What does the cosmic microwave background tell us about the universe's birth?

The answer

The cosmic microwave background (CMB) is electromagnetic radiation filling the universe, emitted approximately 380,000 years after the Big Bang when the cosmos cooled enough for atoms to form and light to travel freely. Discovered accidentally in 1965 by Arno Penzias and Robert Wilson, it has a nearly perfect blackbody spectrum at 2.725 Kelvin. The Planck satellite (2009-2013) mapped CMB temperature fluctuations to 0.00001-degree precision, revealing the seeds of cosmic structure. These tiny variations - quantum fluctuations stretched to cosmic scales by inflation - grew into galaxies and galaxy clusters. Planck data determined the universe is 13.799 billion years old, composed of 68.3% dark energy, 26.8% dark matter, and just 4.9% ordinary matter. Future missions like LiteBIRD aim to detect B-mode polarization signatures from primordial gravitational waves, which would directly confirm cosmic inflation theory.