Dark matter is so named because we cannot see it. So it’s ironic that much we have learned about dark matter has come from studying light, specifically, the cosmic microwave background (CMB). I often share with my students the story of its discovery, which paints a wonderful picture of how science works in practice and how we test scientific theory, although as an astroparticle physicist I do not study it directly.

The CMB story starts with Edwin Hubble, who made one of the most earth-shattering discoveries of the 20th century. In 1929 he found that the universe is expanding. After concluding that the “spiral nebulae” were “island universes” and not part of the Milky Way, Hubble measured their distances using Cepheid variable stars. Except for the nearby galaxies in our Local Group, all of the galaxies he observed were moving away from us, and the farthest galaxies were moving away the fastest.

The relationship of the velocity and distance for galaxies is linear and its slope is known as the Hubble constant, H₀. Hubble found its value to be about 500 km/s/Mpc, which means a galaxy one megaparsec from us will be observed to be receding at 500 km/s. The modern value is 69.32 +/- 0.80 km/s/Mpc. Hubble’s high value was due to errors in distances to galaxies. (Distances in astronomy are notoriously hard to measure.) This universal recession immediately suggested that the universe was nonstatic and evolving, and perhaps had a beginning.

Hubble’s discovery came at a time when a flurry of work was being done to model the universe at large using Einstein’s recently developed theory of general relativity. Einstein first favored a static, nonevolving model. However, Georges Lemaître, a Belgian scientist and Catholic priest, showed that an expanding universe was also a valid solution to Einstein’s field equations. Inspired by the phenomenon of radioactivity, Lemaître proposed that the universe as we see it began from the “decay” of a primeval atom. In his view cosmic remnants from this atom formed the seeds of stars, galaxies, and the other structures in the universe we see today. Lemaître viewed this as a cold process.

In the famous paper published in 1948, Ralph Alpher, Hans Bethe, and George Gamow proposed a model explaining the abundances of the elements that incorporated the expansion of the universe. The early universe, they argued, was hot and dense, and expanded from an initially ultradense state. They successfully calculated hydrogen and helium abundances; however, they erroneously postulated that all heavier elements were created in the early universe through combining neutrons. We now understand that all elements heavier than lithium are created in the core of stars.

One of the most important predictions they made was too quickly forgotten: the initial hot, dense state of the universe should exhibit a leftover radiation field. In their theory, particles were created and annihilated in the early universe, and energy was transferred back and forth to a background of photons or light. Those frequent interactions meant that the universe could be modeled as a perfect blackbody, characterized by some temperature, T. As the universe expanded, this background of photons redshifted (i.e., lost energy). In essence, Gamow and his collaborators predicted the CMB and postulated that this background radiation should have a temperature today of about 5 K.

By the early 1960s cosmology had become a showdown between two competing theories. The big bang model gave the universe a problematically young age, two billion years. This age problem led Fred Hoyle, Hermann Bondi, and Thomas Gold to propose the steady-state theory, which explained Hubble’s expansion by proposing new physics and a static universe that continuously created new matter.

The two theories, big bang and steady state, gave very different predictions about the universe. In a way, the steady-state model was conceptually simpler; it had fewer variable parameters and made more concrete predictions. One of these predictions was the distribution of radio sources at large distances. Measurements of radio sources seemed to disfavor the steady-state model, but the results were not conclusive at that time.
In 1964 astronomers Arno Penzias and Robert Wilson found the smoking gun that finally gave unequivocal evidence for the big bang model. While trying to calibrate a horn antenna at Bell Labs, developed to detect radio waves from satellites, they noticed excess noise in the sky corresponding to a uniform signal 100 times stronger than any background they had expected.

At first this signal frustrated them to no end. They went to extreme lengths, even removing bird droppings from the antenna, to determine the source of this background. After painstaking work, they found that the background was neither from the sun nor our own galaxy. It was extragalactic in nature, but its source remained mysterious.

Finally, when a friend pointed out the work of astronomers at Princeton University who were searching for the CMB, Penzias and Wilson realized what they had discovered. The two groups published joint articles in The Astrophysical Journal describing the discovery and interpreting it as the long-predicted cosmic microwave background radiation.

In 1989 NASA launched the Cosmic Background Explorer (COBE) satellite, which verified two fundamental properties of the CMB. The first was that the radiation is remarkably uniform (isotropic) across the sky; hence the early universe was a nearly perfect blackbody. This discovery vindicated the use of statistical thermodynamics to describe the early universe.

But cosmologists were still puzzled by the uniform nature of the CMB. An extremely uniform CMB suggested an extremely uniform early universe. Why, then, is there structure today? Why isn’t the universe a dilute, uniform cloud of gas?

John C. Mather and George Smoot answered this question with COBE, which also revealed the second fundamental property of the CMB: although the CMB is remarkably isotropic, fluctuations (anisotropies) in temperature do exist. Some of the anisotropies discovered by COBE’s differential microwave radiometer (DMR) were due to our motion relative to the CMB frame and foregrounds, such as emissions from dust in the Milky Way. Once these anisotropies and other backgrounds were removed, fundamental anisotropies on the level of one part in 10⁵ remained. In other words, one patch of the CMB sky differs in temperature from another at the fundamental level by only one 100,000th of a degree.

Those fundamental anisotropies were the seeds of early structure formation; they allow us to figure out the composition and state of the early universe. For instance, the scale of these temperature fluctuations hints at the necessity of dark matter; it is too small to allow ordinary matter time to coalesce into the structures we see today without the help of something like dark matter. The problem is time; ordinary matter becomes charge neutral only at the epoch of recombination, and before that, due to electrostatic forces, matter cannot effectively clump into gravitational wells to begin forming structure. The COBE results showed a need for an electrically neutral form of matter that could jump-start the structure formation process well before recombination.

Mather and Smoot were awarded the Nobel Prize in Physics in 2006 for their measurements of the CMB.

After COBE, we have continued to learn a great deal more about the CMB thanks to the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck satellite missions (among others). Experiments such as BICEP-2 (featured in this issue) are probing cosmic inflation shortly after the big bang using the polarization of the CMB. As a particle theorist, I continue to be amazed by the amount of information about the early universe that can be extracted from the cosmic microwave background.

References
Hubble, E., “A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulas,” Proceedings of the National Academy of Sciences of the United States of America, Volume 15, Issue 3, pp. 168–173 (1929).

Lemaître, Georges,  “The Beginning of the World From the Point of View of Quantum Theory,” Nature 127, 706 (1931).

Alpher, R.,  Bethe, H., and Gamow, G., “The Origin of Chemical Elements,” Phys. Rev. 73 (7),  803–804 (1948). 

Penzias, A. and Wilson, R., “A Measurement of Excess Antenna Temperature at 4080 Mc/s,” Astrophys. J. 142, p. 419–421 (1965).

Dicke, Peebles, Roll, and Wilkinson, “Cosmic Black-Body Radiation,” Astrophys. J. 142, 414–419 (1965).

Reading list for more information
Tegmark, Max, “Doppler Peaks and All That: CMB Anisotropies and What They Can Tell Us,” http://arxiv.org/abs/astro-ph/9511148.

Hu, W., Sugiyama, N., and Silk, J., “The Physics of Microwave Background Anisotropies,” Nature 386, 37–43 (1997).

Wayne Hu of the University of Chicago also has a wonderful introduction to the CMB for beginners: http://background.uchicago.edu/~whu/beginners/introduction.html.

Garret, K. and Duda, G., “Dark Matter: A Primer,” Adv. Astron. 2011; http://arxiv.org/abs/1006.2483.