The power spectrum of temperature fluctuations in the Cosmic Microwave Background – animation
This animation explains how the wealth of information that is contained in the all-sky map of temperature fluctuations in the Cosmic Microwave Background can be condensed into a curve known as the power spectrum.
The temperature of the Cosmic Microwave Background exhibits fluctuations on a variety of angular scales on the sky. The animation shows six different maps that depict fluctuations at different angular scales and correspond to different regions of the curve.
The maps are produced by measuring variations of the temperature of the Cosmic Microwave Background on regions of the sky that are separated by different angles. The power spectrum graph shows the relative strength of fluctuations detected at different angular scales on the sky. The animation shows how the fluctuations at each angular scale correspond to a different portion of the graph.
The fluctuations shown here cover the largest angular scales, starting at angles of ninety degrees, shown on the left side of the graph, through to the smallest scales: just a fraction of a degree. For comparison, the diameter of the full Moon in the sky measures about half a degree.
Since ordinary matter particles and photons were tightly coupled before the Cosmic Microwave Background was released, the temperature fluctuations in the Cosmic Microwave Background are a snapshot of the distribution of matter in the early Universe.
The fluctuations at angular scales larger than a degree (shown in the first three maps) reflect the slightly inhomogeneous distribution of matter on very large scales in the early Universe. If, at the time of decoupling, a photon was in a slightly denser portion of space, it had to spend some of its energy against the gravitational attraction of the denser region to move away from it, thus becoming slightly colder than the average temperature of photons. By contrast, photons that were located in a slightly less dense portion of space lost less energy upon leaving it than other photons, thus appearing slightly hotter than average.
At angular scales of about one degree and slightly smaller (shown in the fourth and fifth maps), the graph shows the imprint and oscillation pattern of sound waves that were present in the fluid of ordinary matter and photons in the early Universe. The interplay between gravity, which pulled together the fluid of matter and radiation, and the radiation pressure of the photons, which pushed it away, caused a series of rhythmical compressions and rarefactions everywhere in the fluid.
At scales smaller than about one tenth of a degree (shown in the sixth map) the oscillating pattern is being damped: this is due to diffusion of photons during their decoupling from matter, which was not an instantaneous process but lasted a few tens of thousands of years. During this time, collisions between photons and other particles were still occurring, although not as frequently as in the previous cosmic epochs: these collisions scattered photons in different directions, damping the imprint of fluctuations on the smallest scales.