The oldest light that we can observe in the universe is known as the cosmic microwave background (CMB) radiation. The CMB is a faint radiation that permeates the entire universe and is considered a remnant of the early stages of the universe, specifically from about 380,000 years after the Big Bang.
At that time, the universe was extremely hot and dense. As it expanded and cooled down, the electrons and protons combined to form neutral atoms, allowing photons to travel freely. This event, called recombination, marks the moment when the universe became transparent to light.
The CMB radiation we observe today is the "afterglow" of this recombination. However, due to the expansion of the universe, the original visible light has been stretched and shifted towards longer wavelengths, now in the microwave range. This radiation provides valuable insights into the early universe, helping us understand its properties and formation. It is considered the oldest light we can directly observe.