One parsec is roughly three and a quarter light-years, and Proxima Centauri, our nearest neighboring star, happens to be around one parsec away. Hmmm, I wonder why that definition was chosen?
And just as your computer can have megabytes and gigabytes, representing a million and a billion bytes respectively, distances can have megaparsecs and gigaparsecs. Start tossing those kinds of words around, and people won't even know you're not a real astronomer.
As Hubble's result implies, we're going to use those: the Andromeda Galaxy, née Nebula, our nearest major neighbor, is about three-fourths of a megaparsec away from us.
So here's what Hubble's calculation reveals: for every megaparsec you get away from the Milky Way, objects at that distance are receding from us an additional five hundred kilometers per second.
At some level, it's not surprising to know that galaxies zip and zoom around. People do it, planets do it, stars do it. Why not the largest collection of all? But something fishy was going on with Hubble's measurements. In the peculiar jargon of astronomy, the individual velocity of any particular galaxy is called its peculiar velocity. So fine, the peculiar velocities of galaxies aren't zero. I suppose it's a little bit of a big pill to swallow to contemplate the motion of these humongous cosmic assemblages, but it's one we can take.
It's the average motion that's troubling. There appeared to Hubble to be a separate, common (dare I say universal?) motion to the galaxies around us. And specifically, away from us, at the very precise rate of five hundred kilometers per second for every megaparsec in distance. This was certainly something new and potentially troubling. Given the raw data, how are we to possibly interpret this result?
The revelation of two Hubbles. Left, a modern view of the Andromeda “Nebula,” which Edwin Hubble conclusively demonstrated was really, really far away. (Image courtesy of NASA / JPL-Caltech.) Right, a long stare with the Hubble Space Telescope at a small patch of sky—equivalent to the small square next to the Moon—reveals a universe infested with these beasts. (Images courtesy of NASA / ESA.)
Option 1: A conspiracy. Galaxies move around randomly, and they just so happen to have the right velocities so that a galaxy, say, twice as far from the Earth is moving twice as fast as its nearer cousin. And they're all moving away from us. Maybe we're the center of the universe, and we're somehow repulsive?
Option 2: An illusion. Astrophysicist Fritz Zwicky, who really knew how to rock a bolo tie and whom we will meet again later, tossed this idea into the discussion. Maybe light just gets tired, like an out-of-shape guy trying to run a marathon. Who knows what the mechanism is, or what the physical implications might be? But in this case perhaps the general redshifting isn't an indication of motion but a loss of energy. Redder light is less energetic than bluer light, so this hypothesis is still quite capable of fitting all of Hubble's data. The recession is a fake; it really isn't motion but an artifact of our poor understanding of physics.6
Option 3: We live in an expanding universe.
You and I know that the answer is door number 3, but it's not easy to flesh out that deceptively simple but radical statement in a single paragraph, so it gets its own section.
One of the most amazing aspects of this saga of the 1920s is how quickly the resolution came. When Copernicus and Kepler first proposed a sun-centered universe, it took another couple of generations before Newton could offer a unifying theoretical theme, a coordinating force (“universal gravity”) that could sufficiently explain the motions and models offered by earlier thinkers.
But in this case, the theoretical basis for Hubble's observations was happening simultaneously—and had even been anticipated before he got the result! That theoretical basis is general relativity, and if you've been wondering when dear old Albert would get fully introduced into the story, well, here he is.
Einstein had long been attracted to the problem of gravity (get it?), and especially to Newton's troubled comments that he didn't fully understand why his relationship holds between massive bodies in the solar system, but that it nonetheless works, so it must have some utility.
I'll have to save a full and proper treatment of Einstein's work and legacy to another book—after all, he did basically found a few distinct branches of modern physics—so here's the short and sweet version (as short and sweet as I can make it7).
General relativity is Einstein's magnum opus, a completely radical take on the underpinnings of gravity, explaining it not as a force per se but as an effect. Instead of gravity being an instantaneous, invisible communication between massive objects, the effects of gravity in Einstein's picture are actually the consequence of a relationship between mass and space-time itself.
Right, space-time. Not space and time. Not space or time. Space-time. A single, unified thing. We don't live in a three-dimensional world; we act our little plays in a four-dimensional stage, with our three spatial dimensions (up-down, left-right, back-forth) and a fourth dimension of time (past-future). This was the revelation of special relativity, one of Einstein's first forays into reshaping our worldview in 1905, but it was another scientist (and Einstein's former teacher), Hermann Minkowski, who later made the leap into unifying the dimensions.8
I mention this because you're going to be seeing the word space-time a lot, and I need to explain what it really is: It's a ruler. It's a way of measuring distances (which doesn't sound like a big deal) in both space and time (which does sound like a big deal). If we agree to meet for coffee at four in the afternoon, the total description of the location (“coffee shop at 4:00 p.m.”) is a precise event, and the construction of space-time allows me to compute how far I have to travel to get to that event: I have to move, say, 3.5 miles west, and I have to wait until it's two hours into the future before the chitchat over lattes can occur.
You can imagine space-time as a grid (in four dimensions, so good luck), marking out locations throughout the entire universe, where “entire” also means the distant past and future.
It's a dance floor. The particles, forces, fields, and energies that populate our universe are the dancers, twisting and twirling away in complicated rhythms and beats. But the space-time floor stays fixed.
Or at least it did in the decade between 1905 and 1915, when Einstein brain-birthed general relativity. The special version recognized certain rules that applied throughout the universe: the speed of light is the fastest anything can travel; moving clocks run slow; different observers will disagree about lengths and intervals; mass and energy are two sides of the same coin; and so on. To make special relativity happen, Einstein had to chuck out the Newtonian framework of gravity; it simply wasn't compatible with the new set of rules.
For example, what if the sun vanished? Light takes eight minutes to make the leap across the vacuum from our star to our squinting eyes, so we wouldn't know the sun had disappeared until eight minutes after the event occurred. But would the orbit of the Earth change in those eight minutes? Would the inertia of the Earth “know” instantaneously, while light lagged behind?
Einstein didn't think so (and to be fair, had Newton been aware of the issue, he probably wouldn't think so either). Rather, something had to carry inertia, to take it from place to place in the universe—to connect motions across the vastness between us.
So what infrastructure exists that permeates the cosmos, allowing all the particles, forces, fields, and energies to interact with each other?
Bingo: space-time.
It turns out the dance floor isn't as solid as we thought it was. It doesn't just stay there, a rigid platform for the drama of the universe to play out on. Using a mathematical tool kit developed in the nineteenth century by Bernhard Riemann, Einstein was able to formulate a view of the universe where the floor—space-time itself—bends, warps, flexes, and curves.
Imagine yourself gliding down the floor in a smooth-as-butter waltz (or a sassy hip-wiggling salsa, if that's more your flavor), but the floor is a trampoline. Your very presence bends the floor underneath you, as it does t
o all the other dancers. Negotiating the limited space in the crowded room is a tricky thing. If you even get near the other dancers, their dance-floor depressions alter your course, veering you away from your intended movement.
And that, my friends, is the Einsteinian picture of gravity. Except in four dimensions. Sorry, folks, but analogies can only take us so far in a space-time world. The presence of matter (and energy!) distorts space-time around the object, bending it. Any other matter (and energy!) encountering that object will have its motion disturbed by that deformation in space-time. That is our gravitational experience.
This picture answers the vanishing-sun riddle: if the sun were to poof out of existence, it would take a while for space-time to “relax” back to its flat state and for the Earth to be released from the gravitational grip of the sun, so our planet would be flung out like a spinning rock cut from its string at the same time that the familiar light in the sky would wink out of existence.
So special relativity gave the world a language of space-time, and general relativity taught us that space-time itself is a dynamic, living, breathing, physical object. It's still a ruler—it's still very good at measuring intervals between events—but that ruler can stretch, flex, and bend.
The game of general relativity is then pretty straightforward. In words, that is; in the mathematics, it's insanely complicated. Gravitational interactions are formulated as a set of ten interconnected nonlinear equations, with one side of the problem describing all the possible ways that space-time can bend, flex, and twist and the other side of the problem describing all the ways the matter and energy can bunch together flow, and twist.
So in most cases you take a given physical system—say, the solar system. You count up all the matter and energy sources you care about (e.g., the sun and planets), you turn the GR crank, and out pops a configuration for space-time. Then you apply what's called an equation of motion to, well, figure out how the objects ought to move. And boom, you've got some dynamics.
What does this game have to do with the universe? Well, the universe can be considered as a single, physical system. It contains a certain arrangement of particles, forces, fields, and energies. All that stuff, when studied from the ultimate long view, smoothed out across the entire universe, will bend the cosmos at those very largest scales. In other words, the contents of our universe will bend creation itself, and that bending will influence motion at the largest scales—say, by sending galaxies flying away from each other.
Einstein was the first to apply his tools of general relativity to questions of cosmology in 1917, just a couple of years after formulating the methods in the first place.9 To be fair, he was only one of only a handful of people who actually understood how to do it, so it might as well have been him.
But 1917 was twelve years before Hubble's result, and Einstein assumed, just like everybody assumed, that we live in a static, eternal universe. The firmament was fixed, just like the ancients thought, but the word “firmament” had taken on a much larger definition.
What's interesting is that general relativity didn't automatically predict a static universe—left to their own devices, the equations naturally suggest a dynamic cosmos, one that's inclined to expand or contract but not stay still. Well, that wasn't going to work. No way was the whole entire universe moving around. So to develop a halfway decent model of the static universe as Einstein and everybody else knew it, he had to plug a somewhat awkward “bonus term” (not his words) into the equations.10
It was a perfectly reasonable decision to make at the time, and nobody gave him any gruff for it. But imagine for a moment a world where Einstein didn't feel compelled to fit the known data—where he let the simplest possible expression of general relativity predict what the universe ought to be like. He would have come out with the dynamic universe a full decade before observations would have backed him up on his bold claim.
Man, he could have been famous.
Thankfully, Einstein wasn't the only one thinking of cosmological problems and using the general relativity tool kit to think those thoughts. Many other theorists toyed and tinkered with Einstein's equations, including Willem de Sitter (a Dutchman with a pointy beard), Alexander Friedmann (a Russian with a caterpillar mustache), George Lemaître (a clean-shaven Belgian Catholic priest), Bob Robertson (an American with the tiniest mustache you've ever seen), and Arthur Walker (an Englishman with no beard).
Their story of attempting to use general relativity to describe the whole universe is long, intricate, and intertwining. The most important point is that it demonstrates that no matter your choice of facial hair arrangement, you too can be a theoretical physicist.
It also provided the mathematical framework for modern cosmology. In an expanding universe, the galaxies only appear to be physically rocketing away from each other. In fact, the fabric of space-time itself stretches like pizza dough, causing every galaxy to separate from every other galaxy (ahem, on average, and that caveat will have some interesting consequences for later chapters). The redshift that Hubble noted thus is due to not the motion of galaxies but the stretching of space-time. Make no mistake; the galaxies really are getting farther away from us, but not on their own agenda—the increasing gulfs between us are like the tectonic spreading of oceans between continents.
As light travels from a distant galaxy to, say, the aperture of a hundred-inch telescope, the expansion of the universe stretches out the light. Farther galaxy = more intervening universe = greater stretching = more redshift. Perfectly matching Hubble's results. No intergalactic conspiracy needed.
The gist is that by the time Hubble announced his findings to the world in 1929, he had done his homework and knew enough to name-drop Willem de Sitter's work in his paper as a possible explanation for the results—that we live in an expanding universe.
Modern cosmology can thus trace its lineage to two founding fathers. One was Einstein himself, the theorist's theorist, who was profoundly unconcerned with the experimental results of tests of his theories—what else could they possibly find but that he was correct? A solid mathematical argument could sway his thinking, but data would only serve to validate his reasoning. Indeed, the one time he toed the observational line by assuming a static universe, he modified his equations in a move he would later call his “greatest blunder.”
The other was the pipe-smoking, eagle-eyed observer of the cosmos, Edwin Hubble. The champion of solid analysis, good statistics, and simple but powerful writing, Hubble was extremely cautious about offering theoretical explanations for the amazing results he achieved—it was only in the closing paragraph of his landmark distance-velocity paper that he remarked that the results might be explained by an expanding universe.
In other times, both past and present, observers and theorists are often at odds, either leapfrogging each other, sneering at the opposite camp in the rearview mirror, or straight-out devolving into fistfights.
But something magical happened in the decades surrounding the world wars—for a short period of time (cosmologically speaking, and also in the sense of the timescales of human achievement and growth of understanding), those who collect data and those who try to explain data were in almost perfect lockstep, sometimes even publishing papers side by side in the same journal issue.
The universe was starting to come into focus. For a brief moment, everything felt good.
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).
Your Place in the Universe Page 10