Since the Big Bang (also known as the initial singularity), the universe has passed through several developmental phases. One of these occurred about 400,000 years after the Big Bang. At that moment, matter changed over from a plasma state (atoms are not yet individualized, and the photons are prisoners of the plasma “soup”; the universe is still opaque) to a visible universe (individual atoms form, photons are liberated). This transition required a temperature of 3000°K. The energy of the photons thus liberated was distributed throughout the universe in an isotropic manner (equally distributed in all directions). The expansion of the universe caused wavelengths of light to shift toward the red end of the spectrum (similar to the Doppler effect; this is known as the “red shift”). The astrophysicist George Gamow calculated in 1948 that the wavelength of photons, since the moment when they were first emitted, had lengthened by a factor of 1000. He deduced from this that in addition to cosmic radiation being emitted by celestial objects, we should also be able to observe isotropic emissions corresponding to a temperature of about 3°K. That was in fact observed in 1965 by Penzias and Wilson. The actual value was 2.7°K. This is considered as the best proof of the theory of the Big Bang.