The CMB Sky Map: Reading the Universe's Baby Picture

Light that has been traveling for 13.8 billion years

For roughly its first 380,000 years, the universe was a hot, dense, opaque plasma - electrons and protons had too much energy to bind into neutral atoms, and photons could not travel more than a short distance before scattering off a free electron. As the universe expanded and cooled below about 3,000 kelvin, electrons and protons finally combined into neutral hydrogen (an event called recombination), the fog lifted, and photons were suddenly free to travel in straight lines. That moment is called last scattering, and the photons released then have been streaming across the universe ever since, redshifted by cosmic expansion from a blistering few thousand kelvin down to today's 2.725 K microwave glow - the Cosmic Microwave Background. It is the oldest light we can observe directly, a snapshot of the universe as an infant.

A sky that is uniform to 1 part in 100,000

The single most striking fact about the CMB, measured by COBE, WMAP and Planck in increasing detail since 1992, is how smooth it is: the temperature is 2.725 K in every direction to within about 1 part in 100,000. That extreme uniformity is itself important evidence for cosmic inflation, since regions of sky that never had time to thermally equalize in a simple Big Bang model still show the same temperature. The interesting physics lives in the tiny residual - after subtracting the average and the dipole caused by our own galaxy's motion, what is left is a mottled pattern of hot and cold spots, typically shown with a color scale stretched to exaggerate fluctuations of order 10⁻⁵ (tens of microkelvin) into a vivid map.

From ripples to galaxies

Those microkelvin ripples are not noise; they are density fluctuations in the primordial plasma, thought to originate from quantum fluctuations during cosmic inflation and stretched to macroscopic scale by the universe's expansion. Slightly denser regions of plasma appear slightly hotter at last scattering, and the same slightly denser regions kept growing under their own gravity for billions of years afterward, eventually collapsing into the galaxies and galaxy clusters seen today. The CMB map is, in a real sense, the seed pattern for all cosmic structure - which is why comparing its statistics to the observed large-scale distribution of galaxies is one of the standard tests of cosmological models.

The power spectrum and acoustic peaks

Rather than describing every pixel of the sky map, cosmologists decompose the temperature fluctuations into spherical harmonics and plot their variance as a function of angular scale - the angular power spectrum, C_ℓ against multipole moment ℓ (roughly, ℓ ≈ 180°/θ, so low ℓ is large angular scale and high ℓ is fine detail):

before last scattering: photon pressure pushes plasma OUT
                         gravity pulls plasma IN
                         → standing sound waves ("acoustic oscillations")

ℓ ≈ 220 (~1° scale)  : first peak — the fundamental compression mode
ℓ ≈ 540, 810, ...    : higher harmonics, alternating compression/rarefaction

peak HEIGHTS  encode the density of baryonic and dark matter
peak POSITIONS encode the geometry (curvature) of the universe

Before last scattering, the photon-baryon plasma behaved like a compressible fluid trapped in the gravitational wells of dark matter: radiation pressure pushed outward, gravity pulled inward, and the tug-of-war set up literal sound waves. At the moment recombination froze everything in place, patches of plasma caught mid-compression or mid-rarefaction at particular length scales left an imprint - a series of harmonic peaks in the power spectrum, the CMB's version of the resonant frequencies of an organ pipe. The position of the first peak, near ℓ ≈ 220, corresponds to an angular scale set by the sound horizon at last scattering, and it is famously sensitive to the overall spatial curvature of the universe: a flat universe puts that peak almost exactly where it is observed, which is one of the strongest pieces of evidence that the universe's large-scale geometry really is flat (or extremely close to it).

A precision instrument for cosmology

Fitting the full shape of the power spectrum - not just the first peak but the relative heights and positions of the second, third and higher peaks - lets cosmologists extract the parameters of the standard ΛCDM model to remarkable precision: the fractions of ordinary matter, dark matter and dark energy, the Hubble constant, the age of the universe, and the spectral tilt of the primordial fluctuations from inflation. The Planck satellite's 2018 results pinned the universe's age at 13.8 billion years and its composition at roughly 5% ordinary matter, 27% dark matter and 68% dark energy, derived almost entirely from measurements of this one microwave sky map and its power spectrum.

Frequently asked questions

Try it live

Everything above runs in your browser - open CMB Sky Map and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.