What is the cosmic microwave background radiation?

The cosmic microwave background radiation (CMB) is the afterglow of the Big Bang, filling the universe with microwave light.

The CMB is a form of electromagnetic radiation that is a remnant from an early stage of the universe, also known as the "Big Bang." It was first discovered in 1965 by Arno Penzias and Robert Wilson, who found a persistent noise in their radio telescope that came from all directions in the sky. This noise turned out to be the CMB, providing strong evidence for the Big Bang theory.

When the universe was very young, it was extremely hot and dense. As it expanded, it cooled down, allowing protons and electrons to combine and form neutral hydrogen atoms. This process, known as "recombination," occurred about 380,000 years after the Big Bang. Before recombination, the universe was opaque because photons (light particles) constantly scattered off free electrons. After recombination, photons could travel freely, and this light is what we now observe as the CMB.

The CMB is incredibly uniform, with only tiny fluctuations in temperature. These fluctuations are crucial because they represent the seeds of all current structures in the universe, such as galaxies and clusters of galaxies. By studying these temperature variations, scientists can learn about the early universe's conditions and the processes that led to the formation of cosmic structures.

The CMB is observed in the microwave part of the electromagnetic spectrum, with a temperature of about 2.7 Kelvin, just above absolute zero. Modern satellites, like the Planck satellite, have mapped the CMB in great detail, providing a wealth of information about the universe's age, composition, and development. This makes the CMB a cornerstone of cosmology, helping us understand the universe's origins and evolution.