The cosmic microwave background

Lore & Background

The first published recognition of the CMB as a detectable phenomenon appeared in a brief paper by Soviet astrophysicists A. G. Dicke at Princeton University, began constructing a Dicke radiometer to measure the CMB. That same year, Penzias and Wilson at Bell Telephone Laboratories in Holmdel Township, New Jersey, built a Dicke radiometer intended for radio astronomy and satellite communication experiments. After a telephone call from Crawford Hill, Dicke said 'Boys, we've been scooped.' A meeting between the Princeton and Crawford Hill groups determined the antenna temperature was due to the microwave background.

Reader's Guide

The accidental discovery of the cosmic microwave background in 1964 by American radio astronomers Arno Allan Penzias and Robert Woodrow Wilson completed work begun in the 1940s. This faint, uniform microwave glow fills all observable space and is not linked to any star, galaxy, or other object; its energy density exceeds that of all photons ever emitted by stars. The CMB is the key experimental evidence for the Big Bang theory. In early Big Bang models, the universe was an opaque fog of hot plasma. As it expanded and cooled, protons and electrons combined into neutral atoms, allowing photons to travel freely—an event called decoupling. These photons have since been redshifted by cosmic expansion. The surface of last scattering is the shell from which these photons now reach us. While the CMB is remarkably uniform, sensitive detectors on COBE, WMAP, and Planck reveal tiny temperature variations. These anisotropies form a characteristic pattern of ripples, and their power spectrum shows peaks: the first peak indicates the universe's overall curvature, while the second and third detail the densities of normal matter and dark matter. The CMB also has polarization, with E-mode and B-mode signals. The E-mode is about ten times weaker than the temperature anisotropy, while the B-mode is even weaker. Polarization arises when incoming radiation has quadrupole anisotropy, exciting electrons to produce polarized light. Additionally, tiny departures from the perfect black-body spectrum, known as spectral distortions, are actively studied for insights into the primordial universe. The CMB contains the vast majority of photons in the universe, outnumbering all others by a factor of 400 to 1.

Did You Know?

• The first peak of the CMB power spectrum determines the overall curvature of the universe.