The cosmic microwave background and inflation

The slightly denser regions eventually grew increasingly denser, as gravity caused them to draw more and more matter from the surroundings. These primordial fluctuations in the density of matter in the early Universe are the seeds of the rich network of cosmic structure – stars, galaxies, galaxy clusters – that we observe today. It is thought that the fluctuations seen in the CMB are a result of the brief period of inflation. The exact details are hidden in the CMB, which Planck will be able to extract.

The photons carry a memory of how matter and radiation were distributed at the time of the decoupling. 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. Vice versa, 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. This is why temperature fluctuations in the CMB reflect the pattern of structure in the matter that was present in the early Universe, right when the CMB was released. The CMB can therefore be considered as the ultimate snapshot of our Universe at the time of recombination.