“Less than 100 years ago, we didn’t know that the universe was bigger than the Milky Way,” Lawrence says. “And here we are talking about what happened 13.8 billion years ago! What happened 10 to the negative 35 second after the Big Bang! Ionized universe! Temperature 3,000 degrees Kelvin! It’s just so cool!”
The field of cosmology has advanced spectacularly since the middle of the last century. Only since 1964, when radio astronomers Arno Penzias and Robert Wilson famously noticed a bothersome hiss in their Bell Labs antenna, has the Big Bang theory found acceptance among the majority of scientists. Badly named (by British astronomer Fred Hoyle who didn’t believe in it), the theory has nothing to do with an explosion. “ ‘Big Bang’ just says the universe was hot and dense a long time ago,” says Lawrence. The theory envisions the early universe as a glowing fog with a temperature of billions of degrees—so roiling hot that atoms could not form and light could not escape for about 370,000 years. That’s when the soup cooled to around 3,000 degrees Kelvin (5,000 Fahrenheit), simple hydrogen atoms formed, and photons were freed.
In the late 1940s, astrophysicists had theorized that the first light waves should still be around, cooled to microwave radiation with a temperature of about five degrees Kelvin (just above absolute zero, where all heat energy vanishes). That cosmic microwave radiation was the hiss first heard by Penzias and Wilson. They found that the signal had the same strength in all directions, measured its temperature at 2.7 degrees Kelvin (-454 Fahrenheit), and won the 1978 Nobel Prize in physics for the discovery.
Theorists further predicted that the radiation would preserve the signature spectrum of the universe’s first light. Twenty years after Penzias and Wilson found the cosmic microwave background, the Cosmic Background Explorer, or COBE, satellite confirmed its telltale spectrum.
“The CMB is one of the best examples of the powerful interplay between experimentation and theory and just how productive and important they are to each other,” Lawrence says. For one thing, the uniformity Penzias and Wilson had seen in the cosmic microwave background temperature presented a puzzle. The temperature is the same, even in regions that are billions of light-years apart. “The fact that these regions have the same temperature means that they were sort of cooked together,” says Charles Bennett, a physics and astronomy professor at Johns Hopkins University in Baltimore. The distant regions, cosmologists believe, must at some point have been in contact, yet that seems impossible since the universe has not existed long enough for light to travel all the way from some regions to others. Solving this puzzle was one of the inspirations for the theory of inflation.
Theory, in turn, makes predictions about the universe that experimentalists can test, mainly through measurements of the cosmic microwaves. “We should be very grateful we have the CMB. It’s our fossil,” says Bennett. Bennett’s first job after graduating from MIT in 1984 was working with an instrument on the Cosmic Background Explorer satellite; at NASA’s Goddard Space Flight Center in Maryland, he was on the team searching the cosmic microwave background for small temperature variations, the results of random quantum fluctuations in the primordial universe. What are those? In quantum field theory, matter and energy at sub-atomic scales constantly pop in and out of existence, creating slight vibrations or perturbations. “Because all the evidence suggests that quantum mechanics is a good description of nature, whatever was going on [in the early universe], you would expect to see these tiny fluctuations in density at 10-to-the-negative-35 seconds after the Big Bang,” says Charles Lawrence.
“That’s just how physics works.”
Inflation amplified the most infinitesimal fluctuations imaginable until gravity took over and consolidated the tiny differences in density into planets, stars, galaxies, and the largest structures we know. As Charles Bennett puts it, “nature turns up the contrast over time.” But signs of those fluctuations should be visible in the first light waves—the CMB—as areas of temperature variation. Inflation predicts that some spots in the sky will have a temperature slightly higher than average (by millionths of a degree); other spots, a slightly lower temperature.
On the scale that COBE was observing, the temperature variations were not easy to see. Project scientist John Mather recalls the first color-coded image teased out. The team spent months checking and cross-checking the results before announcing that COBE had indeed spotted the primitive blueprint of all matter in the universe.
“I said, ‘Here is a discovery that will become the basis of a new subject,’ ” recalls Mather, who shared the 2006 Nobel Prize in physics for the COBE results. “People said we were more worried that we got something wrong than people who disagreed with us. But we had to be absolutely sure.”