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One of the greatest questions in cosmology concerns the formation and evolution of large-scale structure in the universe and the initial conditions and processes that have led to the current observed large structures. The largest gravitationally-bound structures that we know of in the universe are galaxy clusters and superclusters, millions of light-years across, containing massive amounts of matter in the form of stars, planets, black holes, neutron stars, brown dwarfs, and other bodies. Yet according to the Big Bang model, at its earliest stages, the universe was a hot, extremely dense plasma of photons, electrons, and baryons (protons, neutrons, and some heavier particles), constantly interacting with each other with tremendously high energies, and thus unable to form neutral atoms. The early universe was opaque to radiation -- due to the constant collisions of photons with other particles, a given emitted photon would almost instantaneously be reabsorbed (the mean free path of the photons was very close to zero). In other words, radiation was “coupled” to matter. As the universe expanded, the wavelength of the photons stretched, decreasing the energy of the photons and therefore the temperature of the universe down to a threshold temperature, about 3000 K, in which conditions were conducive for electrons and protons to combine to form neutral atoms (predominantly hydrogen, with some helium). As a result of this process, which took place about 380,000 years after the Big Bang, termed recombination, photons were able to travel freely in space and in effect “decoupled” from matter. These photons constitute the cosmic microwave background radiation (CMB). The CMB is almost perfectly uniform, but the matter and dark matter distribution at the epoch of recombination 'imprinted' it with faint temperature differences that can be seen in the map above (courtesy WMAP team).
