Audio Guide：

Around 380,000 years after the Big Bang, as the universe began to cool down, protons and electrons were able to combine and form hydrogen. As the number of delocalized electrons decreased, photons were less likely to be scattered by electrons and able to reach all corners of the universe. This is known as the 3K CMB (cosmic microwave background). The CMB provides information of the early times when the universe was just born and is key to the understanding of the universe’s early evolution. In 1964, two scientists from Bell Labs, Arno Allan Penzias and Robert Woodrow Wilson, first detected the cosmic microwave background. In 1989, NASA launched COBE, or the Cosmic Background Explorer. COBE made comprehensive measurements of the CMB in various wave bands and confirmed that the cosmic microwave background corresponds to 3K black-body radiation. The finding also corroborated the Big Bang Theory. In 2001, NASA launched the Wilkinson Microwave Anisotropy Probe. The European Space Agency launched the Planck spacecraft. Both projects aimed to gain a better insight into the evolution of the universe.