The European Space Agency's Planck satellite is due to launch from French Guiana on May 14th. Pending a successful deployment, Planck will measure the temperature of the cosmic microwave background radiation (CMBR) across the entire celestial sphere, with greater sensitivity and spatial resolution than achieved by its predecessor, NASA's WMAP satellite. The variations in the temperature of the CMBR reflect variations in the density of matter when the universe was 380,000 years old, at the time of so-called 'recombination' when atomic nuclei captured previously free electrons.
New Scientist duly have an article to herald the launch, which claims that "these so-called anisotropies are believed to be due to inflation...During inflation, quantum fluctuations in space-time were extended to cosmological scales: by the time the CMB was released, these fluctuations had led to variations in the distribution of matter across the universe. Denser regions of the universe produced CMB photons slightly colder than average, and vice versa."
In fact, whilst it is claimed by cosmologists that temperature fluctuations more than a few degrees across are the imprint of fluctuations present at the end of the inflationary period, fluctuations smaller than a degree are believed to be the result of acoustic oscillations in the plasma of baryons, electrons and photons present between the end of inflation and the time of recombination. These small-scale fluctuations are therefore the visible remnant of the earliest sound waves in the universe.
For the large angular-scale fluctuations, the denser regions redshifted the light climbing out of those regions, and therefore produce cooler spots in the CMBR; in contrast, for the small angular-scale fluctuations, denser regions were regions where the plasma was hotter, hence these denser regions produce hotter spots in the CMBR.