Cosmic Microwave Background Discovery (1964): Big Bang Echo

The discovery of the cosmic microwave background (CMB) in 1964 revealed a faint glow of radiation that fills the entire universe, left over from its hot, dense beginning. Arno Penzias and Robert Wilson found it almost by accident with a radio antenna at Bell Telephone Laboratories in Holmdel, New Jersey, and their results were published in 1965. The discovery transformed cosmology by providing powerful evidence for the Big Bang theory.

Background: two theories of the universe

In 1929 Edwin Hubble showed that distant galaxies are moving away from us, the expanding universe. Building on Einstein’s general relativity, the Belgian priest and physicist Georges Lemaître had already proposed that the universe began from a compact “primeval atom”. In the 1940s George Gamow and his colleagues Ralph Alpher and Robert Herman developed the idea of a hot early universe. In 1948 Alpher and Herman predicted that radiation from that hot phase should still exist, cooled to a temperature of about 5 kelvin (5 degrees above absolute zero).

The prediction was largely forgotten. Many astronomers instead favoured the steady-state theory proposed in 1948 by Fred Hoyle, Hermann Bondi and Thomas Gold, which held that the universe had no beginning and that new matter was continuously created as it expanded. Hoyle himself coined the name “Big Bang” in a BBC radio broadcast in 1949. Without a decisive observation, the debate remained open.

The antenna on Crawford Hill

Bell Labs had built a horn-shaped antenna on Crawford Hill in Holmdel in 1959 to receive signals bounced off the Echo balloon satellites. About 15 metres (50 feet) long with a square opening around 6 metres (20 feet) across, it was designed to pick up very weak signals while rejecting interference from the ground. When the company moved its satellite work elsewhere, the antenna became available for radio astronomy.

Arno Penzias, born in Munich in 1933 to a Jewish family that fled Nazi Germany, and Robert Wilson, born in Houston, Texas, in 1936, joined Bell Labs in the early 1960s. They planned to use the antenna to measure faint radio emission from our own galaxy. To do so, they first needed to understand every source of noise in their equipment, cooling their receiver with liquid helium so that its own electronic noise would be tiny.

An unexplained hiss

In 1964, observing at a wavelength of 7.35 centimetres (a frequency of 4,080 megahertz), they found a persistent excess of noise equivalent to a temperature of about 3.5 kelvin. It was the same in whichever direction they pointed the antenna, by day and night, and did not change with the seasons. That ruled out the Sun, the Milky Way, nearby New York City and the Earth’s atmosphere as obvious causes.

They checked every component. A pair of pigeons had been nesting inside the horn and had coated part of it with what Penzias politely called “a white dielectric material”. The scientists removed the birds, cleaned the antenna and taped over joints in the metal, but the noise barely changed. They were left with a signal that seemed to come from the universe itself, with no explanation.

The Princeton connection

The answer was only about 50 kilometres away. At Princeton University, Robert Dicke had independently concluded that a hot early universe should leave a background of microwave radiation. His young colleague James Peebles had calculated its likely properties, and two other team members, Peter Roll and David Wilkinson, were building a small antenna to look for it.

Penzias heard about Peebles’ work through the radio astronomer Bernard Burke and telephoned Dicke early in 1965. After the call, according to a widely repeated account, Dicke told his colleagues that they had been scooped. The Princeton group visited Holmdel, saw the measurements and agreed that the Bell Labs scientists had found the radiation they were seeking.

The two teams published companion papers in the Astrophysical Journal Letters in July 1965. Penzias and Wilson’s short report, modestly titled “A Measurement of Excess Antenna Temperature at 4080 Mc/s”, described the observation, while the Princeton paper explained it as cosmic black-body radiation from the Big Bang. The Princeton team soon confirmed the signal at a different wavelength.

Looking back, there had been earlier hints. In 1941 the Canadian astronomer Andrew McKellar noticed that cyanogen molecules in interstellar space appeared to be slightly excited, as if bathed in radiation at a temperature of about 2 to 3 kelvin. At the time nobody connected this to cosmology. Some radio engineers had also recorded small, unexplained excess noise in their equipment without recognising its origin. What made the 1964 measurement decisive was its care and the fact that a theoretical explanation was ready at the same moment.

Key people

• Arno Penzias (1933-2024) and Robert Wilson (born 1936): the Bell Labs radio astronomers who made the measurement.
• Robert Dicke (1916-1997): Princeton physicist who predicted the radiation independently and interpreted the result.
• James Peebles (born 1935): theorist who calculated its properties and later shared the 2019 Nobel Prize in Physics for work in cosmology.
• Ralph Alpher (1921-2007) and Robert Herman (1914-1997): physicists who had predicted the radiation in 1948.

Consequences

The CMB was very hard to explain in a steady-state universe, and most astronomers soon accepted the Big Bang as the best description of cosmic history. In 1978 Penzias and Wilson shared the Nobel Prize in Physics for the discovery.

Later measurements refined the picture. NASA’s COBE satellite, launched in 1989, showed that the radiation has an almost perfect black-body spectrum at a temperature of about 2.725 kelvin, and detected tiny variations of about one part in 100,000. These ripples are the seeds of later galaxies and clusters. John Mather and George Smoot received the 2006 Nobel Prize for this work. The WMAP (2001) and Planck (2009) missions mapped the variations in fine detail and helped fix the age of the universe at about 13.8 billion years.

Myths and facts

Myth: the discovery was pure luck. Penzias and Wilson were not looking for the CMB, but they found it because they were exceptionally careful about measuring and eliminating noise.

Myth: the radiation comes from a single explosion at one point in space. The CMB comes from every direction because the early universe was hot everywhere; it is light released when the whole universe became transparent about 380,000 years after the Big Bang.

Fact: everyone has seen it. A small fraction of the “snow” on an untuned analogue television screen, often put at around 1 per cent, was caused by the cosmic microwave background.

Timeline

• 1948: Alpher and Herman predict a cosmic background radiation of about 5 kelvin.
• 1959: Bell Labs builds the horn antenna on Crawford Hill, Holmdel.
• 1964: Penzias and Wilson detect an unexplained, uniform excess noise of about 3.5 kelvin.
• 1965, early: Penzias contacts Dicke; the Princeton team visits Holmdel.
• 1965, July: Companion papers appear in the Astrophysical Journal Letters.
• 1978: Penzias and Wilson share the Nobel Prize in Physics.
• 1989, November: COBE is launched and later confirms the black-body spectrum.
• 2006: Mather and Smoot receive the Nobel Prize for the COBE results.

Why it matters

The cosmic microwave background is the oldest light we can observe, a snapshot of the universe in its infancy. Its discovery settled the main debate of mid-20th-century cosmology in favour of the Big Bang and opened a field of precision measurement that today tells us the age, composition and shape of the universe.