However, the Universe was expanding and as it expanded, it cooled, as the fixed amount of energy within it was able to spread out over larger volumes. After about 380,000 years, it had cooled to around 3000 Kelvin (approximately 2700ºC) and at this point, electrons were able to combine with protons to form hydrogen atoms, and the temperature was too low to separate them again. In the absence of free electrons, the photons were able to move unhindered through the Universe: it became transparent.
Over the intervening billions of years, the Universe has expanded and cooled greatly. Due to the expansion of space, the wavelengths of the photons have grown (they have been ‘redshifted’) to roughly 1 millimetre and thus their effective temperature has decreased to just 2.7 Kelvin, or around -270ºC, just above absolute zero. These photons fill the Universe today (there are roughly 400 in every cubic centimetre of space) and create a background glow that can be detected by far-infrared and radio telescopes.
NASA's second generation space mission, the Wilkinson Microwave Anisotropy Probe (WMAP) was launched in 2001 to study these very small fluctuations in much more detail. The fluctuations were imprinted on the CMB at the moment where the photons and matter decoupled 380,000 years after the Big Bang, and reflect slightly higher and lower densities in the primordial Universe. These fluctuations were originated at an earlier epoch – immediately after the Big Bang – and would later grow, under the effect of gravity, giving rise to the large-scale structure (i.e. clusters and superclusters of galaxies) that we see around us today. WMAP's results have helped determine the proportions of the fundamental constituents of the Universe and to establish the standard model of cosmology prevalent today, and its scientists, headed by Charles Bennett, have garnered many prizes in physics in the intervening years.
Finally, ESA's Planck was launched in 2009 to study the CMB in even greater detail than ever before. It covers a wider frequency range in more bands and at higher sensitivity than WMAP, making it possible to make a much more accurate separation of all of the components of the submillimetre and microwave wavelength sky, including many foreground sources such as the emission from our own Milky Way Galaxy. This thorough picture thus reveals the CMB and its tiny fluctuations in much greater detail and precision than previously achieved. The aim of Planck is to use this greater sensitivity to prove the standard model of cosmology beyond doubt or, more enticingly, to search for deviations from the model which might reflect new physics beyond it.
The standard model of cosmology was derived from a number of different astronomical observations based on entirely different physical processes. To reconcile the data with theory, however, cosmologists have added two additional components that lack experimental confirmation: dark matter, an invisible matter component whose web-like distribution on large scales constitutes the scaffold where galaxies and other cosmic structure formed; and dark energy, a mysterious component that permeates the Universe and is driving its currently accelerated expansion. The standard model of cosmology can be described by a relatively small number of parameters, including: the density of ordinary matter, dark matter and dark energy, the speed of cosmic expansion at the present epoch (also known as the Hubble constant), the geometry of the Universe, and the relative amount of the primordial fluctuations embedded during inflation on different scales and their amplitude.
Different values of these parameters produce a different distribution of structures in the Universe, and a different corresponding pattern of fluctuations in the CMB. By looking at the CMB, Planck can help astronomers extract the parameters that describe the state of the Universe soon after it formed and how it evolved over billions of years.
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