Cosmic Microwave Background

Origin of the CMB

In the first few hundred thousand years after the Big Bang, the universe was a hot, dense plasma of protons, electrons, and photons. The photons could not travel far before scattering off free electrons, making the universe opaque, much like light scattering in a thick fog. As the universe expanded and cooled, the temperature eventually dropped to about 3,000 Kelvin, cool enough for protons and electrons to combine into neutral hydrogen atoms in an event called recombination. Once most of the electrons were bound into atoms, photons could travel freely without being scattered, and the universe became transparent. The CMB consists of these photons, released during recombination and traveling through space ever since.

At the time of release, the CMB photons had a thermal spectrum corresponding to a temperature of about 3,000 Kelvin, placing them in the visible and near-infrared range. However, the expansion of the universe over the past 13.8 billion years has stretched their wavelengths by a factor of about 1,100, shifting them into the microwave part of the electromagnetic spectrum. Today, the CMB has a nearly perfect blackbody spectrum with a temperature of 2.725 Kelvin, just a few degrees above absolute zero. This is the most precise natural blackbody spectrum ever measured, and its perfection is a powerful confirmation that the early universe was in thermal equilibrium, exactly as the Big Bang model predicts.

Discovery and Early Measurements

The CMB was predicted theoretically in the late 1940s by Ralph Alpher, Robert Herman, and George Gamow, who calculated that the early universe should have produced a thermal radiation background that would be observable today at a temperature of a few Kelvin. Their prediction received little attention at the time. In 1964, Arno Penzias and Robert Wilson at Bell Laboratories in New Jersey detected an unexplained excess noise in their radio antenna at a wavelength of 7.35 centimeters that was uniform across the sky and constant in time. After ruling out all known sources of interference, including pigeon droppings in the antenna, they realized they had inadvertently discovered the CMB. Penzias and Wilson received the 1978 Nobel Prize in Physics for this discovery.

The COBE satellite, launched by NASA in 1989, made two landmark measurements of the CMB. The FIRAS instrument measured the CMB spectrum with extraordinary precision, confirming it as a nearly perfect blackbody at 2.725 Kelvin with deviations of less than one part in 100,000. The DMR instrument detected for the first time the tiny temperature variations (anisotropies) in the CMB at a level of about one part in 100,000, providing the first direct evidence of the primordial density fluctuations that seeded the formation of all cosmic structure. John Mather and George Smoot received the 2006 Nobel Prize for these COBE results.

Anisotropies and What They Reveal

The CMB temperature is remarkably uniform across the sky, varying by only about one part in 100,000 from the average of 2.725 Kelvin. These tiny temperature variations, called anisotropies, encode a wealth of information about the early universe. The largest pattern is a dipole anisotropy of about 3.4 millikelvin caused by the Doppler effect of our motion through space: the CMB appears slightly warmer in the direction the solar system is moving (toward the constellation Leo at about 370 kilometers per second) and slightly cooler in the opposite direction.

After subtracting the dipole and the foreground emission from our own galaxy, the remaining anisotropies form a complex pattern of hot and cold spots on the sky. These spots represent regions of slightly higher and lower density in the primordial plasma, with denser regions appearing slightly hotter and less dense regions slightly cooler. The statistical properties of these anisotropies, particularly their angular power spectrum (a graph showing the amplitude of temperature variations as a function of angular scale), contain precise information about the fundamental parameters of cosmology.

The angular power spectrum shows a series of peaks and troughs that arise from acoustic oscillations in the primordial plasma. Before recombination, gravity pulled matter inward toward denser regions while radiation pressure pushed outward, creating sound waves that propagated through the plasma at about 57 percent of the speed of light. These oscillations were frozen in at the moment of recombination, and their pattern in the CMB reveals the density of ordinary matter, the density of dark matter, the geometry of the universe (flat, open, or closed), the age of the universe, and the rate of expansion. The position of the first peak indicates that the universe is geometrically flat, meaning that parallel lines remain parallel over cosmic distances.

Precision Cosmology from WMAP and Planck

The Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001, mapped the CMB with much higher resolution and sensitivity than COBE, measuring the angular power spectrum with sufficient precision to determine cosmological parameters to within a few percent. WMAP established that the universe is 13.77 billion years old (with an uncertainty of about 60 million years), that ordinary matter makes up about 4.6 percent of the total energy density, dark matter about 24 percent, and dark energy about 71.4 percent.

The Planck satellite, launched by the European Space Agency in 2009, further refined these measurements with even greater sensitivity and angular resolution. Planck data indicate the universe is 13.80 billion years old, with ordinary matter comprising 4.9 percent, dark matter 26.8 percent, and dark energy 68.3 percent of the total energy content. Planck also measured the CMB polarization with high precision, providing independent confirmation of the cosmological parameters and placing constraints on the physics of inflation, the hypothesized period of extremely rapid expansion in the universe first fraction of a second.

One of the most important results from Planck is the measurement of the Hubble constant from CMB data, yielding a value of approximately 67.4 kilometers per second per megaparsec. This value is significantly lower than the value of approximately 73 kilometers per second per megaparsec obtained from direct measurements of nearby supernovae and Cepheid variable stars. This discrepancy, known as the Hubble tension, is one of the most actively studied problems in cosmology, as it may indicate new physics beyond the standard cosmological model or systematic errors in one or both measurement methods.

CMB Polarization and the Search for Inflation

The CMB is not only characterized by its temperature but also by its polarization, the preferred orientation of the electric field oscillations of the microwave photons. CMB polarization arises because photons scattering off electrons at the time of recombination are preferentially polarized in certain directions depending on the local radiation pattern. The polarization pattern can be decomposed into two components: E-modes, which are curl-free patterns similar to electric field lines, and B-modes, which have a swirling pattern similar to magnetic field lines.

E-mode polarization has been measured with high precision and is consistent with the standard cosmological model. B-mode polarization is of particular interest because gravitational waves produced during cosmic inflation would leave a unique B-mode signature in the CMB. Detecting this primordial B-mode signal would provide direct evidence for inflation and constrain the energy scale at which it occurred, potentially connecting cosmology to grand unified theories of particle physics. However, the signal is expected to be extremely faint, and foreground contamination from galactic dust, which also produces B-mode polarization, makes detection challenging. Several ground-based and balloon-borne experiments are currently searching for the inflationary B-mode signal with increasing sensitivity.