The oldest light in the universe

The universe we live in started with the Big Bang 13.8 billion years ago. For the first 380,000 years, it was so hot that light couldn't travel through it. Everything was opaque.

Then, the universe cooled just enough for electrons and protons to combine into hydrogen atoms. The moment that happened, the universe became transparent and all the light that had been trapped was released at once.

That light is still traveling. It has been crossing space for 13.8 billion years, arriving from every direction simultaneously. It fills all of space equally. When you point a detector at any point in the sky, you can detect it.

This light is the Cosmic Microwave Background (CMB). The actual light from that moment of release, still arriving. It surrounds you, arriving from every direction, passing through you right now.

The temperature

The CMB has a temperature of 2.725 Kelvin. Kelvin is measured from absolute zero: the coldest anything can be, the point at which all molecular motion stops. Absolute zero is −273.15°C. So 2.725K is −270.425°C: cold as cold gets.

But the temperature is not perfectly uniform. Across the full sky, the temperature varies by roughly ±300 millionths of a degree. These fluctuations are so small that they took decades of satellite technology to detect. These are shown on this site as Signal Temperatures: the actual Celsius reading at the coordinates you clicked, to seven decimal places.

Those tiny variations are the imprints of quantum fluctuations from the first fraction of a second of the existence of our universe. These regions of the infant universe were infinitesimally denser or emptier than their surroundings. Gravity amplified these variations over billions of years into the large-scale structure of the universe: galaxy filaments, voids, and clusters. Every star. This planet. The temperature you read in the bubble is where all of that began.

The cold spot

The CMB Cold Spot at galactic coordinates l=264°, b=−48° is the largest anomaly in the CMB. It is colder than its surroundings by about 70 millionths of a degree.

It might be nothing — a coincidence large enough to look meaningful but not actually be so. It might be caused by an enormous empty region of space between us and the CMB: a void so vast that light loses energy crossing it, arriving colder than it should. It might be a scar left by a knot in the structure of space itself, formed in the first moments after the Big Bang when the universe was still deciding what shape to take. Or — and this has not been ruled out — it might be where another universe bumped into ours.

None of these explanations has been confirmed. The Cold Spot remains an open question.

The map

The map featured on this site is the Planck 2018 SMICA full-sky temperature map, released by the European Space Agency. SMICA — Spectral Matching Independent Component Analysis — is the algorithm used to separate the CMB signal from foreground interference such as interstellar dust, synchrotron radiation, and other sources. The map was assembled from five years of observations by the Planck satellite, and contains 50 million pixels, each a temperature measurement accurate to a few millionths of a degree.

The map is displayed in Mollweide projection, a way of wrapping the full sphere of the sky onto an ellipse, with the galactic center at the center of the image. Galactic longitude runs left to right. Galactic latitude runs top to bottom. Each block on the grid is 5° × 5° of sky.

The dark and blurry corridor across the center of the map is the galactic plane: the disk of the Milky Way cutting across the center of the map. Our home galaxy emits so strongly at microwave frequencies (from interstellar dust, gas, and charged particles spiraling through magnetic fields) that its signal overwhelms the CMB behind it. The Planck team masked that region out entirely; what you see there is a smooth interpolation, not real data. In this region, we are, in a sense, in the way of our own view.

The codec

Each pixel in the map has a temperature. That temperature is a number. Numbers can be converted to letters. Letters can be searched for words.

The conversion is simple and deterministic: the same coordinates always produce the same letters, always produce the same words. No interpretation. No editorial choice.

temperature (μK)  →  shift to positive  →  scale to 0–255  →  mod 26  →  letter A–Z

A 5° patch of sky contains hundreds of pixels. Each pixel produces a letter. That stream of letters is then searched for the longest real English words that tile it reading left to right. These words emerged from physics. Not so much chosen, but found.

The transmissions

When a block of sky is clicked for the first time, the signal words from that location are passed to an AI that is trained to speak as the CMB itself: 13.8 billion years old and present everywhere at once.

The signal words are used as seeds. A word like LIF may surface as life or lifting. HAP may become happen or perhaps.

The transmission is decoded once, permanently. Every person who visits that patch of sky after the initial decoding reads exactly what the first person to click it received. The CMB won't change in the human timescale, so neither will what it said.

Each visitor may decode one block. Once decoded, that block glows softly on the map. It is a record that someone decoded its signal. The location of the decoder is recorded at city level: not who, only where on Earth the transmission was picked up.

There are 2,592 blocks. Each one has been waiting 13.8 billion years.