Our decades-long examination of the cosmic microwave background provides clues about the Big Bang, inflation, and the very early history of the universe.
The universe used to glow. It still does, if you know how to look for it.
For 60 years, physicists have used enormous radio dishes to detect the oldest light in the universe, barely younger than the Big Bang. This light, known as the cosmic microwave background (CMB), is everywhere in the universe, and it looks almost exactly the same in all directions.
But there are tiny variations in the CMB carrying vast amounts of information about the very early universe – and those variations also point to the origins of galaxies, stars, planets, and us. In the great quest to understand the origin of the universe and everything in it, the CMB is our best clue.
The cosmic microwave background and its origins
The origins of the CMB lie in the earliest days of the universe. Immediately after the Big Bang, just shy of 14 billion years ago, the universe was hot: billions of degrees, hotter than the core of the Sun or the center of a nuclear explosion.
It was so hot that no stable structures could form, not even at the subatomic level – as soon as quarks tried to form a proton, the immense thermal energy of their surroundings would immediately rip them apart. But the universe was also expanding rapidly, and cooling as it did so, first allowing protons and neutrons to form, and then allowing those protons and neutrons to assemble into a few light atomic nuclei.
About 20 minutes after the Big Bang, those nuclei stopped forming as the universe expanded and cooled further, dropping to a hundred million degrees. At that point, the universe was filled with atomic nuclei, electrons, and high-energy photons, all mixed together in a state known as a plasma. (Theories suggest there was also a lot of dark matter, but this hypothetical form of matter, true to its name, doesn’t interact with light, and thus wasn’t a key player in the formation of the CMB.)
The Sun and other stars are mostly plasma, and plasma is opaque to essentially all forms of light – those photons couldn’t get far. Fundamentally, this is because light is an electromagnetic phenomenon, meaning photons (little packets of light) will bounce off and interact with anything that has an electric charge, like the electrons and nuclei that pervaded the universe in its infancy.
Photons were trapped, ricocheting around in that charged soup. For light to get anywhere, the electrons and atomic nuclei had to get together to form neutral structures: atoms. That couldn’t happen until the universe expanded and cooled more, because the thermal energy of the plasma was so high that any electron and nucleus that got together to form an atom were immediately ripped apart by the random jostling of their neighboring particles in the plasma. To escape this trap, the photons had to wait.
That wait was short on cosmic scales, but long on human scales. Nearly 400,000 years after the Big Bang, the glow of the plasma had cooled to red-hot, a mere 3,000 degrees Celsius – about half the temperature of the surface of the Sun.
Once that happened, electrons were finally able to join with nuclei to form atoms without being immediately pulled apart, and the plasma turned into a hot, diffuse gas, transparent to light. The photons of the universe were finally free, streaming out across the nascent cosmos in all directions. These photons – the first light to travel across long distances in space – are the origin of today’s CMB.
Traveling the light spectrum, from infrared to radio waves
The spectrum of visible light that we see with our eyes is only a small part of the total spectrum of electromagnetic (EM) radiation – light comes in many forms, both more and less energetic than the kind of light we can see directly.
Initially, the CMB photons were primarily in the infrared, the part of the electromagnetic spectrum only slightly less energetic than visible light. But as the CMB photons traversed the cosmos over the next 13.8 billion years, the universe expanded, stretching them out and robbing them of energy. They shifted down the EM spectrum to the least energetic form of light: radio, specifically microwaves (so named because their wavelengths are shorter than other forms of radio waves).
Humans didn’t learn to detect radio waves until the late 1800s, and nobody detected the faint-but-omnipresent signal of the CMB until 1964. That year, two physicists working at Bell Labs, Arno Penzias and Robert Wilson, built a highly sensitive radio antenna in Holmdel, New Jersey. They were hoping to detect radio waves bouncing off of early communications satellites, but they encountered a problem: their antenna was picking up an unexpected hum.
They tried to eliminate it. At one point, they trapped a pair of pigeons that had taken up residence in the telescope, thinking the hum was caused by a buildup of pigeon droppings. Penzias and Wilson cleaned out the telescope, but the pigeons came back and the scientists decided they had to shoot them. Still, no matter what Penzias and Wilson tried, the radio hum remained, coming in from all directions across the sky.
Finally, they connected the signal they were receiving with work by Princeton physicists Robert Dicke, James Peebles, David Wilkinson, and Peter Roll, postulating the existence of a pervasive microwave signal as an echo of the then-hypothetical Big Bang. (Dicke and his team didn’t originally come up with the idea – physicists George Gamow, Ralph Alpher, and Robert Herman had first proposed the existence of the CMB nearly 20 years earlier, a fact Dicke later claimed he’d forgotten.)
Hunting for differences in the cosmic microwave background
Penzias and Wilson’s discovery ultimately led to a wide scientific consensus that the Big Bang actually happened. But, as is always the case in science, the answer to one question led to another.
Penzias and Wilson had established that the CMB looked the same in all directions. But cosmologists knew this couldn’t be strictly true. While the CMB was expected to be nearly identical across the sky, it couldn’t be perfectly identical, because that would have meant that the very early universe was perfectly uniform too – and a perfectly uniform universe couldn’t give rise to galaxies, stars, planets, and us.
The hunt was on for the little differences in the CMB, the evidence of our own origins. Over the subsequent decades, a series of increasingly sensitive experiments – conducted from the ground, from balloons, and ultimately from microwave antennae launched into space, far from the warm interference of Earth – discovered that the CMB does indeed have small anisotropies, little variations in different directions. Those differences are quite tiny: just one part in 100,000.
These small variations ultimately stem from slight differences in density in the primordial plasma that gave rise to the CMB. In the billions of years since the CMB was first emitted, those small differences, magnified by gravity, turned into all of the glorious variety of structures in the universe around us.
Inflation and the very early universe
Again, the answer to this question led to another: Where did those small differences in density, ultimately responsible for the formation of all the stars and galaxies in the cosmos, come from?
It’s a deep question, and the best theory we have to answer it is known as inflation. Inflation says that in the first fraction of a second of time, our universe briefly expanded very rapidly, and then slowed down. During that eye-blink of ultra-fast inflation, small quantum fluctuations in the density of the universe were scaled up and frozen in, leading to those one-part-in-100,000 differences.
Inflation is widely regarded as the leading candidate for explaining this and several other mysteries about the very early universe. But the theory is very hard to test. Inflation, if it happened, was essentially the first event in the history of the universe: before the CMB was emitted, before the first protons and neutrons showed up, and arguably before the Big Bang itself (for certain definitions of “Big Bang”).
There is nothing left from that time in the history of the universe – or almost nothing. Those tiny variations in density are one of the only sources of information about the ultra-primordial universe of inflation. And the best source of information about those variations is the CMB itself. As old as the most ancient light in the universe is, it’s a window into an even older time.
That window is out in space – but so are we. The Big Bang happened everywhere in the universe at once, and the CMB is everywhere too, even in the room with you right now.
Grab an old analog radio, turn it all the way past 107.5 on FM, and listen to the static. The peak frequency of the CMB is a thousand times higher than the top of the FM band, but a small fraction of the white noise you hear at that frequency is still coming from the tail end of the CMB, passing through your room on its 14-billion-year journey. That fuzz of omnipresent radio static is the remaining echo of the origin of all things.
Adam Becker is a science journalist with a PhD in astrophysics. He has written for The New York Times, the BBC, NPR, Scientific American, New Scientist, Quanta, and other publications. He is the author of two books, What Is Real? and the forthcoming More Everything Forever. He lives in California.
