Cosmic microwave background

The cosmic microwave background, often called the CMB or relic radiation, is a faint glow of microwave radiation that fills all of space. While a standard optical telescope sees the space between stars as dark, sensitive radio telescopes detect this uniform background. It is not associated with any specific star, galaxy, or object. Instead, it comes from every direction in the observable universe. This radiation is a vital piece of experimental evidence for the Big Bang theory. It serves as a snapshot of the very early universe.

To understand its origin, we must look at the mechanics of the early universe. In its earliest periods, the universe was filled with a dense, hot plasma of sub-atomic particles. This plasma was opaque, meaning light could not travel through it easily. As the universe expanded, this plasma began to cool. Eventually, it reached a stage called the recombination epoch. During this time, protons and electrons combined to form neutral atoms of mostly hydrogen. This event allowed photons to travel freely through space because they could no longer be scattered by Thomson scattering.

Since that moment, the universe has continued to expand. This expansion has caused a cosmological redshift, which means the photons have lost energy over time. We can think of the surface of last scattering as a shell at a specific distance in space. The photons we receive today were originally emitted during that decoupling event. Because of this expansion, the radiation now exists in the microwave region of the electromagnetic spectrum. The CMB has a thermal blackbody spectrum with a temperature of 2.725 K.

While the CMB is remarkably uniform, it is not perfectly smooth. Sensitive detectors have mapped small temperature variations known as temperature inhomogeneities. This structure is called anisotropy. The distribution of these anisotropies can be represented by a power spectrum. This spectrum shows a sequence of peaks and valleys that reveal the universe's properties. The first peak determines the overall curvature of the universe. The second and third peaks provide details about the density of normal matter and dark matter, respectively.

The discovery of the CMB was actually an accident. In 1964, American radio astronomers Arno Penzias and Robert Wilson were using a large horn antenna in Holmdel, New Jersey. They were working at Bell Telephone Laboratories on equipment originally built for Project Echo. They detected an unexplained excess antenna temperature of 4.2 K. After consulting with researchers at Princeton University, they realized they had found the cosmic microwave background. This monumental discovery earned Penzias and Wilson the Nobel Prize in Physics in 1978.

Scientists have used several major missions to study these patterns with increasing precision. The NASA Cosmic Background Explorer, or COBE, orbited from 1989 to 1996 and confirmed the primary anisotropy. Later, the WMAP and Planck spacecraft provided even more detailed maps. These missions helped scientists study polarization, which includes E-mode and B-mode signals. The E-mode signal is about ten times weaker than temperature anisotropy. The B-mode signal is even weaker but may hold even more cosmological data.

The scale of the CMB's energy is truly massive. The CMB contains the vast majority of photons in the universe by a factor of 400 to 1. Its number density is one billion times greater than the number density of matter. The energy density of these photons actually exceeds the energy density of all photons emitted by every star in history. Without the expansion of the universe cooling this radiation, the night sky would shine as brightly as the Sun. Studying the CMB allows us to connect the physics of the very small to the evolution of the entire cosmos.