The Cosmic Microwave Background radiation, or CMB for short, is a faint glow of light that fills the universe, falling on Earth from every direction with nearly uniform intensity. It is the residual heat of creation--the afterglow of the big bang--streaming through space these last 14 billion years like the heat from a sun-warmed rock, reradiated at night. Since the early twentieth century, two concepts have transformed the way astronomers think about observing the universe. The first is that it is fantastically large; the portion of the universe visible today is a sphere nearly 15 billion light-years in radius, and that, we believe, is just the tip of the iceberg. The second is that light travels at a fixed speed. A simple consequence of these ideas is that as you look at more and more distant objects, you're seeing farther and farther back in time--sometimes very far back indeed. When you see Jupiter shining in the night sky, for example, you're looking about an hour back in time, whereas the light from distant galaxies captured by telescopes today was emitted millions of years ago.

The CMB is the oldest light we can see--the farthest back both in time and space that we can look. This light set out on its journey more than 14 billion years ago, long before the Earth or even our galaxy existed. It is a relic of the universe's infancy, a time when it was not the cold dark place it is now, but was instead a firestorm of radiation and elementary particles. The familiar objects that surround us today--stars, planets, galaxies and the like--eventually coalesced from these particles as the universe expanded and cooled.

This residual radiation is critical to the study of cosmology because it bears on it the fossil imprint of those particles, a pattern of miniscule intensity variations from which we can decipher the vital statistics of the universe, like identifying a suspect from his fingerprint.

When this cosmic background light was released billions of years ago, it was as hot and bright as the surface of a star. The expansion of the universe, however, has stretched space by a factor of a thousand since then. The wavelength of the light has stretched with it into the microwave part of the electromagnetic spectrum, and the CMB has cooled to its present-day temperature, something the glorified thermometers known as radio telescopes register at about 2.73 degrees above absolute zero.

Our present understanding of the beginning of the universe is based upon the remarkably successful theory of the Hot Big Bang. We believe that our universe began about 15 billion years ago as a hot, dense, nearly uniform sea of radiation a minute fraction of its present size (formally an infinitesimal singularity). If inflation occurred in the first fraction of a second, the universe became matter dominated while expanding exponentially and then returned to radiation domination by the reheating caused by the decay of the inflaton. Baryonic matter formed within the first second, and the nucleosynthesis of the lightest elements took only a few minutes as the universe expanded and cooled. The baryons were in the form of plasma until about 300,000 years after the Big Bang, when the universe had cooled to a temperature near 3000 K, sufficiently cool for protons to capture free electrons and form atomic hydrogen; this process is referred to as recombination. The recombination epoch occurred at a redshift of 1100, meaning that the universe has grown over a thousand times larger since then. The ionization energy of a hydrogen atom is 13.6 eV, but recombination did not occur until the universe had cooled to a characteristic temperature (kT) of 0.3 eV. This delay had several causes. The high entropy of the universe made the rate of electron capture only marginally faster than the rate of photodissociation. Moreover, each electron captured directly into the ground state emits a photon capable of ionizing another newly formed atom, so it was through recombination into excited states and the cooling of the universe to temperatures below the ionization energy of hydrogen that neutral matter finally condensed out of the plasma. Until recombination, the universe was opaque to electromagnetic radiation due to scattering of the photons by free electrons. As recombination occurred, the density of free electrons diminished greatly, leading to the decoupling of matter and radiation as the universe became transparent to light.

The Cosmic Background Radiation (CBR) released during this era of decoupling has a mean free path long enough to travel almost unperturbed until the present day, where we observe it peaked in the microwave region of the spectrum as the Cosmic Microwave Background (CMB). We see this radiation today coming from the surface of last scattering (which is really a spherical shell of finite thickness) at a distance of nearly 15 billion light years. This Cosmic Background Radiation was predicted by the Hot Big Bang theory and discovered at an antenna temperature of 3K in 1964. The number density of photons in the universe at a redshift z is given by:

where (1 + z) is the factor by which the linear scale of the universe has expanded since then. The radiation temperature of the universe is given by T = T0(1 + z) so it is easy to see how the conditions in the early universe at high redshifts were hot and dense.

The CBR is our best probe into the conditions of the early universe. Theories of the formation of large-scale structure predict the existence of slight inhomogeneities in the distribution of matter in the early universe which underwent gravitational collapse to form galaxies, galaxy clusters, and superclusters. These density inhomogeneities lead to temperature anisotropies in the CBR due to a combination of intrinsic temperature fluctuations and gravitational blue/redshifting of the photons leaving under/overdense regions. The DMR (Differential Microwave Radiometer) instrument of the Cosmic Background Explorer (COBE) satellite discovered primordial temperature fluctuations on angular scales larger than 7° of order T / T = 10-5. Subsequent observations of the CMB have revealed temperature anisotropies on smaller angular scales which correspond to the physical scale of observed structures such as galaxies and clusters of galaxies.

There were three main processes by which this radiation interacted with matter in the first few hundred thousand years: Compton scattering, double Compton scattering, and thermal bremsstrahlung. The simplest interaction of matter and radiation is Compton scattering of a single photon off a free electron, gamma + e- —› gamma + e-. The photon will transfer momentum and energy to the electron if it has significant energy in the electron's rest frame. However, the scattering will be well approximated by Thomson scattering if the photon's energy in the rest frame of the electron is significantly less than the rest mass, h ‹‹ me c2. When the electron is relativistic, the photon is blueshifted by roughly a factor in energy when viewed from the electron rest frame, is then emitted at almost the same energy in the electron rest frame, and is blueshifted by another factor of when retransformed to the observer's frame. Thus, energetic electrons can efficiently transfer energy to the photon background of the universe. This process is referred to as Inverse Compton scattering. The combination of cases where the photon gives energy to the electron and vice versa allows Compton scattering to generate thermal equilibrium (which is impossible in the Thomson limit of elastic scattering). Compton scattering conserves the number of photons. There exists a similar process, double Compton scattering, which produces (or absorbs) photons, e- + gamma ‹—› e- + gamma + gamma.

Another electromagnetic interaction which occurs in the plasma of the early universe is Coulomb scattering. Coulomb scattering establishes and maintains thermal equilibrium among the baryons of the photon-baryon fluid without affecting the photons. However, when electrons encounter ions they experience an acceleration and therefore emit electromagnetic radiation. This is called thermal bremsstrahlung or free-free emission. For an ion X, we have e- + X ‹—› e- + X + gamma. The interaction can occur in reverse because of the ability of the charged particles to absorb incoming photons; this is called free-free absorption. Each charged particle emits radiation, but the acceleration is proportional to the mass, so we can usually view the electron as being accelerated in the fixed Coulomb field of the much heavier ion. Bremsstrahlung is dominated by electric-dipole radiation and can also produce and absorb photons.

The net effect is that Compton scattering is dominant for temperatures above 90 eV whereas bremsstrahlung is the primary process between 90 eV and 1 eV. At temperatures above 1 keV, double Compton is more efficient than bremsstrahlung. All three processes occur faster than the expansion of the universe and therefore have an impact until decoupling. A static solution for Compton scattering is the Bose-Einstein distribution,

where µ is a dimensionless chemical potential. At high optical depths, Compton scattering can exchange enough energy to bring the photons to this Bose-Einstein equilibrium distribution. A Planckian spectrum corresponds to zero chemical potential, which will occur only when the number of photons and total energy are in the same proportion as they would be for a blackbody. Thus, unless the photon number starts out exactly right in comparison to the total energy in radiation in the universe, Compton scattering will only produce a Bose-Einstein distribution and not a blackbody spectrum. It is important to note, however, that Compton scattering will preserve a Planck distribution,

All three interactions will preserve a thermal spectrum if one is achieved at any point. It has long been known that the expansion of the universe serves to decrease the temperature of a blackbody spectrum,

but keeps it thermal. This occurs because both the frequency and temperature decrease as (1 + z) leaving h / kT unchanged during expansion. Although Compton scattering alone cannot produce a Planck distribution, such a distribution will remain unaffected by electromagnetic interactions or the universal expansion once it is achieved. A non-zero chemical potential will be reduced to zero by double Compton scattering and, later, bremsstrahlung which will create and absorb photons until the number density matches the energy and a thermal distribution of zero chemical potential is achieved. This results in the thermalization of the CBR at redshifts much greater than that of recombination.

Thermalization, of course, should only be able to create an equilibrium temperature over regions that are in causal contact. The causal horizon at the time of last scattering was relatively small, corresponding to a scale today of about 200 Mpc, or a region of angular extent of one degree on the sky. However, observations of the CMB show that it has an isotropic temperature on the sky to the level of one part in one hundred thousand! This is the origin of the Horizon Problem, which is that there is no physical mechanism expected in the early universe which can produce thermodynamic equilibrium on superhorizon scales. The inflationary universe paradigm solves the Horizon Problem by postulating that the universe underwent a brief phase of exponential expansion during the first second after the Big Bang, during which our entire visible Universe expanded out of a region small enough to have already achieved thermal equilibrium.

The CBR is the most perfect blackbody ever seen, according to the FIRAS (Far InfraRed Absolute Spectrometer) instrument of COBE, which measured a temperature of T0 = 2.726 ± 0.010 K. The theoretical prediction that the CBR will have a blackbody spectrum appears to be confirmed by the FIRAS observation. But this is not the end of the story. FIRAS only observed the peak of the blackbody. Other experiments have mapped out the Rayleigh-Jeans part of the spectrum at low frequency. Most are consistent with a 2.73 K blackbody, but some are not. It is in the low-frequency limit that the greatest spectral distortions might occur because a Bose-Einstein distribution differs from a Planck distribution there. However, double Compton and bremsstrahlung are most effective at low frequencies so strong deviations from a blackbody spectrum are not generally expected.

Spectral distortions in the Wien tail of the spectrum are quite difficult to detect due to the foreground signal from interstellar dust at those high frequencies. For example, broad emission lines from electron capture at recombination are predicted in the Wien tail but cannot be distinguished due to foreground contamination. However, because the energy generated by star formation and active galactic nuclei is absorbed by interstellar dust in all galaxies and then re-radiated in the far-infrared, we expect to see an isotropic Far-Infrared Background (FIRB) which dominates the CMB at frequencies above a few hundred GHz. This FIRB has now been detected in FIRAS data and in data from the COBE DIRBE instrument.

Although Compton, double Compton, and bremsstrahlung interactions occur frequently until decoupling, the complex interplay between them required to thermalize the CBR spectrum is ineffective at redshifts below 107. This means that any process after that time which adds a significant portion of energy to the universe will lead to a spectral distortion today. Neutrino decays during this epoch should lead to a Bose-Einstein rather than a Planck distribution, and this allows the FIRAS observations to set constraints on the decay of neutrinos and other particles in the early universe. The apparent impossibility of thermalizing radiation at low redshift makes the blackbody nature of the CBR strong evidence that it did originate in the early universe and as a result serves to support the Big Bang theory.

The process of Compton scattering can cause spectral distortions if it is too late for double Compton and bremsstrahlung to be effective. In general, low-frequency photons will be shifted to higher frequencies, thereby decreasing the number of photons in the Rayleigh-Jeans region and enhancing the Wien tail. This is referred to as a Compton-y distortion and it is described by the parameter:

The apparent temperature drop in the long-wavelength limit is:

The most important example of this is Compton scattering of photons off hot electrons in galaxy clusters, called the Sunyaev-Zel'dovich (SZ) effect. The electrons transfer energy to the photons, and the spectral distortion results from the sum of all of the scatterings off electrons in thermal motion, each of which has a Doppler shift. The SZ effect from clusters can yield a distortion of y 10-5 - 10-3 and these distortions have been observed in several rich clusters of galaxies. The FIRAS observations place a constraint on any full-sky Comptonization by limiting the average y-distortion to y ‹ 2.5 × 10-5. The integrated y-distortion predicted from the SZ effect of galaxy clusters and large-scale structure is over a factor of ten lower than this observational constraint but that from "cocoons" of radio galaxies is predicted to be of the same order. A kinematic SZ effect is caused by the bulk velocity of the cluster; this is a small effect which is very difficult to detect for individual clusters but will likely be measured statistically by the Planck satellite.

Bell Labs built a giant antenna in Holmdel, New Jersey, in 1960. It was part of a very early satellite transmission system called Echo. By collecting and amplifying weak radio signals bounced off large metallic balloons high in the atmosphere, it could send signals across long distances. Within a few years, the Telstar satellite was launched. It had built-in transponders and made the Echo system obsolete.

Meanwhile, two employees of Bell Labs had had their eye on the antenna. Arno Penzias (b. 1933), a German-born radio astronomer, joined Bell Labs in 1958. He had done his PhD on using masers (microwave amplification by stimulated emission of radiation) to amplify and measure radio signals from the spaces between galaxies. He knew the Holmdel antenna would also make a great radio telescope and was dying to use it to continue his observations, but he pursued other research while the antenna was booked for commercial use. Another radio astronomer came to Bell Labs in 1962 with the same idea. Robert Wilson (b. 1936) had also used masers to amplify weak signals in mapping radio signals from the Milky Way. The launch of Telstar in 1962 gave both researchers what they wanted: the Holmdel antenna was freed up for pure research.

When they began to use it as a telescope they found there was a background "noise" (like static in a radio). This annoyance was a uniform signal in the microwave range, seeming to come from all directions. Everyone assumed it came from the telescope itself, which was not unusual. It hadn't interfered with the Echo system but Penzias and Wilson had to get rid of it to make the observations they planned. They checked everything to rule out the source of the excess radiation. They pointed the antenna right at New York City -- it wasn't urban interference. It wasn't radiation from our galaxy or extraterrestrial radio sources. It wasn't even the pigeons living in the big, horn-shaped antenna. Penzias and Wilson kicked them out and swept out all their droppings. The source remained the same through four seasons, so it couldn't have come from the solar system or even from a 1962 above-ground nuclear test, because in a year that fallout would have shown a decrease. They had to conclude it was not the machine and it was not random noise causing the radiation.

Penzias and Wilson began looking for theoretical explanations. Around the same time, Robert Dicke (1916Ð1997) at nearby Princeton University had been pursuing theories about the big bang. He had elaborated on existing theory to suggest that if there had been a big bang, the residue of the explosion should by now take the form of a low-level background radiation throughout the universe. Dicke was looking for evidence of this theory when Penzias and Wilson got in touch with his lab. He shared his theoretical work with them, even as he resignedly said to his fellow-researchers, "We've been scooped."

Ironically, Robert Wilson had been trained in steady state theory (which suggested the universe was without beginning or end, unlike big bang theory), and he felt uncomfortable with the big bang explanation of their radio noise. When he and Penzias jointly published their research with Dicke, the Bell Lab researchers stuck to "just the facts" -- simply reporting their recorded observations.

It is ironic, too, that many researchers -- both theoretical and experimental -- had stumbled on this phenomenon before, but either discounted it or never put it all together. This was partly because, as Steven Weinberg wrote, "in the 1950s, the study of the early universe was widely regarded as not the sort of thing to which a respectable scientist would devote his time." Since Penzias, Wilson, and Dicke's work, all that has changed. The measurement of cosmic background radiation (as the Holmdel telescope's noise is now called), combined with Edwin Hubble's much earlier finding that the galaxies are rushing away, makes a strong case for the big bang. By the mid 1970s, astronomers called it "the standard model." Arno Penzias and Robert Wilson received the Nobel Prize in physics in 1978.

In the 1950s, there were two theories to the origin of the universe. The first was called the Steady State Theory. It had been put forward by Hermann Bondi, Thomas Gold and Fred Hoyle and held that the universe was homogeneous in space and time and had remained like that forever -- essentially, that the universe existed in "a steady state."

The rival, more controversial theory sought to incorporate the expansion of the universe into its framework. Edwin Hubble had shown in 1929 that galaxies are moving away from one another at remarkable speeds, implying that the space between galaxies is constantly expanding. A few physicists led by George Gamow had taken this notion and argued that the separation between galaxies must have been smaller in the past. If one stretched the idea to the limit, it meant that the universe had been infinitely dense at one point sufficiently back in time. Using the laws of physics, Gamow and his colleagues were able to show that the point -- which was also infinitely hot -- corresponded to the moment of creation. Everything in the universe had emerged from this incredibly dense and hot state in a cataclysmic event astronomers call "the Big Bang."

The conflict between the theories was resolved by Penzias and Wilson in 1965. They had been using an ultra-sensitive microwave receiving system to study radio emissions from the Milky Way when they found an unexpected background of radio noise with no obvious explanation. It came from all directions and, after repeated checks, it appeared to emanate from outside the Galaxy.

Penzias and Wilson consulted with Princeton physicist Robert H. Dicke, who had theorized that if the universe was created according to the Big Bang theory, a background radiation at 3-degree Kelvin would exist throughout the universe. Dicke visited Bell Labs and confirmed that the mysterious radio signal Penzias and Wilson detected was, indeed, the cosmic radiation that had survived from the very early days of the universe. It was proof of the Big Bang.

As John Huchra, a professor of astronomy at Harvard University and a leading observational cosmologist, put it, "The discovery of the 2.7 degree background was the clincher for the current cosmological model, the hot Big Bang. It opened a window on the Universe at a very, very early time, enabling astronomers and physicists to see the initial conditions from which the beauty of the present-day cosmos sprang." The cosmic microwave background hails from the earliest observable event in the history of the universe, some 300,000 years after its birth. Although the original temperature of the cosmic microwave background was much higher, the expansion of the universe has cooled it to its present value of 2.7 degrees Kelvin.

More than three decades after Penzias and Wilson's discovery, the significance of their finding remains as great. It provided a new tool for exploring the early universe.

A few years ago, NASA sent the Cosmic Microwave Background Explorer (COBE) satellite into orbit to investigate the cosmic microwave background in great detail. The principal scientist of the COBE mission, George Smoot, said, "There is no doubt that Penzias and Wilson's discovery of the cosmic background radiation marked a turning point in cosmology."

Tony Tyson at Bell Labs concurred, saying it was one of the greatest breakthroughs in our understanding of the universe's origin. "Its precise black-body spectrum and uniformity over the sky have ruled out many theories of the evolution of the Universe," he noted. Experiments to analyze the small irregularities in the cosmic background radiation are under way today, in the effort to increase our understanding of the early universe. "This faint microwave radiation continues to be a wellspring of cosmological discovery," he said.

John Bahcall, a leading astrophysicist and professor of natural sciences at the Princeton Institute for Advanced Study, said, "The discovery of the cosmic microwave background radiation changed forever the nature of cosmology, from a subject that had many elements in common with theology to a fantastically exciting empirical study of the origins and evolution of the things that populate the physical universe." He called it the most important achievement in astronomy since Hubble's discovery of the expansion of the universe.