5.1 Discovery and properties of the CMB

Discovery and Properties of the Cosmic Microwave Background (CMB)

The cosmic microwave background (CMB) is the oldest light in the universe, a faint glow of radiation filling all of space that dates back to when the cosmos was only about 380,000 years old. It serves as the strongest piece of observational evidence for the Big Bang theory and acts as a direct window into the physical conditions of the early universe. Its properties, from its nearly perfect blackbody spectrum to its tiny temperature fluctuations, constrain the fundamental parameters of modern cosmological models.

Discovery of the CMB

In 1965, Arno Penzias and Robert Wilson were working with a highly sensitive horn antenna at Bell Telephone Laboratories in Holmdel, New Jersey. The antenna had been built for radio astronomy and satellite communication experiments, not for cosmology.

While calibrating the instrument, they encountered a persistent, low-level noise signal they could not eliminate. The signal had two striking features:

• It was isotropic, meaning it came from every direction in the sky equally.
• It was constant, showing no variation with time of day or season.

They systematically ruled out terrestrial and instrumental sources of interference (including, famously, pigeon droppings nesting in the antenna). Once every known source of noise was excluded, they concluded the signal had to be extraterrestrial in origin.

At the same time, a group of Princeton physicists led by Robert Dicke and Jim Peebles had independently predicted that a remnant thermal radiation from the Big Bang should be detectable at microwave wavelengths. When the two groups connected, the explanation became clear: Penzias and Wilson had accidentally discovered the cosmic microwave background. This discovery earned them the 1978 Nobel Prize in Physics and provided the most direct confirmation of the hot Big Bang model.

Properties of the CMB

Blackbody spectrum. The CMB has an almost perfect blackbody (thermal) spectrum, meaning its intensity as a function of frequency follows the Planck function. The spectrum peaks in the microwave range at a wavelength of approximately 1.9 mm. The COBE satellite's FIRAS instrument measured this spectrum in 1990 and found it to be the most precise blackbody ever observed in nature, with deviations smaller than 50 parts per million.

Temperature. The CMB temperature has been measured at $T = 2.725 \pm 0.001 \, \text{K}$. This is far cooler than the roughly 3000 K temperature at which the radiation was originally emitted. The difference is entirely due to the cosmological redshift: as the universe has expanded by a factor of about 1100 since the CMB was released, the radiation's wavelengths have stretched by the same factor, cooling it to its present value.

Near-perfect isotropy with tiny anisotropies. The CMB is remarkably uniform across the sky. Temperature variations are on the order of 1 part in $10^5$ (roughly $\pm 30 \, \mu\text{K}$). These tiny fluctuations, called anisotropies, are not noise. They represent real density and temperature differences in the early universe that later grew through gravitational instability into galaxies, galaxy clusters, and the large-scale cosmic web.

Polarization. The CMB is not entirely unpolarized. It carries a faint polarization signal generated by Thomson scattering of photons off free electrons near the epoch of recombination. This polarization is decomposed into E-modes (curl-free patterns, well measured) and B-modes (divergence-free patterns, much fainter and still under active investigation as a potential signature of primordial gravitational waves).

Significance in Early Universe Studies

The CMB is often described as a "snapshot" of the universe at the epoch of recombination, roughly 380,000 years after the Big Bang. Before this time, the universe was a hot, dense plasma of protons, electrons, and photons. Photons constantly scattered off free electrons, making the universe opaque. As expansion cooled the plasma below about 3000 K, protons and electrons combined into neutral hydrogen atoms. Photons then decoupled from matter and began streaming freely through space. Those photons are what we detect today as the CMB.

What makes the CMB so powerful for cosmology:

• Confirms the hot Big Bang model. The existence of a pervasive thermal radiation background at the predicted temperature is a direct consequence of a hot, dense early universe. No competing cosmological model has successfully reproduced the CMB's precise blackbody spectrum.
• Demonstrates thermal equilibrium. The near-perfect blackbody shape tells you the early universe was in very close thermal equilibrium, meaning radiation and matter had thoroughly exchanged energy before decoupling.
• Supports the cosmological principle. The high degree of isotropy is consistent with the assumption that the universe is homogeneous and isotropic on large scales.
• Seeds of cosmic structure. The small temperature anisotropies map out the density fluctuations that, over billions of years, gravitationally collapsed to form all the structure we see today.
• Constrains cosmological parameters. The angular power spectrum of CMB anisotropies (how the fluctuation amplitude varies with angular scale) depends sensitively on the geometry of the universe, the baryon density, the dark matter density, the Hubble constant, and other parameters. Precise measurements from the COBE, WMAP, and Planck satellites have been central to establishing the standard $\Lambda$CDM model, which describes a spatially flat universe composed of roughly 5% ordinary matter, 27% dark matter, and 68% dark energy.