CMB Colour Lab

What you are looking at

The cosmic microwave background

The cosmic microwave background (CMB) is the oldest light we can observe. About 380,000 years after the Big Bang, the Universe had cooled to about 3,000 kelvin, cool enough for atoms to form and for light to travel freely. Since then the Universe has expanded, stretching that light into microwaves. Today it arrives from every direction with an average temperature of about 2.725 K.

Why the colours are false

Your eyes cannot see microwaves, so the CMB has no visible red or blue. Maps like this one use a false-colour scale: each number is given a colour so that patterns become visible. The Planck mission's own maps do the same. Change the scale and the picture changes; the numbers do not.

Microkelvins and anisotropy

The map shows how much warmer or cooler each direction is than the average: the anisotropy. These differences are tiny, typically tens to a few hundred millionths of a kelvin (microkelvin, µK). A deviation of +100 µK is about 37 parts per million of the mean. A negative value means cooler than the reference mean, never a temperature below absolute zero.

The blackbody spectrum

The CMB has an almost perfect blackbody spectrum, described by Planck's law. Per unit frequency, B_ν(T) = (2hν³/c²) / (e^hν/kT − 1); per unit wavelength, B_λ(T) = (2hc²/λ⁵) / (e^hc/λkT − 1). The two are the same radiation spread over different units, linked by B_λ = B_ν · c/λ². That is why their peaks sit in different places: at 2.72548 K, B_ν peaks near 160.23 GHz (a wavelength of 1.871 mm) while B_λ peaks near 1.063 mm. One temperature, two correct peaks.

Expansion and cooling

In the standard model, the temperature of this radiation scales as T(z) = T0 (1 + z), where z is the redshift. At z ≈ 1100, around last scattering, that gives about 3,001 K. The lab calculates the spectrum at a chosen epoch from this relationship; it is a model, not an observed movie, and the map always shows today's sky.

The map, its projection and its masks

The data is the Planck 2018 (PR3) SMICA map: a component-separation estimate that combines Planck's frequency channels to remove foreground emission from our Galaxy and other sources. It is a processed estimate, not a raw photograph. Near the Galactic plane and some bright sources the estimate is less reliable, so the product comes with a confidence mask; hatched areas are outside it and have no value in this lab. SMICA also fills (inpaints) a narrow Galactic region; the reconstruction view shows those pixels dotted.

Every point on the oval map is a direction in the sky, in Galactic coordinates, with the centre of our Galaxy in the middle and longitude increasing to the left. The hemisphere view shows the sky as seen from inside. Neither is a picture of a ball-shaped Universe, and the map does not mean we are at a special centre.

The data in this lab

Based on observations obtained with Planck (https://www.esa.int/Planck), an ESA science mission with instruments and contributions directly funded by ESA Member States, NASA, and Canada. Planck Collaboration 2020, A&A 641, A4 (Planck 2018 results IV).

A note on the numbers

Some introductions say the CMB was released about 300,000 years after the Big Bang at about 3,000 °C. This lab follows ESA's Planck explainer: about 380,000 years and about 3,000 kelvin (roughly 2,700 °C). Kelvin and degrees Celsius are not interchangeable at these values.