Cosmic Microwave Background
The cosmic microwave background (CMB) radiation is our baby picture of the universe. It is the earliest light that we can see and reveals the primordial conditions in the early universe. For the past century, we have searched for, discovered, and refined our knowledge of the CMB using ground and space telescopes, but the history of its discovery predates our ventures into space.
The CMB Image
The CMB is light that pervades the universe. We see the light in microwave wavelengths. In the image above, we see a mottled structure. This is a mapping of temperature taken directly by the telescope. However, the difference in temperature between the red spots and blue spots is in the hundreds of thousandths of a degree. The telescope can pick up temperature differences on the order of millionths of degrees. So, to all other unsophisticated instruments, this light appears to be one temperature: 2.7 Kelvin (equivalent to -270°C or -455°F).
The Planck CMB all-sky image, showing a map of the temperature at microwave wavelengths. Red areas are warmer than blue areas, but the difference is only a hundred thousandth of a degree.
These imperceptible temperature differences tell us a lot about the nature of the universe and, ultimately, the large-scale structure we see today.
Unlike the all-sky maps in the infrared, visible, and other wavelengths, all the CMB maps have been processed such that the foreground light from the inside the Milky Way and the local universe has been removed. This reveals the CMB at all points in the sky without the overshadowing band of the Milky Way.
Origin of the CMB
We call the CMB a “baby picture” because its origins are from a time when light began to travel across the universe—a time shortly after the Big Bang. The universe was born 13.8 billion years ago when the Big Bang occurred. The primordial universe of subatomic particles quickly formed into atomic nuclei of hydrogen, helium, and lithium. Electrons and light scattered off these nuclei in this early epoch when we say the universe was opaque—light was scattering off these particles and could not travel across space.
Around 380,000 years after the Big Bang, the expanding universe cooled to about 3,000 Kelvin. At this temperature, the free electrons were able to combine with the hydrogen nuclei (protons) to form neutral hydrogen. At this point, like a fog lifting, light was able to travel across the universe without scattering off all these particles—the universe becomes transparent to light. At this point the CMB light peaked at a wavelength of about 1 micron in the infrared and, as we mentioned above, the gas temperature was about 3,000 Kelvin. The CMB would have glowed orange-red in visible light.
However, since that time, the universe has expanded 1,000 times in size, and the light within has been redshifted toward longer and longer wavelengths. Today, the peak CMB light has redshifted 1,000 times, so it’s now in the microwave with a wavelength of 1 millimeter (1 micron × 1,000 = 1 mm). The corresponding gas temperature over that expansion is also 1,000 times less, so instead of 3,000 Kelvin, it’s now 3 Kelvin, or the more precise 2.7 Kelvin.
While the fog lifted 380,000 years after the Big Bang when electrons combined with protons to form hydrogen, to continue our analogy, we are still subject to an upper cloud deck. The CMB light itself is the limit of what we can see—it prevents us from seeing an earlier time in the universe because light did not travel freely in the universe before that time. The CMB light is our observational limit and an imprint of the universe at that time.
Read more on the Chronology of the Universe.
Seeds of Structure
The tiny differences in temperature we see in the CMB image may be thought of as infinitesimally small fluctuations in the density of the universe at that time, 380,000 years after the Big Bang. These are the seeds of the large-scale structure we see today—the walls, filaments, and superclusters. These seeds will grow, by gravity, into the present structure. How that happened remains a mystery to us, and is surely the subject of a future Nobel Prize.
Cosmological Constraints
Mapping these cosmic fluctuations yields information about the nature of the universe. Measuring these fluctuations tells us about the density and composition of the universe, the nature of the expansion of the universe, and knowing the matter and energy of the universe, we can use Einstein’s Theory of General Relativity to understand the rate of expansion then turn the clock back and deduce the age of the universe, which stands at around 13.8 billion years. The CMB confirms for us the Big Bang Theory of the formation of the universe.
Mapping Efforts
The CMB has been mapped by three main space telescopes: the Cosmic Background Explorer (COBE), the Wilkinson Microwave Anisotropy Probe (WMAP), and the Planck mission.
COBE operated between 1989 and 1993 and returned the first detailed all-sky image of the CMB. WMAP gathered data from 2001 through 2010 and offered a much clearer picture of the CMB. The Planck mission ran from 2009 to 2013 and is the best image to date of the CMB.
Each successive mission increased the quality of the maps and refined our understanding of the universe. COBE’s effective resolution in this map is 10° while the Planck map has 5-arcminute resolution.
A three-panel image of the CMB over the same region of the sky. The top panel is from COBE (1992), the middle panel is the WMAP data (2003), and the bottom map is the Planck image (2013). Each successive mission increased the resolution and our understanding of the universe.
Image Placement
Each CMB all-sky image is placed on a sphere, like all other all-sky images. However, rather than confined to the area around Earth, we place this sphere at the boundary of the observable universe. In our comoving space, this is around 45 billion light years from Earth.
This is not an accurate representation of the CMB light. In reality, the CMB light pervades the universe—it is all around us all the time. Because these are two-dimensional images we are placing in a three-dimensional space, we need to make a choice how we visualize them. We decided to allow the image to represent two concepts: the CMB itself and the limit of our observable universe. But, it’s important to remember this light is everywhere in the universe.
The CMB image wrapped on a sphere at the limit of the observable universe. The Quasars appear in the center of the 45-billion-light-year-radius sphere. Everything inside the CMB sphere is that part of the universe we can see. We do not recommend flying out to this vantage point, but we want to show it to see the extent of the observable universe in the context of the quasars.
Discovery
In 1948, astronomers Ralph Alpher (1921–2007), Hans Bethe (1906–2005), and George Gamow (1904–1968) published their assertion that the gas in the early universe must have been very hot and dense and that this gas should be present throughout today’s universe, albeit much cooler and less dense.
Alpher searched for this cool gas, but it would be another sixteen years before it was discovered, not by astronomers but by two physicists working at Bell Telephone Laboratories in New Jersey. In 1964, Arno Penzias (1933–2024) and Robert Wilson (b. 1936) were trying to communicate with a recently launched communications satellite and could not remove “noise” from their transmissions. This weak hiss was a constant nuisance that was present during the day, the night, and throughout the year. This fact ruled out possibilities such as equipment interference, atmospheric effects, or even bird droppings on the radio telescope built to communicate with their satellite.
The Horn Antenna, located in Holmdel, New Jersey, on the former Bell Labs campus, was the instrument that Arno Penzias and Robert Wilson used to discover the cosmic microwave background light. The bizarre looking horn antenna was designed to listen for satellite communications and radio signals (microwaves are within the broader radio spectrum). The horn is focused into the control room on the right, where the detector was located. Credit: Brian P. Abbott
Penzias and Wilson tried their best to remove this noise but were unsuccessful. In the end, they acknowledged that the faint microwave signal must be real and is not from some defect or artificial interference.
In the meantime, researchers down the road at Princeton University were on Alpher’s trail, investigating this gas from the early universe. They maintained that the hot radiation would have been redshifted into the radio range of the electromagnetic spectrum. Furthermore, astronomers expected this radiation to be thermal, or what astronomers call blackbody radiation. An object is said to be a blackbody when it emits all the radiation it absorbs. In the early universe, with only free electrons and nuclei (protons and neutrons), light scattered off electrons and would have produced a blackbody spectrum.
If the signal detected at Bell Labs corresponded to a blackbody, it would have a temperature of about 3 Kelvin. But the Bell Labs observations could not confirm that the radiation was in fact from a blackbody, and they could not conclude with certainty that this was the radiation left from the Big Bang.
Penzias and Wilson were awarded the Nobel Prize in 1978 for their discovery. The observational and theoretical work of many others led to the investment in missions to see this light in higher resolution in order to better understand the cosmological parameters that describe the nature of the universe. COBE was launched in 1989, and first quantified the “structure” seen in the light. The COBE team received the Nobel Prize in 2006 for their findings. Since then, missions of increasing resolution on the sky offered a more accurate understanding of the origins and characteristics of the universe.