Cosmic microwave background

Lore & Background

The cosmic microwave background was first predicted in 1948 by Ralph Alpher and Robert Herman, who reasoned that the expansion of the universe would stretch high-energy radiation from the Big Bang into the microwave region, down to about 5 K. The first published recognition of the CMB as a detectable phenomenon appeared in a brief paper by Soviet astrophysicists A. G. Doroshkevich and Igor Novikov in spring 1964. In 1964, David Todd Wilkinson and Peter Roll, colleagues of Robert H. Dicke at Princeton University, began constructing a Dicke radiometer to measure it. That same year, Arno Penzias and Robert Woodrow Wilson at Bell Telephone Laboratories in Holmdel Township, New Jersey, built a Dicke radiometer intended for radio astronomy and satellite communication experiments. Their antenna, constructed in 1959 for Project Echo, showed an excess 4.2 K antenna temperature on 20 May 1964 that they could not account for. After a telephone call from Crawford Hill, Dicke said 'Boys, we've been scooped.' A meeting between the Princeton and Crawford Hill groups confirmed the antenna temperature was due to the microwave background. Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery.

Reader's Guide

The cosmic microwave background is the key experimental evidence for the Big Bang theory. In Big Bang models, the early universe was filled with an opaque fog of dense, hot plasma. As the universe expanded, this plasma cooled until protons and electrons combined to form neutral atoms, making the universe transparent. This decoupling event released photons that have since traveled freely, growing less energetic due to cosmological redshift. The CMB is remarkably uniform across the sky, isotropic to roughly one part in 25,000, with root mean square variations just over 100 μK after subtracting a dipole anisotropy from the Sun's peculiar velocity. Its blackbody spectrum at 2.72548±0.00057 K uniquely characterizes the radiation intensity. The CMB contains the vast majority of photons in the universe—about 411 photons per cubic centimeter—and its energy density exceeds that of all photons emitted by all stars in history. Without the expansion of the universe cooling the CMB, the night sky would shine as brightly as the Sun. The anisotropy structure, measured by experiments such as COBE, WMAP, and Planck, holds important information about the early universe: the first peak determines the overall curvature, while the second and third peaks detail the density of normal matter and dark matter, respectively. The CMB also has polarization in E-mode and B-mode, and tiny spectral distortions from the black-body law are expected, containing information about the primordial universe and structure formation.

Did You Know?

• The CMB was accidentally discovered in 1964 by Arno Penzias and Robert Woodrow Wilson using a Dicke radiometer built for satellite communication experiments.
• The CMB has a thermal black body spectrum at a temperature of 2.72548±0.00057 K.
• The CMB contains about 411 photons per cubic centimeter, and its energy density exceeds that of all photons emitted by all stars in the history of the universe.
• The first peak in the CMB power spectrum determines the overall curvature of the universe, while the second and third peaks detail the density of normal matter and dark matter.