The Large Horn Antenna and the Discovery of Cosmic Microwave Background Radiation

As part of the APS historic sites initiative, on December 9, 2008, APS Vice-President Curtis Callan presented a plaque to Bell Labs to commemorate the discovery of the Cosmic Microwave Background Radiation (CMB) that provided evidence for the Big Bang. Bell Labs radio astronomers Arno Penzias and Robert Wilson were using a large horn antenna in 1964 and 1965 to map signals from the Milky Way, when they serendipitously discovered the CMB. As written in the citation, "This unexpected discovery, offering strong evidence that the universe began with the Big Bang, ushered in experimental cosmology." Penzias and Wilson shared the Nobel Prize in Physics in 1978 in honor of their findings.

The CMB is "noise" leftover from the creation of the Universe. The microwave radiation is only 3 degrees above Absolute Zero or -270 degrees C,1 and is uniformly perceptible from all directions. Its presence demonstrates that that our universe began in an extremely hot and violent explosion, called the Big Bang, 13.7 billion years ago.

In 1960, Bell Labs built a 20-foot horn-shaped antenna in Holmdel, NJ to be used with an early satellite system called Echo. The intention was to collect and amplify radio signals to send them across long distances, but within a few years, another satellite was launched and Echo became obsolete.2



With the antenna no longer tied to commercial applications, it was now free for research. Penzias and Wilson jumped at the chance to use it to analyze radio signals from the spaces between galaxies.3 But when they began to employ it, they encountered a persistent "noise" of microwaves that came from every direction. If they were to conduct experiments with the antenna, they would have to find a way to remove the static.

Penzias and Wilson tested everything they could think of to rule out the source of the radiation racket. They knew it wasn’t radiation from the Milky Way or extraterrestrial radio sources. They pointed the antenna towards New York City to rule out "urban interference", and did analysis to dismiss possible military testing from their list.4

Then they found droppings of pigeons nesting in the antenna. They cleaned out the mess and tried removing the birds and discouraging them from roosting, but they kept flying back. "To get rid of them, we finally found the most humane thing was to get a shot gun…and at very close range [we] just killed them instantly. It’s not something I’m happy about, but that seemed like the only way out of our dilemma," said Penzias.5 "And so the pigeons left with a smaller bang, but the noise remained, coming from every direction."6

At the same time, the two astronomers learned that Princeton University physicist Robert Dicke had predicted that if the Big Bang had occurred, there would be low level radiation found throughout the universe. Dicke was about to design an experiment to test this hypothesis when he was contacted by Penzias. Upon hearing of Penzias’ and Wilson’s discovery, Dicke turned to his laboratory colleagues and said "well boys, we’ve been scooped."7

Although both groups published their results in Astrophysical Journal Letters, only Penzias and Wilson received the Nobel Prize for the discovery of the CMB.

The horn antenna was designated a National Historic Landmark in 1990. Its significance in fostering a new appreciation for the field of cosmology and a better understanding of our origins can be summed up by the following: "Scientists have labeled the discovery [of the CMB] the greatest scientific discovery of the 20th century."8

©2009, Alaina G. Levine

1 Hu, Wayne, "An Introduction to the Cosmic Microwave Background", Lecture given at the Institute for Advanced Study, October 25, 1996.

2 "A Science Odyssey: People and Discoveries: Penzias and Wilson discover cosmic microwave radiation 1965," PBS.

3 "This Month in Physics History, June 1963: Discovery of the Cosmic Microwave Background," APS News, July 2002.

4 Schoenstein, Ralph, "The Big Bang's Echo," All Things Considered, NPR, May 17, 2005.

5 Ibid.

6 Ibid.

7 Ibid.

8 Ibid.