You make a planet, and the planet will want to collect an atmosphere and some satellites. You make a galaxy, and the galaxy will attract the flows of gas surrounding it. You make a universe full of mass, and that mass will pull, resisting the primordial urge to continue expanding and separating away from itself. The universe may have been born in a state of expansion, but matter is the stereotypical stubborn donkey, refusing to budge. Depending on the amount of mass, the expansion may just gently slow down over the eons, or it may give up entirely, stopping the expansion and pulling the universe in on itself.
Whatever the long-term fate of the universe, at this stage a million years into its history, the density of matter was high enough to begin slowing down the expansion, which was probably a new and uncomfortable sensation for the cosmos. That's the ultimate picture of the universe after matter's triumph—in the postrecombination cosmic landscape, we live in an expanding (less vigorously, but still pretty spry for a creature a million years old) universe, flooded with a reddening population of photons and an abundance of newly forged hydrogen and helium atoms, which simply floated around dumbly.
About that structure. As we saw when astronomers and physicists first started cooking up our modern view of cosmology, they adopted what would become known as the cosmological principle: our universe is pretty much the same from place to place. What started as a handy trick for simplifying the crazy-hard mathematics of general relativity sort of evolved into a standing rule, taking a cue from Copernicus and turning our non-specialness into a generic facet of our cosmos. The universe is large and generally uncaring about us and really quite boring.
This principle of cosmological insignificance—perhaps the greatest differentiator between the cosmologies of our ancestors and our modern view—was validated in a big way by the detection of the cosmic microwave background. As predicted, it was pretty much unchanged throughout the sky. And 41,253 square degrees of the same dang thing is a pretty clear signal that even the most clueless could pick up on. The evidence validated the core assumptions of the big bang model.
But at some scale it breaks down. Sure, if you compare different parts of the universe at big scales, those parts look statistically the same. But below those scales you have galaxies, stars, planets, people, pumpkin spice. And those are most certainly not the same everywhere you go. The universe isn't wall-to-wall stars; there are gaps. The Milky Way looks different from Andromeda, and way different from the Magellanic Clouds.
There's structure. There are differences. There are regions of incredible density and places with only a few scant hydrogen atoms. There are stars and not-stars. The universe is not a completely uniform tomato soup—it's a chunky beef stew.
How can we resolve this apparent paradox? To fit the observations (the raw data that all our ideas are obedient to), we need a universe that's smooth at large scales but coarse at small ones (“small” being a relative term here). In other words, we need to explain the simultaneous and partially contradictory existence of both a sky dotted with stars and a background of uniform microwave radiation.
Indeed, one of the main tools in our cosmological arsenal, inflation, just makes the situation worse—at least at first blush. One of the most important jobs of inflation was to solve the so-called horizon problem by ensuring that everything in the universe had enough time to coordinate to get everyone looking the same before whooshing out and making the big giant cosmos that we're all used to by now.
So if inflation is a super-homogenizer, a primordial smoothie blender, how can there still be chunks of fruit floating around and getting stuck in the straw?
The answer is…inflation. Yup, at it again, with a major trick up its quantum mechanical sleeve. The same hammer that cosmologists use to solve our scientific shortcomings when it comes to understanding the large-scale properties of the universe also explains the structure contained within it.
When I first introduced inflation a million years ago, I talked about it in half-circumspect terms, like I wasn't really believing what I was telling you. And if I remember correctly, I promised I would return to inflation and provide some better evidence for it. And here we are.
When inflation was first dreamed up by Alan Guth in the 1970s, he didn't really have the puzzle of structure formation in mind. Astronomers hadn't yet made deep enough extragalactic surveys to map out significant structures in our universe, and the formation of stars and galaxies was a problem for other people. In fact, for a while cosmologists were seriously considering the possibility that cosmic strings, weird and twisty fractures in space-time itself related to magnetic monopoles, might provide the mixing necessary to stir up some density differences in the early universe.1
Alas, it was not to be.2 Instead it was the humble inflaton, the highly hypothetical quantum field that drove inflation before the close of the first second. You see, it didn't just inflate space-time, fulfilling its cosmological duty to solve all sorts of nasty artifacts of the early big bang picture. It had a bonus side effect: it inflated everything.
Remember those quantum fields that permeate all of space-time? And how we can never look at individual particles the same way again? I know it was just a chapter ago, but it was pretty heavy stuff. The world around you is infested with these writhing, contorting, vibrating quantum fields, and even in their ground state—the state, by definition, of minimum energy—they support a massive amount of energy.
If you take a box and empty out all the particles and radiation, you have an empty box. But the quantum fields soak into space-time itself, so your box is still full of them. They're just not “active,” with no major waves on them, so there are no particles bouncing around between the walls. But ground states don't mean zero-energy states. That's a rookie classical physics mistake, and you need to start thinking like a quantum mechanic if you're going to make it in this universe.
Even in their ground state, the quantum fields are abuzz with activity, humming just below the level of perception. They normally don't affect the physics of our everyday world or even the subatomic realm: all our normal interactions happen “on top of” this ground state, so we don't notice any difference. It doesn't matter if you build your house on the seashore or on top of a mountain—if you want to get to the second floor, you still have to climb a flight of stairs.
But we know this “vacuum energy” (one of the names we give to this ground state of the quantum fields) exists because it does occasionally interact with our world. If an atom absorbs a bit of radiation, causing an electron to jump to a higher energy level, what makes that electron want to come back down? The higher energy level is perfectly stable as long as the electron isn't jostled. Well, it's the vacuum energy, the background hum of the quantum universe, which provides the necessary jostling to knock the electron off its ledge and back down to a lower-energy state. There's also the Casimir effect, where two parallel metal plates will attract each other in a vacuum, due to the quantum fields on the outside overwhelming the ones on the inside and creating an inward pressure. It's a bit too complicated to fully explain here but serves as a needed reminder that vacuum energy is indeed a thing.3
I'm going to switch metaphors now. Instead of a background hum, I want you to think of vacuum energy as a background fuzz. You might be tempted to call an empty patch of pure space perfectly smooth and featureless, but the quantum fields, sitting in their ground state, provide a subtly textured backdrop to the painting of our universe. If you zoom in on the sub-sub-sub-microscopic scale, all the way down to the Planck length, you'll see that space-time isn't flat—it's bumpy, constantly roiling and boiling with these quantum motions. The features in space-time are driven by the effervescent hum of the fields: they are constantly changing their energy state, and the addition or subtraction of energy, no matter how fleeting, affects the space-time around them.
When inflation inflated the universe into ridiculous proportions in the first second of the universe, it also inflated these quantum fluctuations, enlarging them out of
the realm of the microscopic and into the world of the merely small.
Throughout the radiation era, through the collapse of atoms in recombination, and into the dark ages of matter domination, these seeds, these wrinkles in density, persisted and grew. What's more, new perturbations (introducing the technical term because that's how we roll) began entering the observable universe. This is an important part of the story of inflation and how we know it's (relatively) accurate, so here comes another metaphor.
Let's say you're in a big concert hall, listening as the orchestra plays, I dunno, the Brandenburg Concertos, or that opening theme from Game of Thrones. The room is full of sound, from short-wavelength high-pitched flutes to long-wavelength low-pitched brass. In an unexpected flash, the concert hall undergoes a phase transition and experiences inflation, growing exponentially larger in a fraction of a second. The walls of the room, the orchestra itself, and even your friends get carried away from you. The hall is now much larger than the limit of what you can see—you're completely isolated in your observable patch.
Along with the concert hall, the sounds emanating from the instruments were stretched too, most of them reaching such incredibly long wavelengths that they don't fit inside your observable bubble. In essence, they're frozen, too big to resonate in your universe and be heard. But over time your bubble grows, continuing to expand. As it grows larger, it catches up with the frozen-in sound waves, and as soon as the waves fit, they vibrate the air molecules and you can hear the sound again.
One by one, tone by tone, frequency by frequency, you're able to piece back the last moment of the orchestra, starting with higher frequencies that fit easily in your bubble, then over time the lower and lower pitches. Each successive note perturbs the air in a different wave, resonating at longer and longer scales.
The act of inflation pulled some fundamental quantum fluctuations outside our observable universe, but as our cosmos expanded it “caught up” to continually larger fluctuations. Once they (jargon mode on) “entered the horizon” (jargon mode off) they could begin influencing the motion of matter and energy.
And we can see the influence of those quantum fluctuations, inflated well beyond their usual scale, like a disturbing macro photograph. The enlarged fluctuations left an imprint on the cosmic microwave background, where minute density differences, planted in that first instant of the big bang, persisted through the first few hundred thousand years of cosmic history. Those density differences were small but detectable. By the 1990s, detailed measurements of the cosmic microwave background revealed temperature differences of one part in ten thousand.4
More missions followed and flew, and today we have incredibly high precision maps of that relic light.5 We're finding a universe that's homogeneous—the cosmic microwave background is essentially the same no matter the patch of sky—but not completely so. There are tiny, subtle differences in temperature, and the statistical properties of those variations match precisely what we expect from this inflationary picture. While we can't see behind the great firewall of the CMB, the cataclysmic events of previous epochs rippled and shook through the young universe, leaving their stamp on that first flash of light.
The pieces of evidence that indicated an inflationary paradigm for an early universe took decades to emerge after the initial accidental detection of the CMB by Penzias and Wilson in the 1960s. That's because these temperature differences are incredibly small, and you need to measure good chunks of the sky before you get reliable enough statistics to claim a result. Hence science at an industrial scale is needed—no more of this amateur-hour business. But after enough seriously hard work with space-based missions such as COBE (the Cosmic Background Explorer), WMAP (the Wilkinson Microwave Anisotropy Probe) and Planck (just…Planck), nature eventually revealed the secrets of the primordial universe as written on the face of the microwave background.
Notice that this will be a common theme running forward—of discoveries and observational insights coming less and less from individual astronomers or quick-thinking theorists, and more and more from teams of dedicated professionals with armies of grad students laboring underneath them. There's a social commentary on the state of modern science somewhere in there, but I won't pursue that (in this book).
The end result is this: It seems, by all lines of available evidence, that something like inflation occurred in our past, and that the event of rapid expansion enlarged fundamental quantum fluctuations to macroscopic scales. And these small differences in density and temperature planted the kernels for much larger structures.
All it takes is a little gravity and a lot of time.
In our universe, with interactions governed by gravity at large scales and long times, the rich get richer and the poor get poorer. If there's a random spot with just a tiny bit more density than average, it will have a slightly stronger gravitational pull, and nearby matter will be attracted to it. As more matter piles on, the density difference grows larger, and the corresponding gravitational attraction becomes stronger, encouraging more nearby neighbors to join the party.
So over time, over the course of hundreds of thousands of years, stretching into the millions, pockets of gas in the universe began to condense and grow larger, and the large spaces in between them slowly, steadily emptied out. The process may have been seeded by exotic quantum forces, but by now it was driven by the entirely benign and predictable gravitational pull.
There was movement in the dark.
Slowly, silently, stealthily, throughout the era of these dark ages, long after the light from the cosmic background had shifted below the visible, a web of matter began to weave itself. Starting from the microscopic, clumps of hydrogen and helium atoms arose, accumulating and growing steadily larger, forming dense clumps a light-year across, tens of light-years across, thousands and millions of light-years across. The first major structures in our universe began to form.
All in darkness, in a cooling and expanding cosmos, over millions of years the slow but relentless process of gravity drove matter to collect in on itself, generating small-scale differences set against a large-scale backdrop of a homogeneous universe.
And in one small corner of the universe, in a pocket of gas no different from any other, the densities reached a critical point. As matter accumulated on itself to form larger structures, the internal compressions deep in their cores grew and grew, eventually reaching a point where the gravitational pressures overwhelmed the electrostatic repulsion of the hydrogen atoms, forcing them together in a nuclear embrace, igniting a fusion reaction.
The first star was born. The cosmic dawn had arrived.
The first stars to appear in the universe are thought to be massive beasts, easily a hundred-plus times more massive than the sun. We think they arrived on the scene around the time that our home was celebrating its hundred-millionth birthday, less than a hundredth of its current age. Compared to the exotic, cacophonous pace of earlier stages in cosmic evolution, events take an achingly slow time to wind up in the older, colder, slower cosmos of more recent eras.
In the sweet joys of astronomical conventions, these stars are known as Population III (or simply “Pop-3” if you want to sound fluent in astro-jargon), even though they're the first generation. Stars like our own sun are called Population I, with the intervening generation Population II no matter the ordering scheme. These first stars are something special—they are made of pure, unrefined hydrogen and helium, without a trace of heavier elements.
The first stars were the first intense sources of light to appear in the cosmos since the dense, hot plasma that filled the universe before recombination. One by one, stars began to ignite across the cosmos, flooding its volume with an intensity of light that hadn't been seen for millions of years.
While I'd love to give you more details, the formation of the first stars is shrouded in mystery. Part of the problem is observational—as far we can tell (and we've looked really, really hard) there are no Population III stars remaining in our present-day un
iverse. Since bigger stars lead shorter lives due to the increased ferocity of their nuclear reactions, that's a clue that they must have been heavy beasts. It's only with our deepest surveys that we even catch a glimpse of the first galaxies, which formed eons after those first stars. Those young galaxies may—emphasis on the may—contain a few retirement communities of the first stellar generation residing in them, so some of the combined light that makes its way across the vastness of time and space into our telescopes could show a trace of that older population. In recent years there have been tantalizing hints, but nothing supremely conclusive.
The other roadblock to knowledge of the first stars is theoretical. Earlier epochs in the universe were satisfyingly linear, which in scientific circles means it's easy to calculate and make predictions from the math. It's well-behaved and housebroken. You can solve problems the good old-fashioned way, with pencil and paper. Even inflation itself can be understood, at least at the broad-brush level, on the chalkboard. The predictions for the temperature of the cosmic microwave background were calculated by hand. The growth of density perturbations under the influence of gravity, the addition of new disturbances as the universe grew larger, our model of the young universe for the first millions of years, the lot of it, are all shaped by relatively simple physics and straightforward math.
But stars are not linear creatures. Complex flows of gas and matter are not easily solved with a piece of chalk. Predictions for the relationships between matter, radiation, and nuclear forces are not for the faint of heart. Understanding this epoch requires serious horsepower in the form of sophisticated computer simulations that attempt to recreate the physics of these early epochs. It's a tough business, recapitulating the universe in silicon, and usually requires a wealth of observational data to constrain and restrict the theoretical models. But with a lack of observations, the simulations are leading the way, with theorists tweaking input parameters and models of physical processes to glean some sort of useful information about those first stars.6
Your Place in the Universe Page 15