So far in our tale we've been following two threads. One has been about humanity's general confusion when it comes to the goings-on of the night sky, and our attempts—usually feeble, but occasionally breathtaking in scope—to measure and understand what's going on up there. The other thread has been a biography of the universe itself as we currently (don't) understand it, starting in the black box of the Planck epoch and proceeding through the splitting of the forces, the incredible dynamics of inflation, and the rise of matter over antimatter.

It's time for these two threads to—briefly—meet. The next major event in the history of our cosmos is a watershed transformation, a clear dividing line between the exotic forces and energies of its youth, and the beginnings of a structure that we will eventually grow to call familiar.

In the timeline of our more recent history, the expansion of the universe had just been uncovered, and in the coming decades, debates would swell over how best to interpret Hubble's stunning results. But in the 1960s, a key observation would be made—a simple collection of data that cemented our modern picture of the grand history of our universe: the big bang model.

By the time our universe was twenty minutes old, it had already experienced the most dramatic phase changes it would ever experience. Imagine, if you will, that in the instant after your birth you immediately experience growth spurts, puberty, and the onset of middle age—complete with graying hair—before the doctors have even cut the umbilical cord. While in terms of the linear passage of time, the universe has a long, long future ahead of it, by the end of the nucleosynthesis era, it had already experienced the most exciting things that could happen to it. It was then doomed to a relative retirement and decline for the rest of its days.

Corresponding with the change in character of the universe is a change in important timescales. When the universe was dominated by exotic merged forces like the electroweak interaction, the operational physics was set by the speed of that dominant player, and phases would begin and end in the blink of a cosmic eye. But once the protons, neutrons, and electrons that we're familiar with in our daily lives were finally manufactured, the universe took its first slide into the slow lane of life.

It was inevitable, really. Continued cosmic expansion means continued cooling. At lower densities and lower temperatures, more familiar physical interactions take precedence. Strong and weak nuclear have each had their turn, and now and for the next few hundred thousand years, it's time for electromagnetism to take charge (when my editor left a note saying, “I see what you did there,” I realized that this was perhaps the only unintentional pun in the entire book).

Indeed, if you add up all the stuff remaining in the hot early universe, it's mostly composed of photons, the ubiquitous carrier of the electromagnetic force. The cosmos at this stage is a proper plasma, the same state of matter you would find in a lightning bolt or the interior of the sun (please don't go personally looking for this; just take my word for it).

In a plasma you've got protons, neutrons, electrons, and photons, all bouncing around together angrily. And some of the protons and neutrons have hooked up to form helium or lithium, and the energy of the surrounding soup is now too feeble to break them apart.

But the battle between matter and antimatter was a Pyrrhic victory—only one proton per billion survived the great primordial war, leaving the numbers of normal matter severely underpopulated. So the plasma was strongly dominated by the photons, the light. Within this opaque energetic soup, atoms (yes, finally, atoms) would try to form when a stray electron would get caught in an orbital around a proton or other nucleus. But as soon as the bond would form, a home-wrecking photon would slam in to destroy the newfound relationship, slapping the electron away and back into the mix of singles.

Nothing could withstand the domination of the photons. Strong nuclear was too short-range, and in the expanded cosmos, it could only find its influence confined to the nuclei. Weak nuclear, as exotic and essential as it can be, was always a pushover. And gravity? By far the weakest of the forces, billions upon billions (upon a few more billions) of times weaker than even the so-called weak force, it was hopeless against these interactions.

But gravity did have one thing going for it—it was playing the long game, fighting with guerilla tactics, laying traps and ambushes so that the tyrannical photons would eventually sow the seeds of their own downfall and forever be relegated to cosmic irrelevance.

Gravity won its ultimate victory by fighting dirty. It couldn't hope to free the helpless atoms from the relentless electromagnetic onslaught in one-to-one battles. But Einstein, de Sitter, Friedmann, Lemaître, and all the others had discovered that gravity, and gravity alone, was sufficient to describe the dynamics of the universe at the very largest scales. While other forces may govern small interactions (like the formation of an atom an hour into the history of the cosmos or the rhythm of your heartbeat billions of years later), gravity is the only force that operates at infinite range and affects all things, regardless of size, shape…or electric charge.

That's the key. Photons could win individual battles, but the war was over before electromagnetism even started marshaling its forces. That's because the universe is, on balance, electrically neutral. For every negative charge, there's a corresponding positive charge out there, somewhere. So on average, any large-scale electromagnetic interactions simply cancel out. There's no coordination, no grand strategy, no overarching plan.

And gravity had a plan. Gravity was driving the expansion of the universe—that's the ultimate lesson of general relativity—and expansion makes the density drop for everyone. For matter, it's a simple cubic relationship. Put one particle in a box, you have a density of…one particle per box. Expand the box by doubling each side, you now have the equivalent of 2 × 2 × 2 = 2³ boxes, so your density has dropped to one particle per eight boxes.

This goes for matter and radiation in equal measure, so at first blush it seems like a wash. But photons pick up an extra interaction from the expansion of the universe. They get elongated by the stretching of space-time itself—as the universe grows, their wavelengths grow longer as well. In other words, the light redshifts, which is what Hubble observed when he examined the light from distant galaxies in the 1920s.

And here's the kicker: redshifted light has less energy.

It's happening in our universe now, and it happened in the universe long ago. As the universe aged, the photons not only diluted, but also lost energy. It was an endurance race between matter and radiation: who could outlive the other in the inexorably evolving cosmos? This was gravity's wicked plan all along; the radiation was simply born to lose.

Because radiation had such overwhelming numbers, this was indeed a long struggle, the longest such war for survival that the universe had yet seen. But gravity is gravity is gravity: quiet and unassuming, but relentless.

The universe expanded and cooled. The densities of matter and radiation dropped. The primordial plasma lost its ferocity. Year by year, the photons grew tired, their influence over matter diminishing.

And 380,000 years into the history of the cosmos, when our observable universe was about one-billionth its present volume, radiation gave up the fight for good.

The first atoms were born.

The name “big bang” was coined by the sharp-minded and sharper-tongued astronomer Fred Hoyle, who didn't exactly take a shine to this expanding-universe business.1

Hoyle had a good point—Edwin Hubble's 1929 observations of a general redshifting of light from distant galaxies only suggested an expanding universe. But the “conspiracy” option, where we are the literal center of the universe with all the galaxies literally flying away from us in a predetermined pattern in order to reproduce a straight relationship between distance and redshift, was never really under serious consideration.

Why? Because Copernicus, that's why. Hundreds of years ago, “Hey, folks, maybe the universe isn't focused on us, just saying” was a radical, thought-provoking notion worthy of much narrowing of the eyes and muttering under the breath. But the eventual success of that (initially flawed) picture, plus decade after decade of the universe rubbing our noses in it—proving over and over that the fantastic energies and vast scales really don't care about us—led, by the twentieth century, to a much more cautious generation of astronomers.

For them, it was preferred to assume that we're not special. Better safe than sorry, I suppose. This central conceit of modern cosmological thinking goes by a few names, like the Copernican principle or the mediocrity principle, and we'll return to the topic in sobering discussion later.

What about “tired light,” the concept that it's not expanding space that's sapping the energy from light, shifting it to redder hues, but simply that light loses energy as it travels? One challenge to this idea is that in order to make light grow tired, you must have that light interact with some sort of substance sprinkled in the intergalactic gulfs—say, magical redshifting pixie dust, purely for example. Since the light will bounce off that magical redshifting pixie dust, the light must also scatter. So images of distant galaxies must be slightly fuzzier than closer ones, because their light has had more interactions with the magical redshifting pixie dust. Plus, that same magical redshifting pixie dust must be sprinkled inside our own galaxy too, so stars on the far side of the Milky Way should be redder and fuzzier than our closer neighbors.

The instruments of the first half of the twentieth century didn't have quite the measuring sophistication to conclusively rule out tired light, but the concept never really caught on. There were no known physical mechanisms for making light tired, and it conflicted with everything else we knew about the photons among us. Even Fritz Zwicky, the bolo-rocking astrophysicist who cooked up the idea, tossed as many varieties of models into his paper as he could think of (and crossed a few off the list in the very same paper), in the spirit of “Let's make sure we don't leave any stone unturned before we jump into an expanding universe.”

For those of you with highly skeptical hearts, don't fret. More modern astronomers with instruments of sufficient sophistication have indeed followed up on these lines of thinking and found them to be less than fruitful. Tired light is a tired idea.2

Expanding universe it is, then. But perhaps not necessarily the big bang “primordial atom” as cooked up by the Catholic priest Lemaître in his application of relativity to the universe. After all, the cosmos having an “origin” did seem bit too close to Genesis for some, and didn't we move past all this using-the-Bible-to-support-our-arguments line of thinking since the days of Kepler?

Enter Fred Hoyle, an amazingly brilliant astronomer who, as far as I can tell, decided to take up the mantle of Curmudgeon Superior from Galileo and seemed to openly work against his own best interests, burning bridges faster than he could build them. He led vital work into the nature of how stars function, but in this story of the universe's story, he serves as the devil's advocate against the consensus growing around Hubble and Einstein's cosmological offspring.

And, like before, I have a soft spot in my heart for the die-hard skeptics in history, even when they become so cantankerous that nobody invites them to any parties. They're annoying, but oh so useful.

The ultimate too-cool-for-school kind of guy, Hoyle often took the opposite position to whatever was popular with his fellow scientists. I must say this was an awesome tactic, because (a) science needs healthy debate and skepticism to survive, and (b) he was smart enough for it to pay off most of the time.

But if a cosmic conspiracy and tired light were off the table for cosmological consideration, what possible alternative explanation was there? You couldn't argue with the data—the results of Hubble and company were too squeaky-clean for any charges of shenanigans. But you could always argue against the theory. Not general relativity itself—by the 1930s, Einstein's theory had already trounced any other potential challengers to the title of Explainer of Gravity—but there was one little crack that Hoyle identified. A small one, but big enough for him to drive a wedge into it and force the scientific community to hold it, take a breath, are you sure?, before jumping off the cliff.

The big mental hurdle that you have to leap, the metaphysical pill you have to swallow, the elephant in the room that you have to address if you want to take this big bang picture seriously is that the universe has, fundamentally, finite age. It has a beginning. There is a specific moment, in the countable past, when the universe switched from not existing to actually existing.

If you're religiously minded, that's not such a big deal. But Hoyle wasn't arguing against the so-called big bang (and even though he didn't intend the tag to be derisive, given his cantankerous nature I can't help but see the corner of his lips curl when he coined the phrase during a BBC radio show in 1949) theory on religious grounds. Far from it. “Everything we see in the universe came from, well, somewhere” isn't the most scientific of statements, but on its face it's not malignant.

Instead, Hoyle challenged the cosmic establishment to go all the way to the finish line when they insist that the universe doesn't care about us. If you're going to elevate Copernicus such that his name gets stuck in front of the word “principle,” the thinking goes, then you need to finish what you started.

We are not the center of the universe. We are not special. We do not have a special vantage point on the heavens—our view is, statistically, just like anybody else's. Ergo, from our perspective it looks pretty much the same in every direction. In the jargon, our universe is isotropic.

You can take it one step further and assert/assume that on average, at the largest scales, the universe is generally the same from place to place. It is homogeneous, like the milk you buy from the store. Therefore, nobody is the center. There is no special location in the universe that is wildly different or elevated or distinct from any other. Again, I'm repeating the phrase on average, at the largest scales because this is cosmology: you can only think big about these kinds of questions.

These two ideas combined, that the universe is both isotropic and homogeneous, form the backbone of general relativity's insights into the cosmos, and hence they are often referred to as the cosmological principle. They are the basic assumptions needed to simplify Einstein's nasty equations enough that you can get work done with the mathematics, and they serve as fundamental statements about the nature of our universe and our role in it.

Hoyle and colleagues rattled the cages: you want a boring universe, where nothing is very different from place to place? That's fine, that's great, that's wonderful. So then shouldn't the universe be pretty much the same from time to time as well? In other words, shouldn't our cosmos be the same through space and time together, like in this concept of space-time that everyone is so excited about?

The refutation to the big bang's cosmological principle was a perfect cosmological principle that rested on the assumption that the universe is eternal and unchanging, that it is indeed homogeneous, through both the vastness of space and the deepness of time. This was the default position just a few decades earlier, before Hubble astounded the world, so why let his results spoil the fun?

This might have ended up just empty words, but like I said, Hoyle had chops. Together with some colleagues he formulated an attractive alternative to the big bang—the steady-state model.3 In this picture, requiring only a small and innocuous alteration to the equations of general relativity, matter is continuously created in the universe, with the rate of creation matching the outward expansion. Thus as the universe continues to grow fatter, its density remains constant—there are always new partygoers joining the big bash as the room gets bigger.

Steady-state cosmology fit the data just fine. Arguments that it seemed too absurd to have matter popping into existence all the time (“Where does your stuff come from?”) were met with sharp rejoinders—the big bang model also posited the spontaneous creation of matter (“Where does your stuff come from, pal?”). It simply stretched the instant, fiery explosion of matter into a long, drawn-out slow burn. A simmer rather than a boil.

The match was set, between the perfect cosmology of the steady-state picture and the finite-aged cosmos of the big bang. And through the late 1940s and into the 1950s, there was no clear winner.

After 380,000 years of waiting, electrons could finally join their hadronic cousins and form the first atoms. Before this time, the radiation had already diluted to the point that matter was the dominant player, but still it fought its losing, helpless battle, preventing the formation of atoms. Finally, though, it called it quits; matter and radiation would never affect each other on cosmic scales again.

This remarkable event would have just flashed by in a haze known to us only dimly via equations and simulations, like all the other major transitions before it, except that the universe was now, for the first time, transparent. Before the formation of atoms, the universe was filled with hot, dense plasma. Just as the radiation prevented the atoms from forming stable long-term bonds, the thick dilution of matter prevented the radiation from traveling freely. A photon would attempt to make a great leap at light speed, only to run smack-dab into a klutzing electron.

But now that neutral hydrogen and helium had formed, deliciously transparent, light had room to move. Over a relatively brief window of time, about ten thousand years or so, the fog of the primordial universe lifted, and a more recognizable, more clear universe became the norm.

For obscure historical reasons, physicists refer to this event by the name recombination, as if this were the second time that atoms got together in the universe, which it wasn't, unless you count being squished into an exotic quark-gluon plasma as “together.” I personally prefer photon decoupling or, less formally, the best fireworks show ever.

The light emitted was literally white-hot, corresponding to a blackbody temperature of about three thousand kelvin, about half the temperature of the surface of the sun.

I know, I know. Blackbody temperature? It's perhaps one of the most confusing terms in all of physics (and that's saying something). It comes from the devices used in the nineteenth century to study the radiation emitted from as-black-as-possible objects, objects that drank in as much of the surrounding radiation as possible and were at a fixed temperature. Perhaps a more descriptive term is thermal radiation, or maybe even warm and/or hot stuff radiation.

All stuff gives off radiation of some form. All that wiggling, jiggling, and rotating at the molecular level releases some of that energy in the form of light. Since some wiggles and jiggles are bigger or smaller than other wiggles and jiggles, the radiation emitted covers a broad spectrum, with a distinct peak depending on the temperature.

For example, you. At a temperature of ninety-eight degrees Fahrenheit, you are emitting all sorts of radiation, most of it in the infrared—hence why infrared goggles are so handy for seeing people in the dark. But you're also giving off a little bit of microwaves (enough to be detectable by a standard household satellite dish) and even visible light (not enough to be seen, but it's there).

The cooler an object is, the longer the wavelength of the majority of light it gives off. The hotter, the shorter. The full description of blackbody (aka thermal) radiation was cracked by Max Planck, a name we already encountered as we tried to come up with a numbering system to describe the earliest moments of the universe. In the process of describing blackbody radiation, he also inadvertently invented quantum mechanics, but that's a story for another chapter.

At the moment of the separation between radiation and matter 380,000 years into the history of our universe, the cosmos was in almost perfect equilibrium. Radiation and matter were bouncing around ferociously, and those countless interactions created an essentially ideal blackbody scenario. Thus when the light was finally released, it carried that imprint, perfectly mimicking a laboratory device at a temperature of three thousand kelvin.

That primordial light permeated the cosmos. Truly for the first time, the densities had dropped so much that it could travel for countless light-years before interacting with a stray bit of matter. It soaked the universe but was no longer a part of it. And it was bright, like having the surface of the sun surrounding you on all sides. Indeed, this sudden release of radiation generated more photons than all the stars will produce, ever, in the entire future history of the cosmos.

But that event was a long time ago. We aren't bathed in white-hot radiation from the early universe. What happened? The quiet but inexorable expansion of the universe happened. Gravity didn't just win; it rubbed radiation's nose in it. With the continued expansion, the radiation was stretched and stretched, redshifted down just like any other long-distance photon in the universe. The primordial light was still there, bathing the sky, but no longer in the visible range of the human eye.

By the late 1950s, steady state was starting to look a little unsteady. Newly developed radio telescopes were beginning to peer into the deep cosmos, and their initial results revealed an unexpectedly high number of intense radio sources, at great distances, and a relative radio silence nearby. That's a hint—only a hint—that the universe may be the same in space, but different in time. If light takes a certain amount of time to travel from place to place, then the further we look, the deeper we see into the past. If these radio emitters, known as quasars, are out there but not around here, then that means they were more common in the past. Perhaps then the universe has changed character with time.

Around the same time that Hoyle was pooh-poohing the big bang, other scientists were working out the full physical implications of such a radical universe. Four in particular—Ralph Alpher, Robert Herman, George Gamov, and Robert Dicke—semi-independently came to a remarkable conclusion. If the universe were smaller in the past, then it must have been hotter. Eventually, at some distant time, it should have been so small, dense, and hot that it was a plasma.

But at a specific time, a switch would have flipped, juuuust as the universe cooled enough to the right amount, and the cosmos would have gone from plasma to not-plasma. And that radiation has a calculable temperature, based on our knowledge of plasma physics (which was all the rage at the time), but that temperature would be reduced by the present epoch.

They initially calculated a temperature of a bare few degrees above absolute zero—apparently our present-day universe is indeed very cold—corresponding to a peak blackbody wavelength firmly in the microwave band. Additionally, we should be completely soaked in this radiation; if our universe is truly homogeneous, it should fill out the sky equally in all directions, with nary a deviation in sight.

So all they needed to do was design, build, test, and operate a microwave antenna and search for this “cosmic microwave background,” and they'd be set.

At the same time, two engineers for Bell Labs, Arno Penzias and Robert Wilson, who, bless their hearts, knew absolutely nothing about cosmology, were designing, building, testing, and operating a microwave antenna for their own industrial purposes.

It was the “testing” part that was giving them trouble. It was the first time in human history that someone had attempted to construct such a microwave detector, so I can't fault them for making things up as they went along. They built everything perfectly, but try as they might, they couldn't get rid of a constant background hiss from their instrument.

They tried the usual things. Turning it off and on again. Making sure the cables were plugged into the right spot. Replacing said cables. Testing for interference.

They tried the unusual things. Calling up the nearby army base to ask—politely—if they were transmitting at these frequencies. Cleaning the pigeon poop off the antenna. Just shooting all the dang pigeons.

I'm sure it was the oddest thing they'd ever seen. No matter where they pointed the antenna, no matter the time of day, no matter the season, there was this constant background static.

After years of banging their heads against the wall, they wondered if this hiss might be real—and might be extraterrestrial. So they sent out some feelers to the astronomical community, and before long the Dicke crew caught wind of it. They met with Penzias and Wilson. They chatted. They both came to the same conclusion: they found it. The cosmic microwave background. The afterglow of the big bang itself.

The result was two side-by-side papers, a flash heard around the world. One paper, written by the physicists, summarized the current state of the art in big bang thinking, this astounding insight that our universe was fundamentally different in the past than it is today, and that this difference is detectable and measurable.4 The other, written by the engineers, summarized the observations.⁵

Penzias and Wilson won a Nobel Prize for their work in failing to find a source of static hiss in their fancy antenna. There's a good chance you've probably never heard of the others before you read their names a few paragraphs ago.

C'est la vie.

The cosmic microwave background, or CMB for those in a hurry, was the nail in the coffin for Hoyle's steady-state theory. The perfect cosmological principle, as lovely as it sounds, doesn't appear to apply to our universe. While Hoyle would continue to fight the good fight past 1965, the games he would have to play to reconcile the steady-state model with the overwhelming abundance of data were stretching far too thin.

Steady state predicted that the universe ought to be the same in the past. But here we are, bathed in relic radiation generated billions of years ago. It was found across the sky, almost perfectly matching a blackbody spectrum (indeed, it's the most perfect blackbody found in nature, besting even humanmade ones), with almost no variation from point to point. It couldn't be generated by stars or galaxies—their distribution is far too lumpy to explain the smoothness of the background signal. It truly appeared to be a background, a source of light sitting behind everything else we can see.

If you could put on microwave goggles, you could detect this bath of radiation—if only faintly. Although the CMB is the largest reservoir of photons in the universe, our universe is very, very large nowadays. But build a simple microwave receiver, and you'll pick it up. If you've ever encountered an old rabbit-ears TV that's stuck between channels, you've seen with your own eyes this fossil from a distant age—about 25 percent of the static in our lives comes from the cosmic microwave background.

The cosmic microwave background pretty much killed off any other competing theory as well. Nothing else could fit, no other idea could make the cut. The raw observational data from the past three decades were simply too overwhelming.

Our universe was different in the past, and it will be different in the future. This is the ultimate, if initially unpalatable, answer to Olbers’ paradox. Why aren't we surrounded by an infinity of stars covering every square degree of the sky above? Because at a certain time in the past, there were no stars. Our universe may be old, but it's only so old.

The cosmos may be infinite in size (we'll get to that later), but it certainly isn't infinite in time, and it's growing larger every day. In the past it was smaller, hotter, and denser. How did it arise? What were the earliest moments like? These are very difficult questions to answer, if indeed they are valid questions at all.

But they are questions we'll eventually have to face, because the facts of our observations push us to that inevitable, uncomfortable conclusion.

Have your own personal theory of the history of the universe? That's fine—science thrives on creativity. But the knife of observation is sharp and is perfectly willing to cut your precious idea down to size. If you want your cosmology to work, you have to explain the existence and properties of the cosmic microwave background. Its presence is inarguable and its implications unavoidable.

While the subject was difficult to think about, at least theory and observations were in accord…for a brief moment. The angst of the nineteenth century was slowly dissolving as new understanding poured forth from the chalkboards—and now computers!—of theorists and the instruments of observers. To be clear, nobody really enjoyed the answers they were getting, but at least the picture of the universe was clicking into place.

The consternation about the complex nature of the stars was still there, and I'll resolve that tension in later chapters. It was replaced instead by a growing despair in the true scale, both in time and space, of the universe.

In some ways, a finite age to the universe is more troubling than the alternative. In an infinite (either in the static or steady sense), at least you can take comfort in the fact that this is just the way things are—that the universe simply persists, unchanging, through the deepness of time. But with the big bang, we know now that the universe has a past…and a future. And that both of these are different from the present.

While the picture of the cosmos was starting to sharpen into an unpleasant focus, at least things were making sense. Gravity, the feeblest of forces, was able to shape and govern the majesty of the heavens—Newton would have never guessed the magnitude of his initial insight! Over time, the discovery of the nuclear forces would help us understand the earliest epochs in our cosmic history, and also the mysterious processes in the hearts of stars.

The universe was revealing itself to be larger, more complex, and made of deeper stuff than we ever realized before. And while it looked, for a brief moment, like we had solved some of the largest riddles of our age—the true scope of the cosmos—seeds of mystery were already beginning to grow.