The oldest light, in your hand
Colour bar: colder −300 µK to +300 µK warmer
CMB Maps shows the cosmic microwave background, the oldest light we can see, across the whole sky. Every map is drawn on your device from the missions’ own data: COBE, WMAP, the final 2018 Planck map and Planck’s nine frequency maps.
For iPhone and iPad with iOS 17 or later. It also runs on Mac with Apple silicon and on Apple Vision Pro as an iPad app.
The whole sky glows faintly in microwaves, from every direction. This glow is the cosmic microwave background (CMB). It is the oldest light we can see, but not the Big Bang itself: it was set free about 380,000 years after it, when the universe had cooled to about 3,000 K and its hot gas turned transparent.
Since then the expansion of the universe has stretched its waves about 1,090 times. Today the glow matches a temperature of 2.7255 K, just above absolute zero.
The glow is almost the same everywhere. Its ripples, slightly warmer and colder spots, differ from the average by only a few parts in 100,000. On Planck’s map a typical spot is about 110 µK, millionths of a kelvin, from the average. The spots mark slightly denser or thinner gas, the seeds of later galaxies. On large scales, denser gas looks colder.
Two things closer to home change what we see. We move through the glow at about 370 km/s, so one half of the sky looks slightly warmer; the maps on this page have that taken out. Our own Galaxy also glows in microwaves, in a band across the middle of the sky, in front of the CMB. The CMB maps have most of it taken out.
Our eyes cannot see microwaves, so every map here is in false colour, and its colour bar says what the colours mean. On the CMB maps each colour stands for a temperature difference from the average; on Planck’s single-band maps it stands for how bright that band is. A whole-sky map is the sky around you, every direction at once, flattened into an oval. The oval keeps areas true, so a patch covers the same space anywhere on it.
COBE, WMAP and Planck each scanned the whole sky for years, mapping the oldest light. Each saw finer detail than the last. Compared at the same detail, they agree: WMAP’s map with Planck’s, and COBE’s with both within its noise, away from our Galaxy.
Colour bar for all three maps: colder −200 µK to +200 µK warmer
COBE’s DMR compared the sky’s temperature in different directions, at 31.5, 53 and 90 GHz. In 1992 its team announced the first ripples. Its noise is bigger than the signal, so most small spots on its map are noise, not sky.
The middle band is not clean CMB: our Galaxy’s leftovers mix in.
WMAP scanned the whole sky for nine years, from 2001 to 2010, measuring the temperature difference between two directions in five frequency bands. Its team mixed the bands to cancel most of our Galaxy’s glow.
1° map — reliable only on scales above about 10°
Planck scanned the sky from 2009 to 2013 in nine frequency bands, from 30 to 857 GHz, and its team combined them into a map of the CMB.
About 1.5 % of the map is filled in for display, not measured, where our Galaxy and bright sources hid the CMB.
One patch of sky, 28° across, south of the Galactic plane at l 355°, b −50°, as each mission mapped it, on the same colour scale. It lies 50° from the plane of our Galaxy, and nothing in it is filled in.
Colour bar for all three close-ups: colder −200 µK to +200 µK warmer
Measured as a temperature, the ripples are the same in every band, but our Galaxy’s glow changes from band to band: it glows brightly at low and high frequencies, and in the middle the cosmic background shows through best. Planck’s team combined all nine bands to cancel the Galaxy and keep the ripples. Here are three of the bands, with our Galaxy still in them.
Colour bar, stretched: fainter −500 µK to +50,000 µK brighter
Colour bar, stretched: fainter −500 µK to +10,000 µK brighter
Colour bar, stretched: fainter 0 MJy/sr to 1,000 MJy/sr brighter
CMB Maps has three parts, each for one idea.
Planck’s 2018 map of the whole sky, drawn on your device from the data. Drag it, pinch to zoom in as far as the map’s own detail goes, and fly to 20 landmarks.
Tilt your iPhone to fade from COBE to WMAP to Planck, on one colour scale, and feel each map click into place. Between the stops the label names the mix, and a slider, taps and keys do everything tilting does.
Tap anywhere to read the map there. At the Cold Spot, Planck’s map reads “Colder than average, about −270 µK · 10 in 100,000”, with its position on the sky.
Planck mapped the sky in nine bands, from 30 to 857 GHz. Our Galaxy’s glow changes from band to band, and at 353 GHz its dust takes over.
Around you puts you at the centre of your own sky, here towards Orion and Taurus, with constellation lines. On iPhone and iPad, Look around turns the view as you move your device.
A timeline of twelve moments, from the 1948 prediction of the glow to a mission planned for the 2030s. Two charts, a glossary, and every number in the app with its source.
The maps in these screenshots are false colour, drawn by the app from the data. Credits: ESA, Planck Collaboration (CC BY-NC 3.0 IGO); NASA / WMAP Science Team; SIMBAD database, operated at CDS, Strasbourg, France (ODbL 1.0); IAU and Sky & Telescope magazine (Roger Sinnott and Rick Fienberg), figures by Alan MacRobert (CC BY 4.0); d3-celestial (BSD-3-Clause). Details on The data.
Two charts in Learn show what the light itself tells us. Here they are, drawn from the same published data.
COBE’s FIRAS instrument measured the difference between the sky’s glow and a precise blackbody it carried on board. A blackbody spectrum is the light of something at a single temperature, and its shape is set by that temperature alone. FIRAS’s measurements lie on a blackbody curve at about 2.725 K, the FIRAS team’s own figure. They stray from it by less than 50 parts per million of the peak (root mean square). The error bars are far too small to see here: the peak is about 18,000 times taller than a typical one.
Why it matters: a hot, dense young universe should leave exactly this kind of light behind, and an expanding universe keeps it a blackbody as it cools. NASA’s archive puts it plainly: “This observation matches the predictions of the hot Big Bang theory extraordinarily well.” Combined with other measurements, FIRAS gives today’s figure, 2.7255 K. The glow is brightest at about 160 GHz, counting brightness per unit of frequency.
The power spectrum shows how strong the ripples are at each size: big patches of sky on the left, small specks on the right. The dots are Planck’s 2018 measurements and the curve is the best-fitting model. The tallest peak, at multipole ℓ = 220, comes from spots about 0.8° across, wider than the full Moon, which is about half a degree.
The peaks come from sound waves in the young universe, which squeezed and stretched its hot gas. Patches caught at full squeeze or full stretch when the light set out became the strongest ripples.
Why it matters: the size of the strongest spots fits a universe in which space is flat on the largest scales. From Planck’s measurements of this light, cosmologists worked out that the universe is 13.8 billion years old.
CMB Maps first came out in 2014. As you tilted the phone, it faded between three pictures: NASA’s COBE and WMAP images and ESA’s first Planck image of the CMB. It echoed a 2007 NASA fact card with a lenticular front. Version 2.0 is a complete rewrite and its first update since 2014. The maps are now drawn from the data themselves, and the Classic 2014 deck keeps the old look.