What is the oldest light we can see?

The answer

The oldest light in the universe is the Cosmic Microwave Background (CMB), released about 380,000 years after the Big Bang when the universe cooled enough for electrons and protons to combine into neutral atoms, allowing photons to travel freely for the first time. This ancient light has been stretched by the expansion of the universe from visible light into microwave radiation. The Planck satellite mapped the CMB with extraordinary precision, revealing tiny temperature fluctuations that seeded the formation of all galaxies, stars, and planets.