The Cosmic Microwave Background, or CMB, was discovered in 1964. This is the afterglow of the Big Bang and it is as near as scientists can get to observing the event that brought the universe into existence, 13.8 billion years ago. Linking the structures observed in the universe to the features discerned in the CMB remains a key challenge for cosmologists.
“I always think of space-time as being the real substance of space, and the galaxies and the stars just like the foam on the ocean.” George Smoot
The first great breakthrough came from the Cosmic Microwave Background Explorer, known as COBE, a NASA satellite launched in 1989. The detectors on COBE, designed and run by George Smoot, John Mather, and Mike Hauser, were able to find the oldest structures in the visible universe, described by Smoot as “wrinkles in time.” These wrinkles in otherwise uniform space were once dense regions containing the matter that would form stars and galaxies. They correspond to the large-scale galaxy superclusters and great walls seen in the universe today, and add weight to the inflationary model of the early universe proposed by American Alan Guth.
The CMB is a flash of radiation that was released about 380,000 years after the Big Bang, at the time the first atoms formed. The expanding universe had cooled enough for stable ions (positively charged nuclei) of hydrogen and helium to form, and then, after a little more cooling, the ions began to collect electrons to make neutral atoms. The removal of free electrons from space led to the release of photons (particles of radiation).
Those photons are visible now as the CMB. The CMB comes from the whole sky, without exception. It has redshifted (the wavelengths have stretched), and it now has wavelengths of a few millimeters, while the original radiation’s wavelengths would be measured in nanometers (billionths of a meter). One of the key observations of the CMB came in the 1970s, and removed any doubt that it was an echo of the Big Bang. This was the discovery that the thermal spectrum of radiation from the CMB tallied very closely with that of a theoretical black body.
Black bodies do not really exist—they cannot be made and no object observed in the universe functions as black bodies do in theory. However, the CMB is the closest match that has ever been found.
A black body absorbs all radiation that hits it. Nothing is reflected. However, the absorbed radiation adds to the thermal energy of the object, and this is released as radiation. In 1900, German Max Planck, the founding figure of quantum physics, showed that the spectrum of radiation released by a black body is entirely dependent on temperature.
In an everyday example of radiation varying with temperature, an iron bar glows red when first heated. Heating it more makes it orange, and eventually the bar will glow “blue hot.” Metalworkers learn to roughly judge the temperature of iron by its color. The metal is not particularly close to a black body in the theoretical sense, but stars and other astronomical objects are a much closer match to a black body, and so the color, or wavelengths of their emissions, can be compared to the thermal spectrum of a theoretical black body to give a relatively precise temperature.
The temperature of the CMB today is a chilly 2.7 K. The thermal spectrum at that temperature contains no visible light, which is why space looks black to human eyes. However, the spectrum has redshifted (stretched) over time as the universe has expanded. Extrapolating back to the moment the CMB was emitted gives an original temperature of about 3,000 K. The color of radiation at this temperature is orange, so the CMB started out as a flash of orange light that shone out from every point in space.
The early observations of the CMB suggested that it was isotropic, which means that its spectrum is the same everywhere. In cosmology, the terms density, energy, and temperature are somewhat synonymous when discussing the early universe. So the isotropic nature of the CMB suggested that, in those early days, space had a uniform density, or spread of energy. However, this did not tally with the developing theories of the Big Bang, which demanded that matter and energy were not evenly spread through the young universe, but had been concentrated together in places. These denser areas, or anisotropies, were where the stars and galaxies formed. COBE was sent into space to take a close look at the CMB to see if it could find any anisotropies, to find out whether the CMB changed, however slightly, depending on where it looked.
A mission to study the CMB from space had been in the planning stages since the mid-1970s. Construction of COBE began in 1981. It was initially designed to enter polar orbit (its orbit passing over both poles). However, the Challenger disaster of 1986 grounded the shuttle fleet, and the COBE team had to look for another launch system. In 1989, the satellite was launched using a Delta rocket, and it was placed in a sun-synchronous geocentric orbit —orbiting in a way that saw it pass over each place on Earth at the same time of day. This worked just as well as a polar orbit in that it allowed COBE to point away from Earth and scan the entire celestial sphere, strip by strip.
The spacecraft carried three instruments, all protected from the sun’s heat and light by a cone-shaped shield, and chilled to 2 K (colder than space itself) using 100 gallons (650 liters) of liquid helium. George Smoot ran the
Differential Microwave Radiometer (DMR), which mapped the precise wavelengths of the CMB, while John Mather was in charge of FIRAS, the Far-InfraRed Absolute Spectrophotometer, which collected data on the spectra of the CMB. These two experiments were looking for anisotropies. The third detector on COBE had a slightly different goal. The Diffuse Infrared Background Experiment, run by Mike Hauser, found galaxies that were so ancient and far away that they are only visible by their heat radiation (or infrared).
COBE’s instruments created the most accurate map of the CMB to date. However, it was not a simple surveying job. Smoot and Mather were interested in primary anisotropies—that is, the density differences that were present at the time the CMB formed. To find these, they needed to filter out the secondary fluctuations caused by obstacles that lay between COBE and the edge of the universe. Dust clouds and the effects of gravity had interfered with the radiation on its long journey to Earth. The data from the three instruments were used to detect and correct these so-called secondary anisotropies.
“[COBE has made] the greatest discovery of the century, if not of all time.” Stephen Hawking
After 10 months in space, COBE’s helium ran out, which limited the function of the two infrared detectors, but the DMR continued working until 1993. By 1992, the COBE team’s analysis had shown what they were looking for. The CMB, and thus the early universe, was not a uniform blob of energy. Instead it was riddled with tiny but significant fluctuations. The differences were minute, with density variations of about 0.001 percent. However, the pattern was enough to explain why the contents of the universe are clustered together, while the rest of space is made from vast empty voids.
Since COBE, two subsequent missions have added detail to the picture of the CMB. Between 2001 and 2010, NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) mapped the CMB to a higher resolution than COBE. Then, from 2009–2013, the ESA’s Planck Observatory produced the most accurate map to date.
Every wrinkle on the map is the seed from which an entire galaxy formed about 13 billion years ago. However, no known galaxy can be seen forming
in the CMB. The CMB radiation detected today has traveled from near the edge of the observable universe over the course of most of the age of the universe. Astronomers can only see 13.8 billion light-years away, but most of the universe now lies farther away than that. The galaxies forming in the CMB are now far beyond what can be observed, and are receding faster than the speed of light.
After a childhood in Florida and Ohio, Smoot began his career as a particle physicist working at MIT. His interests switched to cosmology and he moved across the country to the Lawrence Berkeley National Laboratory. It was there that Smoot studied the CMB and developed ways of measuring its radiation.
Smoot’s early work involved fitting detectors to high-altitude U2 spyplanes, but in the late 1970s, he became involved in the COBE project to take his detector into space. After his success with COBE, Smoot cowrote Wrinkles in Time with Keay Davidson to explain the discovery. Smoot won the Nobel Prize in 2006, along with John Mather, for his work on COBE. He reportedly gave his prize money to charity. However, three years later, Smoot won an even greater sum when he bagged the $1 million jackpot on the TV game show Are You Smarter Than a 5th Grader?