The oldest known light from space is often referred to as the cosmic microwave background radiation (CMB). The CMB is electromagnetic radiation that permeates the entire universe and is considered the remnant heat left over from the Big Bang, which is the prevailing scientific theory of the universe's origin.
The CMB originated when the universe was around 380,000 years old, during a phase called recombination. Before this time, the universe was extremely hot and dense, and photons (particles of light) were constantly interacting with charged particles, preventing them from freely traveling. As the universe expanded and cooled, protons and electrons combined to form neutral atoms, and the photons were able to propagate through space without significant interference.
The CMB represents the moment when the universe transitioned from a hot, opaque plasma to a transparent state. It is a faint glow of microwave radiation that fills the entire universe uniformly and is present in all directions. The CMB has been detected and measured by various space-based and ground-based telescopes, most notably the Cosmic Background Explorer (COBE), the Wilkinson Microwave Anisotropy Probe (WMAP), and the Planck satellite.
The CMB provides crucial evidence supporting the Big Bang theory, as its characteristics match the predictions made by the theory. By studying the properties of the CMB, scientists can gain insights into the early universe, such as its age, composition, and structure.
So, the oldest known light from space is the cosmic microwave background radiation, which originated around 380,000 years after the Big Bang and has been observed and studied extensively by cosmologists.