Light from the Past: Recombination and the Cosmic Microwave Background
The universe becomes visible
When atoms formed
After the early universe cooled enough, electrons could combine with nuclei to form neutral atoms. This event is often called recombination. Once neutral atoms formed, light no longer scattered constantly off free electrons, so the universe became transparent.
Why this matters
The oldest light we can observe directly comes from this time, giving us a snapshot of the early universe.
The cosmic microwave background
The cosmic microwave background, or CMB, is faint radiation coming from every direction in space. It is relic light from the early universe, stretched by expansion into microwave wavelengths.
CMB basics
Why did the universe become transparent after recombination?
When electrons joined nuclei, there were fewer free electrons to scatter photons.
Correct answer: Free electrons became bound into atoms, so photons scattered less often
What does CMB stand for?
It is the afterglow radiation from the early universe.
Correct answer: Cosmic Microwave Background
Tiny patterns, huge meaning
The CMB is not perfectly uniform. Tiny temperature differences across the sky encode information about density variations in the early universe, which later grew into galaxies and clusters.
The CMB is best described as:
The CMB is a nearly uniform background glow left over from the early universe.
Correct answer: Relic radiation from the early universe
What kind of evidence does the CMB provide for cosmology?
Its existence and structure strongly support the Big Bang model.
Correct answer: Evidence for a hot early universe and its later evolution