A photograph of the whole sky, taken when it was 3,000 kelvin.
For its first 380,000 years the universe was fog. Photons could not travel a hand's breadth before colliding with a free electron; the cosmos glowed opaque, like the inside of the sun. Then it cooled below 3,000 K, electrons and protons combined into neutral hydrogen, and in a single cosmic instant the fog cleared. The light released that day has been travelling ever since. This map is that light — the farthest, oldest thing any instrument can see, a wall of glow surrounding us in every direction.
The specks are the seeds of galaxies.
The colour is false. The real sky at this wavelength is smooth to a part in a hundred thousand — so we amplify the ripples ten-thousand-fold to make them visible. Cold spots mark where matter was slightly denser and gravity had begun to pull; warm spots mark the rarefied troughs. Every cluster, every galaxy, every star grew from one of these barely-there wrinkles. The map is a Mollweide projection: the entire celestial sphere flattened into one ellipse, poles pinched to the top and bottom edges.
Before there was light, there was a ringing.
The plasma before recombination behaved like a fluid under two competing forces: gravity pulling it in, radiation pressure pushing it out. That tension set the whole universe ringing — standing sound waves frozen in place the moment the light was let go. Sort the spots by size and those harmonics appear as peaks. The first and loudest, near one degree, fixes the geometry of the universe as flat to within a fraction of a percent. Everything we know about the composition of the cosmos is read out of these ripples.