CMB anisotropies
/ C-M-B an-eye-SOT-ruh-pees /
If you photograph the microwave sky and stretch the contrast far enough, a stunningly uniform glow reveals a faint mottled pattern — a delicate patchwork of slightly warmer and slightly cooler spots, like the freckling on an egg. The cosmic microwave background is almost perfectly smooth, the same temperature in every direction, but not exactly. Those tiny departures from perfect smoothness are the CMB anisotropies, and reading them has become one of the most precise ways we have to measure the whole universe.
'Anisotropy' simply means 'not the same in all directions.' The CMB's average temperature is 2.725 degrees above absolute zero, and the anisotropies are deviations of only about one part in 100,000 — differences of tens of millionths of a degree from spot to spot. These warmer and cooler patches are a direct image of the primordial density fluctuations at the moment of last scattering: slightly denser regions were a hair hotter, slightly emptier ones a hair cooler. So the speckled map is a photograph of the seeds of all cosmic structure, frozen when the universe was 380,000 years old. (One must first subtract a larger, smooth lopsidedness caused simply by our own motion through space, the so-called dipole.)
These faint freckles are extraordinarily informative. Statistically analyzing the sizes and arrangement of the hot and cold spots — especially a series of preferred spot sizes called the acoustic peaks — lets cosmologists weigh the universe's ingredients with stunning precision: how much ordinary matter, how much dark matter, how much dark energy, and the geometry of space (the CMB tells us space is flat to within a fraction of a percent). Satellites named COBE, WMAP, and Planck mapped these anisotropies in ever finer detail, and their measurements are a cornerstone of modern cosmology. In a real sense, much of what we claim to know about the contents and history of the universe is read from this faint two-degrees-above-absolute-zero speckle.
If the whole microwave sky were the smooth surface of a calm sea, the anisotropies would be ripples just a hundred-thousandth as high — utterly invisible without instruments built to feel millionths of a degree. The COBE satellite first detected them in 1992; the discovery was called 'looking at the face of God,' because that faint texture is the imprint of the seeds of every galaxy.
The faint hot-and-cold pattern in the CMB encodes the universe's contents and geometry.
The anisotropies are real cosmological signal, but a much larger apparent unevenness — the dipole — is just our own motion through space and must be removed first. The true ripples are only one part in 100,000.