Your Place in the Universe

by Paul M. Sutter

  Except when it matters, as in the case with dark energy. We know very well (by now, at least) how much vacuum energy, if it is the culprit here, is driving the accelerated expansion. It's a very tiny and definitely not infinite number: somewhere in the ballpark of 10−30 grams in every cubic centimeter of the cosmos—ten measly hydrogen atoms per cubic meter, give or take, spread across the entire universe. That's all it takes to give us the spectacular and stupefying accelerating-cosmos results.

  When we attempt a naïve prediction of this number based on quantum field theory by, say, only adding up frequencies to some reasonable threshold like, I dunno, the Planck scale or whatever, out pops a number clocking in at 1090 grams per cubic centimeter. That's wrong. Very wrong. Deeply, uncomfortably, you're-obviously-missing-something-fundamental-about-the-universe wrong.

  So maybe quantum fields aren't the best path forward to explaining dark energy. But even if we can't get the number right (or even close to kind of right), dark energy still acts as if it were a vacuum energy. Even if the true source is something else, accelerated expansion has the behavior—just unfortunately not the magnitude—of space-time-filling quantum fields permeating all of the cosmos.

  Basically, we're stuck.

  There's another way to frame this problem in general relativity, which might or might not be helpful, depending on your level of pessimism. This framing can be sourced directly to old Einstein himself. Remember how, when he first manufactured relativity in the general sense, he didn't know about the expanding universe? And how the equations gave him the flexibility to toss in an extra constant to maintain a static cosmos? And then as Hubble astonished the world, he scrubbed out the so-called cosmological constant when nobody was looking?

  Well, here we are again, finding ourselves in a situation where it looks like we need to add that constant back in after all. Not to maintain a static universe against any movement, but to power the accelerated expansion. The reason this might help is that it's an alternative way to formulate the conundrum, which might lead somewhere promising, since a cosmological constant sits on the “curved space-time” side of the relativity fence. The downside is that a cosmological constant is perfectly identical to a vacuum energy, which sits on the “matter and energy” side of the equations of general relativity. So you haven't actually moved anywhere.

  Still, could dark energy really be a fault of gravity rather than some vague, poorly understood part of our universe? It's the same question we had with dark matter, and it has the same problems. As nauseating as vacuum energy is, there appears to be (at the time of this writing) no modification and/or extension of general relativity that competently explains dark energy.7

  Part of the problem is that we're in the dark (ha!) not just theoretically but observationally. It's been a couple of decades since dark energy first burst onto the scene in scores of supernova detonations seen across the universe. Astronomers are not idle folks, and in those years they've collected heaps of evidence and come to the conclusion that yup, it's still there.

  Take, for instance, those baryon acoustic oscillations, the fancy-pants term for the rockin’ sound waves in the early universe. When recombination smoothed the troubled waters of the young cosmos, those waves got frozen in, leading to a slight—but detectable—impression in the arrangement of galaxies on very (very) large scales. That impression acts as a standard ruler, as if you tossed a bunch of yardsticks (metersticks in the international print of this book) around the cosmos. Since we know the size of the oscillations, we can compare that to the size we measure in the sky, and reconstruct the expansion history at those points.

  It's a completely independent way of targeting dark energy, and it reveals…dark energy. Indeed, if dark energy hadn't happened to our universe, galaxies would have continued their cosmological building program, erecting ever-larger structures, and that process would have washed out any primordial imprints altogether. The very fact that we can still detect that faint impression leads us to conclude that dark energy is happening.

  Despite decades of our best observational efforts, we've only been able to, at best, confirm the existence of dark energy, without actually learning a lot more about it. Has it changed with time, or is it truly a cosmological constant? Is it connected at all to dark matter? Are there multiple sources of dark energy? If it's a vacuum energy, why can't we get the numbers to come out right? Is dark energy related to inflation, which was driven by another (mysterious) quantum field, and if so, why are these effects separated by billions of years and orders of magnitude in energy?

  All questions, no answers.

  Our first dart thrown at the dark energy board is miles off, but we don't have any other decent (even half-decent; heck, we'd even accept 1 percent decent at this stage) ideas.

  The modern picture of our universe, as painted by hundreds of observations and experiments independently searching for answers and cross-checking each other, from the cosmic microwave background to distant supernovae to the weights of clusters of galaxies, is cold and bleak: 13.8 billion years old, composed of less than 5 percent normal (light-loving) matter. One-quarter dark matter and three-quarters dark energy. It's geometrically flat, but the expansion is accelerating, for reasons we don't fathom.

  We call it “concordance cosmology,” as it's the result of many different lines of research that all point to the same bleak conclusions.8 It's a completely different universe from the one explored hundreds of years ago. That universe was complicated and messy, but small and hot. Cozy and alive. The universe revealed in the modern age is old and slow, well past its prime and dominated by mysteries piled on mysteries. It turns out that the efforts of generations of scientists over the course of centuries have barely even scraped the cosmological surface.

  But the universe is not dead yet, and we have unfinished business.

  Against that backdrop of an old, cold, dark, expanding cosmos, fires still burn. Normal matter may make up less than 5 percent of the contents of the universe, and indeed you could erase out all the baryons in existence and the long-term history and fate of the universe would largely go on unchanged, but baryons do deserve some special discussion. After all, even with the development of neutrino and gravitational wave astronomy, stars and light are our primary views into the celestial realm. That was true hundreds of years ago at the birth of modern cosmology, and sometimes old tricks are still good; hundreds of years from now, I'm willing to bet that the good old-fashioned optical telescope will be at the forefront of astronomical research.

  It may feature a mirror the size of small planet, but the basic gist will still be the same.

  Four hundred years ago, Galileo Galilei revolutionized our understanding of—and our place in—the universe using a simple, small telescope. Three hundred years later, Edwin Hubble accomplished the same feat using a much larger version of the same instrument. The thirty decades between them saw an almost-annual revision of the cosmic cast of characters as new vistas were opened, new maps were charted, and new mysteries developed.

  By the late nineteenth century, two major questions began to crystallize: how do stars work, and how big is the universe? We've been following the second question for the past few chapters, which at first had some surprising but respectable answers (“larger than you thought”) but quickly led to discoveries that made it seem like nature was just playing a cruel joke on us (“by the way, your kind of matter doesn't matter”).

  Answers to the latter question were gradually taken up by a fervent group of astronomers and physicists who, over time, eventually gained self-awareness and named themselves cosmologists, studiers of the universe itself. At first a somewhat fringe and hand-wavey discipline, known for its startlingly inaccurate measurements, the field grew into respectability, and even popularity, with the discovery of the cosmic microwave background and large-scale maps of the cosmos.

  But the traditional astronomers weren't asleep at the aperture in the twentieth century. Through careful (scientific, even) observat
ions, scientists around the world and through the decades unraveled the mysteries behind stars and galaxies. Don't get me wrong—there are still about a million things we don't understand about the baryonic, light-loving world, but we've come a long way.

  When we last left the nineteenth century, we turned to matters of cosmological interest. But there were still so many puzzles. So many varieties of colors and sizes of stars, in all sorts of groupings and collections, some sprinkled evenly throughout the galaxy, others clumped together. Some were isolated loners; others, complicated pairings, triplets, or more. New stars would appear, burning fiercely for days or weeks before fading back into obscurity. Some would pulse rhythmically over the course of months—or minutes!—without hesitation or interruption.

  And then there were the nebulae—thin, wispy veils of dust and gas, sometimes associated with stars and sometimes alone. Some kinds of nebulae were later violently reclassified as entire galaxies, with creative classification schemes rapidly applied to the new order of celestial objects. Others maintained their original cloudy moniker but still held their secrets well.

  Deeper probes sketched the shape of our own Milky Way as a thin disk, with twisting spiral arms and a bulging egg-yolk center, orbited by satellite galaxies and clumps of red, dead stars. More observations, especially with the opening of X-ray and radio astronomy, revealed even stranger creatures like pulsars and quasars, some within and some without our home galaxy.

  The more nuclear fires were collected and categorized, the more the intellectual fires burned in the hearts of astronomers around the world: how does it all work, and are we connected, even in the slightest, to the celestial realm?

  The revealed answer is frustratingly scientific: no, but also yes, in a technical sense.

  Let's get the “no” part over with: the stars are almost incomprehensibly distant from us and do not affect us, here on the surface of the Earth, in any remotely conceivable way. Sorry, Kepler. There's no divine order to it all, and the position of a particular star or planet at the moment of our births does not have anything to do with anything. The forces that govern the motions of celestial objects are complex and chaotic. The regular patterns in the night sky are a coincidental effect of the Earth's rotation and revolution, not caused by anything fundamental in their nature.

  Even the planets, close enough to be considered “ours,” are thoroughly remote and isolated balls of gas and rock. Sure, technically gravity's influence is infinite: right now, as you read, you're feeling a slight tugging from the massive gravity of Jupiter, the largest of the solar system planets. But even that mighty giant, with its 317 Earths’ worth of mass, is so far away that the gravitational attraction of this very book has more influence on you.

  And the stars themselves? You can imagine experiencing our sun up close, its surface a roiling inferno, a cathedral of plasma and radiation. From the Earth, a serene yellow ball. The same ferocity placed light-years away? A pinprick of light, a literal point in the night sky, without any dimension, easily overwhelmed by nothing more than a streetlamp.

  But in a strange twist of fate, those distant stars are more than just tiny points of light sometimes visible in a dark enough sky. They're our cosmic cousins.

  Given the winding and interconnected nature of scientific research, it's hard to pick a singular watershed moment when we first started to make the big connections that signaled a change in how we view the world. But for the sake of convenience and narrative simplicity, which I'm sure you'll appreciate, I'll start with two fellas named Ejnar Hertzsprung and Henry Russell and their handy little diagram of stars.

  Before 1910, stars were just stars. Astronomers were used to the incredible variety on display in the heavens but had gotten little further than simply naming them. Red giants, white dwarfs, blue supergiants, and so on weren't the most creative or romantic names, but they served their functional purposes well. But how were these stars of different sizes and colors connected together? Complicating this connection was the fact that stars also had different levels of brightness, sometimes due to their very nature and sometimes due to their different distances from the Earth. If I didn't tell you how far away I was, you couldn't tell me how intrinsically bright my flashlight was, and vice versa.

  Ejnar and Henry (not a crime-fighting duo, as their names might suggest) took a stab at boiling down all this mess into its essential essences and trying to discern what really mattered when it came to stars. Working with catalogs of thousands of stars, they applied a variety of methods to figure out their distances and hence their true brightness. Plotting those thousands of stars on a diagram comparing the brightness to the color revealed a puzzling pattern. Perhaps the most puzzling part of that pattern was the peculiar fact that there was a pattern at all.1 Stars didn't just pick a random number from the brightness and color lotteries at the moment of their birth—almost all stars lived along a narrow diagonal strip, with brighter stars shifting to bluer colors. The largest stars formed a horizontal branch above that strip, and the white dwarfs were isolated in their own little island.

  Fantastic, a pattern! Drat, another mystery. C'est la vie scientifique. This wasn't a solution to the problem of stars but, at least, a major, major clue. Which is a good enough start, I suppose. If I were Arthur Conan Doyle, I'd probably have my character say something really clever right now.

  It's always amusing to read about a theory that everybody at the time figured was probably wrong, but nobody had any better ideas. In this case, the original thought was that perhaps stars start big, red, and fat, then slowly contract over their lives. The steady gravitational collapse provides a source of energy, powering their radiative expenditures.

  Slight hitch: this process can make a star like the sun burn for ten or twenty million years, tops, and the biologists and geologists at the time were already pointing out that the Earth was a billion years old at the low end.2

  The solution came a decade later, first suggested by (Sir) Arthur Eddington—of “let's go on a safari to test Einstein's relativity” fame3—once we figured out the whole nuclear physics and quantum mechanics and mass-equals-energy business happening at the subatomic level. It's a complicated story (of course), but let's take a high-level stroll.

  The sun is a giant ball more than one hundred times wider than the Earth (in fact, there are boiling pustules on its surface larger than our entire planet), clocking in a whopping 333,000 Earth-masses of mostly hydrogen, a good fraction of helium, and a sprinkling of some heavier junk. Imagine yourself sitting in the center of that behemoth. It's just a little bit intense: the hydrogen plasma soup is crammed so tightly it's 150 times denser than water, the pressure is a headache-inducing 250 billion times greater than at sea level, and the temperature is a scorching 15.6 million Kelvin. I know, I know, an August day in Ohio can feel like this, but it's nothing compared to the sun's core.

  With the hydrogen atoms crammed so tightly together, they overcome their natural electric hatred for each other and—replacing descriptions of complicated nuclear chain reactions with the simple word “fuse”—fuse into helium. Wonderful, the intensity of the sun's core is cooking hydrogen into helium; what does that get us? It gets us liberated energy, that's what. That's because the inputs into the reaction (four individual protons) are slightly more massive than the outputs (two protons and two neutrons glued together into a helium nucleus).

  It's the gluing that's doing the work to provide the difference. Look at it this way: it takes energy to get your hands in there and rip apart a helium nucleus, so the formula is “helium + energy = separate parts,” which can handily be rearranged as “helium − separate parts = energy.” That difference in energy is precisely a difference in mass, just like Einstein taught us. So speaking of these reactions in terms of mass differences or energy differences is all the same, because mass and energy are equivalent, and the sun can glue together protons and spit out energy in the form of radiating photons.

  There are also some extra products from the r
eactions, like positrons and neutrinos, and it's the detection of the neutrinos that lets us peek into the core itself and verify that yes, the fusion party is still raging deep inside the sun.4

  This is a very efficient process, trading a little bit of mass for a lot of energy, enabling the sun to burn for billions of years and also enabling the biologists to say they told us so. Fine, we'll give them that one. To speak specifically for once, at the end of each nuclear chain reaction, the end products are 0.7 percent less massive than when they started, giving 26.73 million electron volts (hey, remember those?) of energy. Every single second, the sun chews through six hundred million tons of hydrogen, giving an energy production rate that exceeds a staggering 1026 watts. Yes, there's an awesome word for it: one hundred yottawatts. That's a lotta watts.

  Of that incredible photonic output, most gets dumped into empty, useless space, and a measly 0.000000045 percent, or about one part per ten billion, actually strikes the Earth, half of which makes it to the surface during the day, providing the ultimate power source for all life on this planet.

  Once we cracked the nuclear code, the Hertzsprung-Russell diagram fell into place, and it turns out we had stellar evolution completely backward: as stars age, they grow larger and more luminous. Who would have guessed? As a star consumes hydrogen, lumps of inert helium ash grow like tumors in its core. To compensate for this and keep the fusion party going, the temperature of the core rises, increasing the fusion rate and hence the size and luminosity of a star. Have a few duds join your party? Just crank up the music to compensate—that'll do the trick.