Your Place in the Universe

by Paul M. Sutter

  So despite the breaking of the cosmic dawn, we're still relatively in the dark when it comes to first formation of stars. The bigness of the stars is assumed to be generally correct, though, because of the nature of the clumping of material in these protogalaxies (there wasn't a lot of mixing going on that could steal away energy) and because direct hydrogen fusion is relatively inefficient, so you can pile a lot of stuff on before it kicks in. But estimates vary wildly—simulations suggest anywhere from forty to three hundred times the mass of our sun.7

  It's tough to say at this early stage (in understanding) what happened at this early stage (in the evolution of the universe). But we do know that stars formed relatively early on in our cosmic history, and that it led to another, perhaps final, transition.

  For the first time in a long, long time, there was a new source of energy. The newborn stars flooded their surroundings with high-energy photons the likes of which hadn't been seen since the previous, plasma-filled epoch. Until those stars formed out of the murky depths of cold and slippery hydrogen, the only meaningful source of radiation was the quickly dimming and redshifting afterglow of the cosmic background. While those numberless photons could still be felt, they didn't pack nearly the punch they used to—their influence over the lives of matter had long since waned.

  In its early, heady days, the universe was a violent place. Here, still-forming protogalaxies crash into each other, igniting a round of brilliant star formation and presumably some spewing of gas. (Image courtesy of NASA.)

  But these new stars? With potentially gargantuan sizes reaching hundreds of times the mass of the sun? They were powerhouses of light and brilliance. What's more, those first stars began clumping together. The process of structure formation laid down in the inflationary era didn't stop with the ignition of nuclear furnaces in the hearts of those first stars—they were simply the heralds of greater things to come.

  With the first few hundred million years, either soon after the formation of the first stars or concurrently with them (we're not exactly sure), the first galaxies began to collect themselves. The great “island universes” first confirmed by Edwin Hubble were already on the cosmic scene at a relatively early time. The progenitors of what would eventually evolve into our own Milky Way or our neighbor Andromeda had already begun coalescing, studded with sparks of massive stars.

  The first galaxies were probably small and relatively dim. They hosted intensely luminous stars, but not many of them. They hadn't had billions of years to accumulate new materials from the surroundings. They didn't have efficient pathways for mixing gas and dust (and “dust” wasn't even really a thing back then) to produce batches of hundreds or thousands of stars in a single go. But they did, as far as we can tell, already host black holes.8

  As with everything else in this epoch, we're not exactly sure how the first black holes formed. Nowadays black holes form from the deaths of massive stars, when the explosive nuclear energy in their cores can no longer support their own crushing gravitational weight. And—hey!—there are tons of giant nuclear gasballs floating around in the cosmic dawn. So it's easy to suspect that they'd eventually die, spectacularly, and leave behind a population of black holes with masses a few dozen times the mass of the sun, or thereabouts.

  But we're not seeing evidence for these ancient small black holes, the same kind that fat stars leave behind every single day. We're seeing evidence for much bigger monsters: the supermassive black holes. The width of a solar system, these hulking beasts lurk in the hearts of galaxies and tip the scales somewhere north of a few tens of millions times the mass of the sun. They're thought to grow from mergers of many smaller black holes, plus the occasional gorging on surrounding interstellar gas. They're very common in the present-day cosmos; we think every galaxy hosts such a black heart, even the Milky Way.9

  The name for ours is Sagittarius A*, for the morbidly curious.

  Of course it's hard to see black holes directly, either in our neighborhood or in the distant reaches of the early universe. They're black, space is black—not a lot of contrast. But when one of these black beasts is actively feeding, when gas is collecting and swirling around its great maw, the extreme gravitational compression of the material itself causes it to shine fiercely. These are the most powerful engines in the known universe, easily outshining a supernova. We have a few names for them—active galactic nuclei, quasars, etc.—but they all mean the same thing: when black holes feed, the material they consume screams in blazing agony.

  And we see them in the young universe. Really young, when the cosmos isn't even a billion years old. That's strange; the biggest black holes that can power such emissions take multiple generations of stars to be born, live, die, leave behind a remnant, then have multiple remnants collect in the galactic center and merge. That takes time, right?

  In fact, all the characters in this act of the play—the stars, the black holes, and the galaxies—all seem to come in much more quickly than we expected. The universe is showing signs of a certain maturity despite its youthful age.

  Is the formation of complex structures just faster than we normally think? Did it occur faster in the early universe for some reason we haven't fathomed? Are there other forces at work? Perhaps the initial massive black holes condensed out of primordial hydrogen straight away, bypassing the conventional route of stellar death. Perhaps these clumps seeded the early, rapid formation of those young galaxies as well. Maybe the universe just acted different when it was a kid, despite operating under pretty much the same physical principles as today.

  Who knows. As of the time I'm writing this, it's an active area of research. The physical dark ages may have ended with the ignition of the first stars, but when it comes to human knowledge of the first billion years, we're still lacking a lot of light.

  Like I said, the biggest challenge to studying this epoch—the dark ages and the cosmic dawn—is just that: there isn't a lot of light. Up until the recent uses of neutrino detectors and gravitational wave observatories, the only way for us to access the universe was through electromagnetic radiation. Light, in all its glorious forms. Different kinds of light, like the infrared that Herschel accidentally found, or X-rays, ultraviolet, or radio, reveal the full variety of physical processes afoot in our cosmos.

  When it comes to the dark ages, nothing's alight. And even though the cosmic dawn is defined as the moment when stars arrived on the scene, (a) there aren't a lot of them, and (b) they're stupid far away. What's most irritating for the modern astronomer is that we have so much information around this era. At the early-universe side, we have the cosmic microwave background, a flood of primordial photons, up there amid the most well-studied celestial object in human history.

  At the modern-universe side, we have, well, the universe. Stars and galaxies aplenty, buzzing with activity, glowing in every radiation band conceivable. A noisy, messy, chaotic universe. It perplexed our astronomical ancestors, but lately we've become rather good at building gigantic surveys to map out large swaths of the local cosmos in one go.

  But the dark ages? There's simply not enough light to see that far (at least with current instruments—upcoming missions like the James Webb Space Telescope should be able to pick out some of the first creatures in our universe).10 It would be completely, utterly hopeless—a case where the observers just walk up to the theorists and shrug—if not for one thing, a small quirk of quantum mechanics.

  Neutral hydrogen, the kind that inhabited the universe after the recombination event (making neutral hydrogen was, I guess, the entire point of recombination), is as simple as simple atoms can be: a single proton and a single electron. Those two particles have mass, charge, and spin. Remember spin? If you don't, go reread chapter 7 right now.

  Welcome back. Left to its own devices, hydrogen likes to have the proton spin one direction and the electron spin the other—that's its preferred lowest-energy ground state. Of course every once in a while, the hydrogen atom can get a kick, flipping the electron over s
o that it spins in the same direction as the proton. Since this state just has a teensy-tiny bit more energy than the true ground state, it's actually quite stable. In fact, there usually should be no reason at all for the electron to flip back around. It just has no quantum mechanical incentive to do so.

  But another gem of quantum mechanics is that if you wait long enough in the subatomic world, unexpected things can happen. Energy barriers can be broken. Walls can be tunneled. Particles can appear where they please. And if you give a neutral hydrogen atom a few million years to think about it, it can spontaneously flip its own electron over and settle back down into its usual up-down spin state.

  That means a release of energy from one quantum level to another. That means there's emission of radiation with the exact same energy. That radiation is deep in the radio—around 1400 megahertz. Put your hands out in front of you and pretend you're holding a basketball. I'm serious: put down this book and pretend you're holding a basketball. I don't care if you're in public. We need to do this together.

  Thank you.

  That's about twenty-one centimeters, the wavelength of radiation emitted by neutral hydrogen via this subtle quantum mechanical trapeze act of the electron. We routinely use this so-called 21-cm radiation to map out pockets of neutral hydrogen in modern-day galaxies, and it might just give us a window into the dark ages. The severe challenge is that this primordial signal is weak, just the barest whisper in the deep background, and it's shouted over by all the other nearby sources of radio emission. That said, as I write the hunt is on for this peek into our distant past using massive radio telescope arrays. Even if a confirmed, reliable detection is made soon, though, it will be many years before a solid analysis can be performed.11

  The study of the dark ages and cosmic dawn is perhaps the last frontier of modern cosmology, the great unknown and unmapped realm between the nearby galaxies and the cosmic microwave background itself. That said, we can still manage to make progress. We're trying our best, OK?

  The young universe was filled with neutral hydrogen, from metaphorical corner to metaphorical corner. But look around the universe today; as I hinted at just above, there are only pockets of neutral gas remaining. Indeed, most of the hydrogen and helium in present-day galaxies is in the form of a plasma. The sun's a ball of plasma. The interstellar gas is a plasma. The material that floats around and between galaxies is a plasma, a superthin yet still hot plasma.

  We know that the gas in the young universe just after recombination was neutral. We know that the gas in the present-day universe, 13.8 billion years later, is not. How did it get reionized, and how the heck are we supposed to figure it out?

  Perhaps it was that first generation of stars, massive and angry, hot enough to spew out ultraviolet radiation. That kind of light packs enough punch to knock an electron off an atom. Perhaps it took a lot more than that; maybe it was the supernova death of those stars that injected enough energy and X-ray radiation into their surroundings. Possibly it was the combined might of millions of stars—the first galaxies—to transform the cosmos. Probably those massive black holes, forming surprisingly early, did the trick. As gas fell into their horizons, it compressed and heated, releasing torrents of radiation in the process.

  Whatever the cause, about a billion years into its history, the universe underwent its last major metamorphosis, an era we call the Epoch of Reionization. Slowly, starting from pockets of stars and protogalaxies, the final veil was lifted in the universe as neutral hydrogen and helium were reenergized and reverted back into their plasma state, a state they have retained until today.

  As with everything else in the dark ages, we don't know a lot about this process. We knew it had to happen, because we know the state of the universe before and after this transformation. And while we don't know the details yet, we do know that this transmutation was messy, ugly, and awkward.

  After a billion years, the universe hit puberty.

  Compare yourself at ten years old and twenty years old. Chances are you're a radically different person. Compare yourself at twenty versus thirty. You're probably a little rounder around the middle but otherwise unchanged. That's what puberty does—it transforms you from a kid into a protoadult. The Epoch of Reionization was when the universe finally grew up. Done were the days of wild phase transitions and late-night plasma parties. In were the days of mortgage payments and bad backs.

  In the dark ages, the universe was threaded with a dense, warm soup of neutral gas. If you could transport yourself there, it would look totally unfamiliar and alien, as strange as any of the earlier epochs. After the cosmic dawn and reionization, the universe looked like…the universe. Vast, dark, transparent, and dotted with galaxies swirling with billions of glittering stars. Much like you since your twentieth birthday, it hasn't physically changed much since then, at least in comparison to younger days.

  After a billion years, after the last of the neutral gas had been swept away and left to cower in small pockets inside galaxies, after the cosmos had once again been filled with light and heat and warmth, the greatest structure in the universe began to coalesce.

  Fritz Zwicky knew that something fishy was going on.

  It was the early 1930s, and in the years following Hubble's spectacular and surprising result—that galaxies are things and that they are, on average, redshifting away from us, implying that we live in an dynamic, expanding universe—astronomers had undergone a quick change in heart. With gigantic new telescopes like the hundred-incher on Mount Wilson, which Hubble used as a massive eye to peer beyond the limits of the Milky Way, astronomers went from debating the very existence of extragalactic objects to racing to catalog as many of them as possible.

  If you can't beat ’em, join ’em, I guess.

  Now in survey mode, astronomers reclassified known spiral nebulae as spiral galaxies (although the term “nebula” would still persist for some time, thankfully we've been able to drop that anachronism by now; otherwise our discussion of cosmology would be even more saddled by the presence of yet another historical-jargony albatross) and started mapping the heavens for as many deepest-sky objects they could find.

  Zwicky, the Swiss astronomer renowned for his acumen, creative (and sometimes crazy) thought processes, and prickliness, took a special interest in a group of galaxies in the direction of the constellation Coma Berenices.

  Now, even the most ardent astronomy enthusiasts will admit that there's basically nothing interesting happening in the constellation. Heck, even its name, Berenice's Hair, doesn't exactly inspire the wonder and majesty we typically associate with the night sky. There are a few Messier objects, but otherwise it's an unremarkable patch of darkness.

  Unless you look deeper. The universe in that particular piece of the sky is swarming with galaxies, hundreds of them, much more than other random directions in the sky. That itself is an intriguing fact to note, which astronomers in the first half of the twentieth century surely did: Galaxies outside the Milky Way are not scattered around randomly and uniformly. No, there are vast empty patches and what appear to be clusters of these galaxies. The astronomers at the time had no idea of what to make of this, but as we've already begun to see in the previous chapter, the solution will become apparent to them soon enough.

  Zwicky diligently mapped, measured, and cataloged as many galaxies in one cluster in particular, the Coma Cluster, as he could; a veritable cosmic butterfly case of extragalactic creatures. All those galaxies had roughly the same distance from our home, which was the first clue that it wasn't an accident of optics that led to their clustering in the sky. No, these galaxies were associated with each other in deep space—they lived together.

  Since the galaxies lived together, Zwicky guessed that this cluster must be stable. If you saw a random group of people at a random time of day at a random house, you might guess that those people aren't total strangers collected together by pure chance—they're probably a family. They've been together for a while. Sure, it might just be a guest-fil
led house party on a Thursday afternoon, but it's not likely. This argument is from statistics, and it's a pretty useful one in cosmology. Structures that don't last long (in cosmological terms) simply won't persist long enough for us to catch their light in the small window of time that we've been observing the deep heavens.

  It's a safe assumption: if you see something, it's already been there a long time.

  Zwicky cleverly used that assumption to start answering a very simple but deeply difficult question: how much does that cluster of galaxies weigh? How massive is it? It's one of the most straightforward questions we can ask of anything, even celestial objects. It's high on the standard list of questions for describing anything, really. You get a baby announcement, and you get a few key pieces of information: name, sex, length…and weight.

  Astronomical objects aren't gendered, so as soon as we name something (like, say, the Coma Cluster), our next job to break out the rulers and scales. Which is hard, because things in space are typically gigantic and far away. But Zwicky used an old physics trick known as the virial theorem. Originally developed in the nineteenth century, it connects kinetic to potential energies within a system of particles bound together.

  We'll jump right to Zwicky's application of the theorem to explore what all that jargon means. First off, the “particles” are going to be entire galaxies. Yes, galaxies are gigantic, but they're peanuts compared to the size of a cluster. The kinetic energy is related to the speed of every galaxy—the faster the galaxies are buzzing around, the greater the kinetic energy. The potential energy here is provided by gravity—the mutual interaction of all the galaxies provides the glue holding them together.