After recombination, about 380,000 years post-Big Bang, photons could travel freely. This forms the cosmic microwave background, our first observable light. It illuminates early matter distribution and tiny density fluctuations. The CMB provides a map of the universe’s infancy. It’s the oldest observable radiation, capturing the “moment the lights turned on.” Without this epoch, we couldn’t study the universe’s early structure.
It matters because the first light preserves information from the universe’s infancy.
It also serves as a baseline for testing cosmological theories.
The universe’s first light still travels to us today.
Wilkinson Microwave Anisotropy Probe [wmap.gsfc.nasa.gov]