The Cosmic Microwave Background (CMB) is electromagnetic radiation that fills the universe, dating back to about 380,000 years after the Big Bang. It represents the oldest light in the universe that we can observe.
In the early universe, everything was so hot and dense that photons (light particles) couldn't travel freely. As the universe expanded and cooled, protons and electrons combined to form neutral hydrogen atoms. This allowed photons to travel freely for the first time, creating the CMB that we observe today.
The CMB was accidentally discovered in 1965 by Arno Penzias and Robert Wilson, who detected unexpected microwave radiation that seemed to come from all directions in space. They were awarded the Nobel Prize in Physics in 1978 for this discovery, which provided strong evidence for the Big Bang theory.
The CMB has an almost perfectly uniform temperature of 2.725 Kelvin (-270.425°C), just slightly above absolute zero. This uniformity is remarkable, with variations of only about 1 part in 100,000.
The tiny temperature fluctuations in the CMB (shown as different colors in the visualization) are incredibly important. They represent regions of slightly different density in the early universe, which eventually grew into the galaxies and large-scale structures we see today.
The remarkable uniformity of the CMB across the entire sky poses a puzzle: regions on opposite sides of the observable universe couldn't have been in causal contact with each other given the age of the universe. This "horizon problem" is one of the reasons scientists developed the theory of cosmic inflation.
The CMB is often described as the "echo" or "afterglow" of the Big Bang. Its existence and properties provide strong evidence that the universe began in an extremely hot, dense state and has been expanding ever since.
Detailed measurements of the CMB allow scientists to determine fundamental properties of the universe, including its age (13.8 billion years), geometry (flat), and composition (5% ordinary matter, 27% dark matter, 68% dark energy).
The pattern of temperature fluctuations in the CMB reveals the seeds of cosmic structure. These tiny variations in density eventually grew through gravitational attraction to form galaxies, galaxy clusters, and the cosmic web.
The Cosmic Background Explorer was the first satellite to detect the tiny temperature fluctuations in the CMB. This discovery earned George Smoot and John Mather the 2006 Nobel Prize in Physics. COBE provided the first evidence that the early universe contained the seeds that would grow into the structures we see today.
The Planck satellite provided the most detailed map of the CMB to date, with unprecedented precision. Its measurements refined our understanding of the universe's composition and supported the standard model of cosmology, including the existence of dark matter and dark energy.