Cosmic Microwave Background
The thermal afterglow of the Big Bang emitted 380,000 years after the universe began, marking when primordial plasma sound waves froze into place.
This relic radiation is the thermal afterglow of the Big Bang, emitted approximately 380,000 years after the universe began when it cooled enough for electrons to bind to nuclei. It represents the epoch where sound waves in primordial plasma froze into place, creating the baryon acoustic oscillations that serve as a cross-check for cosmological models. Precise maps of this radiation, such as those from the Planck satellite, have provided cosmologists with highly constrained values for fundamental cosmological parameters.
What this means in real life
Imagine a hot oven cooling down after being turned off—the heat radiates outward. The CMB is like that ancient heat from the universe's first moments, now cooled to near absolute zero and stretched across billions of light-years.
What it isn’t
It is not a visible glow or aurora you can see with your eyes. It's microwave radiation—the same frequency used in microwave ovens—detected only by sensitive radio telescopes, not ordinary telescopes or cameras.
Commonly misused online
Often described as 'the edge of the universe' or 'the universe's boundary.' In reality, it's radiation from a specific moment in time (380,000 years after the Big Bang), not a physical edge or wall.