Scientific Method —

50 years with the Big Bang’s “smoking gun”

Back in 1964, a couple of guys in New Jersey were puzzled by an odd hum.

The microwave horn in New Jersey where Wilson and Penzias first detected the hum left over from the birth of the Universe.

Today, Alcatel-Lucent, the corporate entity that inherited Bell Labs, is celebrating one of the lab's most momentous discoveries: the cosmic microwave background, a remnant of the Big Bang. Since its discovery, the study of the cosmic microwave background's details has continued to pay dividends in our understanding of the Universe and its formation, with the latest results being released just this year.

We're a long way from 1964, where pigeon droppings in New Jersey were an experimental complication.

Seeing the background

The story of the cosmic microwave background's discovery is anything but a sudden eureka-like breakthrough, which makes putting an exact date on it a pointless exercise. Back in 1964, Bell Lab's Arno Penzias and Robert Wilson weren't looking to do astronomy; they were trying to do microwave communications, something directly relevant to Bell Lab's parent company. The problem they had was that no matter where they pointed their equipment, there was a steady hum of noise.

What followed was a long, awkward slog of trying to get rid of it. Scanning the skies showed that the noise wasn't associated with any particular direction, ruling out a single source (the site in New Jersey wasn't too far from New York City, which would have been an obvious candidate). That suggested a hardware source, which is where the pigeons that hung out inside the huge horn of the receiver come into the story. But cleaning up after the pigeons didn't get rid of the hum, nor did chilling down the hardware to get rid of thermal noise. Penzias and Wilson were stumped.

Fortunately, they didn't stay stumped for long. As it turns out, researchers at nearby Princeton University were considering a search for energy left over from the Big Bang. By a few seconds after the Big Bang, the energy density of the Universe had dropped enough so that the first atomic nuclei could form, but it would take hundreds of thousands of years for the Universe to cool enough so that electrons could combine with them to form neutral atoms. That event would have released energy, which would have been redshifted into the microwave end of the spectrum.

Theorists had predicted this, but nobody had figured out a way of searching for it. Thanks to some discussions with colleagues, Penzias and Wilson gradually came to realize what they had found: the background radiation left over from this event. After getting in touch with the Princeton researchers, both teams managed to publish in the same issue of the Astrophysical Journal in 1965. The Nobel Prize followed in 1978.

Inflation and COBE

That's where things stayed for some time. By all appearances when measured from Earth, the microwave background was even—consistent with the large-scale evenness of the Universe. But on smaller scales, the Universe was looking rather lumpy, with giant clusters of galaxies and huge voids. A variety of theoretical ideas were presented to explain this situation, with the leading candidate being inflation. Inflation would allow the early Universe to be small enough to even out, while blowing it up so rapidly that tiny quantum fluctuations would become the huge structures we see in the modern Universe.

Many versions of inflation posited that the modern lumps would have correlates in the cosmic microwave background—tiny fluctuations in the energy of the microwaves. Finding these fluctuations became the mission of the Cosmic Background Explorer (COBE) satellite. Even as it was launched in 1989, scientists were still debating whether the fluctuations would be large enough to be detectable with then-current technology.

By 1992, however, the COBE team was able to announce that it had seen the seeds of the modern Universe in the cosmic microwave background thanks to minuscule fluctuations that differed from the main signal by one part in a thousand. In 2006, team leaders John Mather and George Smoot shared the Nobel Prize for this discovery. In retrospect, COBE can be viewed as the start of the era of precision cosmology.

WMAP, Planck, and BICEP

That was the start—but it's far from the finish. While COBE nailed down the presence of fluctuations in the energy, the power spectrum of the cosmic microwave background—a series of wiggles in the black-body radiation curve of the Universe—contains additional details about the Universe. The precise shape of these wiggles would tell us the age of the Universe, the amount of matter (dark and regular) it contains, the Hubble constant that measures its expansion, and more.

In 2001, NASA launched the Wilkinson Microwave Anisotropy Probe (WMAP) to provide a detailed map of the cosmic microwave background fluctuations and to examine its power spectrum. In 2010, after years of scanning and multiple data releases, WMAP was retired while still operational. Combined with other results that had come in from other sources, WMAP provided the contents of our Universe: 71 percent dark energy, 24 percent dark matter, and just under five percent of ordinary matter—the stuff that makes up everything we see around us and all the stars of the Universe.

Shortly before its retirement, the European Space Agency launched Planck, its cosmic microwave background explorer. Planck has provided a higher-resolution map of the cosmos than WMAP, with increased sensitivity to a number the features of the background. It also was designed to provide a map of the polarization of the microwaves.

That polarization is a key test of inflation. Although our Universe is consistent with what we expect from inflation, that doesn't provide direct evidence of the event. The process of inflation, however, would have created gravity waves that spread out along with space, influencing the polarization of the first light that we can detect, the cosmic microwave background.

Before Planck's data could be fully analyzed, however, the explorer's research team was beaten to the punch by the Background Imaging of Cosmic Extragalactic Polarization (BICEP) telescope at the South Pole. Some scientists have raised questions about the validity of these results, however, which means that replication will likely be key to their acceptance.

That's why researchers aren't yet done looking at the cosmic microwave background. Beyond the BICEP questions, there's also an odd feature, nicknamed the "axis of evil," which has yet to be explained—in fact, we're still debating whether it needs to be explained.

All of which means that the door hasn't shut yet on the information we can glean from the cosmic microwave background. Beyond that, given all that it's provided over the years, it wouldn't be at all surprising if an enterprising scientist doesn't eventually figure out how to extract a bit more from it.

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