Cosmic Microwave Background

The Cosmic Microwave Background (CMB) is the oldest light in the universe, emitted when the universe was just 380,000 years old. It provides crucial evidence for the Big Bang theory and insights into the universe’s composition and evolution.

Discovery

The CMB was discovered accidentally in 1965 by radio astronomers Arno Penzias and Robert Wilson, who were testing a sensitive antenna. They detected a persistent microwave signal coming from all directions in space, which they initially thought was interference.

The signal was quickly identified as the predicted afterglow of the Big Bang by physicists Robert Dicke and Jim Peebles.

Characteristics

• Temperature: 2.7 Kelvin (-270.4°C), slightly warmer in some directions
• Wavelength: Primarily microwaves, peaking at about 1 millimeter
• Uniformity: Extremely uniform across the sky, with tiny variations

What the CMB Tells Us

The CMB radiation was emitted when the universe cooled enough for electrons and protons to combine into neutral atoms. Before this “recombination,” the universe was opaque due to scattering of light by free electrons.

The tiny temperature variations in the CMB (about 1 part in 100,000) represent density fluctuations that eventually grew into the galaxies and galaxy clusters we see today.

Modern Observations

Satellites like COBE (1990s), WMAP (2000s), and Planck (2010s) have mapped the CMB with increasing precision. These measurements have determined:

• The universe’s age: 13.8 billion years
• Composition: 5% normal matter, 27% dark matter, 68% dark energy
• Geometry: The universe is flat
• The universe began with rapid inflation

The CMB remains one of the strongest pieces of evidence supporting the Big Bang theory and the standard model of cosmology.