The cosmic microwave background, explained

The cosmic microwave background is the faint residual light from the very beginnings of the universe. It’s often referred to as the afterglow of the Big Bang. 

At over 13 billion years old, it is the oldest light in the universe—which makes it a tantalizing object of study for scientists seeking clues about our origins.

What is the cosmic microwave background?

The cosmic microwave background is the faint residual light from the universe's very beginnings. It’s often referred to as the afterglow of the Big Bang. 

At over 13 billion years old, the cosmic microwave background, or CMB, is the oldest light in the universe, which makes it a tantalizing object of study for scientists seeking clues about our origins.

“It’s like having a time machine to go back to the first moments of our universe,” said renowned UChicago and Argonne National Laboratory cosmologist John Carlstrom, who has led several experiments to map the cosmic microwave background.

Where does the cosmic microwave background come from?

The very early universe existed in an unimaginably hot and dense state, with temperatures over a trillion degrees, and densities higher than those inside an atomic nucleus. 

This hot, dense soup initially contained only the most elementary particles, like electrons and neutrinos. All the while, though, the universe was expanding and cooling, and by the time it was one second old, this expanding soup already contained protons and neutrons—though it was still too hot for atoms to form. 

"You can imagine the universe at that time as a glowing fog that you couldn't see through," said Fermilab physicist Don Lincoln.

After about 380,000 years, the universe had cooled down to "only" about 5,400 degrees Fahrenheit (3,000 degrees Celsius).  Though still very hot by human standards, this was cool enough that protons and electrons bound together to form atoms. Atoms interfere less with light than loose charged particles (like protons and electrons), so the light now had a clear path to travel without being scattered. 

This light, from the earliest moments of the universe, has persisted largely unperturbed over the course of our cosmic history. It comprises the cosmic microwave background.

"So by studying and looking at the universe in those wavelengths, we can look at the early universe—essentially, capture a baby picture,” explained Clarence Chang, a physicist with the University of Chicago and Argonne National Laboratory.

As space expanded over time, the universe cooled down, and matter began to coalesce—eventually evolving into the stars, the sun, the Earth, and us.

What does it have to do with microwaves?

The cosmic microwave background is so named because it was first discovered by scientists measuring light in the microwave spectrum—the same kind of light your kitchen microwave uses to cook food!

Later tests showed the cosmic microwave background actually contains light across many parts of the electromagnetic spectrum, not just microwaves. 

When the background light we observe began its journey to us over 13 billion years ago it had a spectrum similar to sunlight, but because the light is stretched out with the expansion of the universe, it has shifted to much longer wavelengths.

Can you see the cosmic microwave background with the naked eye?

You cannot see the cosmic microwave background with the human eye, just like you can’t see the radiation your microwave oven produces. 

"[Even] typical camera technology, such as what’s used in our phones—even pushed to its extreme—still doesn’t see these photons," Chang said. "So we have to make a new technology to do it." 

However, some people might remember the static “snow” on old analog television sets if they weren’t picking up a channel clearly. A little of this static was actually the TV picking up the cosmic microwave background.

What does the cosmic microwave background tell us about the universe?

Scientists are using measurements of the cosmic microwave background to explore big questions, including: what is the origin of our universe? How did all the structures we see today form? And what is the fate of our universe? 

“The history of the universe—and the physics that govern its evolution—are encoded in the cosmic microwave background,” Carlstrom said. “These are big, big topics in physics, many of which we don’t know how to get at any other way.”

Scientists view the cosmic microwave background as a window onto phenomena such as dark energy, dark matter, and cosmic inflation—the rapid expansion of the universe that occurred in the first tiny fraction of a second of the universe’s existence. 

By detecting subtle changes in the cosmic microwave background’s temperature and polarization, scientists can see how these variations in the early universe correlate to those in the density of matter seen today, 13 billion years later. The differences "can be imagined as seeds for where galaxies would eventually grow," as the European Space Agency puts it.

Studying patterns in the cosmic microwave background can also reveal information about what this light has encountered as it traveled over billions of years across the universe.

“[This light] has interacted with all of the structures that have formed since these early times. So we can study the very subtle imprints that these structures make in the cosmic microwave background to actually tease out the processes and the physics that have happened between when it was emitted and today,” explained Lindsey Bleem, a physicist with UChicago and Argonne.

Measuring light from the cosmic microwave background is also a key method for determining the Hubble Constant, a number that describes how fast the universe is expanding. The exact figure is a matter of debate, but it points to the universe's age and history. 

Perhaps the most exciting target of cosmic microwave background searches is a background of what are termed primordial gravitational waves—ripples in space-time from the earliest moments of the universe. 

If our current best model for these earliest moments—that there was an epoch of extremely rapid expansion in the early universe—is correct, we should be able to see the imprint of primordial gravitational waves produced by this inflation in the cosmic microwave background. If so, it would be what some have called a “smoking gun” for the theory of inflation. 

When was the cosmic microwave background discovered?

Two researchers at Bell Labs in New Jersey accidentally discovered the cosmic microwave background in the mid-1960s. The Big Bang theory was only a few decades old at this point, and the notion that the universe was dynamic and evolved from a hot dense state was not widely shared. "It was still just an idea—popular in some circles, detested in others," recalled Princeton University researcher P. James Peebles. 

Peebles and others at Princeton, led by Robert Dicke, began developing an experiment to see whether the cosmic microwave background existed. 

Meanwhile, over at telecommunications company Bell Labs, Arno Penzias and Robert Wilson were conducting experiments with an antenna designed to detect radio waves. To improve the signals, they sought to remove interference. But even when they turned the antenna to empty space, a buzzing noise persisted, day and night. 

The static wasn't coming from the nearby military base. It wasn't coming from the pigeons they found nesting in the antenna, either. No earthly source seemed to explain the buzz. 

Eventually, after connecting with the team at Princeton, Penzias and Wilson determined the interference had a cosmic origin and its existence was consistent with the theory of the cosmic microwave background. The researchers published their findings in 1965; Penzias and Wilson won the Nobel Prize in Physics for their discovery in 1978.

How do you study the cosmic microwave background? 

Scientists have designed specialized instruments and telescopes, both large and small, to study the cosmic microwave background. 

The detectors in these telescopes are designed specially to capture the cosmic microwave background's long-wavelength light, and they can map it in exquisite detail—both on spacecraft and from Earth. 

In the early 1990s, a space-borne instrument called the Cosmic Background Explorer, or COBE, mapped the cosmic microwave background and detected minute temperature variations—as small as one part in 100,000. The Wilkinson Microwave Anisotropy Probe (WMAP) launched in 2001, and the 2009 Planck satellite subsequently refined this map of the cosmic microwave background. Meanwhile, ground-based experiments including the South Pole Telescope (run by a collaboration led by the University of Chicago), the BICEP/Keck family of instruments, and the Atacama Cosmology Telescope, have made extremely precise measurements from Earth. 

Today, a new and powerful array called the Simons Observatory is taking data near the Atacama Cosmology Telescope in Chile. Meanwhile, a coalition of institutions is building a project known as the South Pole Observatory, which will combine a new generation of detectors at the South Pole Telescope with the BICEP experiment to observe the cosmic microwave background in greater detail than ever before. Among their goals is to search for the ripples from the Big Bang itself, known as primordial gravitational waves. 

The South Pole and Atacama Desert are choice locations for these experiments because the dry air and high altitude minimizes interference from water molecules in the Earth’s atmosphere.