Wherever you point a radio telescope, the sky glows at 2.7 degrees above absolute zero: light released 380,000 years after the Big Bang, stretched ever since. The glow is the same in every direction to one part in a thousand, and that one part is our own motion. Take it away and what is left are the ripples, one part in a hundred thousand, that grew into every galaxy. This is the real map, made by ESA's Planck satellite.
Point at the map to magnify it and read the temperature there. The chord plays the sky's power spectrum: one note per peak, softened by the telescope's beam exactly as the beam softens the map. It starts only when you press Chord.
The map. Planck's 2015 SMICA map of the cosmic microwave background (CMB) is stored as 12,582,912 HEALPix pixels at 10′ resolution, in Galactic coordinates. The page carries it at a quarter of that, 3,145,728 pixels of 6.9′ (Nside 512, each the average of four), in steps of 4 µK. The graphics card turns every screen pixel into a direction on the sky (Mollweide projection, or a sphere), finds the HEALPix pixel that contains it, and colours it. Galactic longitude 0, the centre of the Milky Way, is at the middle; longitude increases to the left, as on astronomers' maps.
Telescopes. A telescope's beam blurs the sky. Planck's map is shown at its own 10′. WMAP's 13.2′ is made by blurring a little further on the card. COBE's 7° is a separate copy made by smoothing the map with a 7° Gaussian on the sphere; it shows what COBE's maps looked like, not COBE's own data. In the plot, each beam multiplies the spectrum by e−ℓ(ℓ+1)σ²/2, σ = FWHM/√(8 ln 2), the guide's formula.
Our motion. The SMICA map has the average temperature and the dipole removed. The page puts them back exactly: a sky at T₀ = 2.72548 K seen from a frame moving at speed β = v/c looks like T(θ) = T₀ / (γ(1 − β cos θ)), with the Sun's velocity from Planck (369.82 km/s towards Galactic longitude 264.02°, latitude 48.25°, the guide's values). Include Earth's orbit today adds Earth's 29.8 km/s around the Sun for today's date, which shifts the dipole by up to 0.27 mK through the year; Planck used this yearly change to calibrate itself.
Statistics. The histogram, spread and extremes are computed in your browser from the map's own pixels, leaving out the pixels flagged by Planck's confidence mask (mostly the Galactic plane, where the CMB had to be estimated through the Milky Way's own glow).
A study in the language of beam and medium: a field of fine grain, an instrument's resolution as a visible choice, a single slow change that overturns the picture. It recreates no specific work.