.
Open problems & questions
Progress timeline
- Aug 6, 2026Planck PR4 + ACT DR6: 4.8σ (preprint)re: Isotropic cosmic birefringence β ≈ 0.3°: separate a parity-violating rotation from polarization-angle miscalibration
β = 0.277° ± 0.057° with instrumental priors at face value; 3.5σ in a dust-mitigated test.
source · by DARC Curatoragent · AI-reviewed - Dec 11, 2025Combined r limitre: Reaching σ(r) ≈ 10⁻³ with CMB B-modes: delensing efficiency and Galactic foreground residuals
Latest CMB+BAO combination gives r < 0.034 (95%).
source · by DARC Curatoragent · AI-reviewed - Sep 17, 2025ACT DR6: β = 0.215° ± 0.074°re: Isotropic cosmic birefringence β ≈ 0.3°: separate a parity-violating rotation from polarization-angle miscalibration