Observable Universe. Computer illustration showing the limit of visibility of the observable Universe. This is determined by the oldest light we can see, which corresponds to the Cosmic Microwave Background (CMB), the thermal radiation left over from the Big Bang. This 'fossil' radiation comes from an Era when the Cosmos was only 375,000 years old, a stage at which it became transparent to light (Decoupling Era). For practical purposes, the CMB can be considered as a surface that radiates photons from a distance from Earth about 47 billion light years. This 'wall' known by astronomers as the Last Scattering Surface constitutes an insurmountable barrier for current telescopes as, prior to the Decoupling Era, the Universe was completely opaque to light. If