Your Place in the Universe

by Paul M. Sutter

  Oh, and we've begun to map the universe within the Zone of Avoidance. While visible light gets snuffed out by our dusty galactic center, other wavelengths like radio and infrared sail right through, giving us the barest hint of the galaxies sitting on the far side of the Milky Way.

  Combining the initial mappings within the Zone and a more respectable definition of a supercluster reveals what's going on with the Great Attractor. Remember how I said we live in a hierarchical universe? Yes, I'm repeating myself because it's that important.

  Structures in our cosmos assemble themselves from smaller objects, and the Great Attractor is just the latest phase of that extragalactic construction project. The Local Group and a few other objects are collecting together toward the Virgo Cluster, which sits at the heart of the Virgo Supercluster. But the Virgo Supercluster is just a side branch of a much larger collection: the Laniakea Supercluster. And that supercluster has its own heart at the center: the Norma Cluster.

  When you look in the direction of the Great Attractor, you're looking right at the Norma Cluster. But the Attractor isn't so much a thing as a place, the focal point of a process set in motion billions of years ago. Given enough time, every galaxy, group, and cluster within the Laniakea complex will collapse into a single uber-massive cluster.

  Except it won't.

  The cosmic web is not long for this universe. It's a transient, effervescent feature. A relic of the past, still persisting in the present but doomed in the future.

  We live in a hierarchical universe, but we also live in an expanding universe. Structures like galaxies, groups, and clusters form despite this expansion. Although every galaxy is moving away from every other galaxy, this is only on average. Tiny instabilities in the early universe grow by gravity and, despite it being the wimpiest of the forces of nature, give it enough time and the pull of Newton and Einstein will do its indomitable work, growing structures bit by gaseous bit.

  But the expansion is inevitable, and as I'm about to tell you in the next chapter, it's getting worse. Galaxies are still in motion, set off by gravitational interactions initiated in the distant past, but that motion is slowly grinding to a halt. Five billion years ago, the engines of creation shut down.

  The Local Group will never reach the Virgo Cluster. The Virgo Supercluster will never reach Norma. Laniakea will never fully condense, and eventually it will be ripped apart.

  In a few tens of billions of years, the cosmic web, with its beautiful, intricate lacework of filaments, walls, and knots—the largest pattern found in nature—will be gone.

  All gone.

  When the universe was about nine billion years old, on the eve of the formation of our own solar system, after spending ages in calm complacency slowly spinning the cosmic web from loose threads of galaxies and dark matter, a crisis erupted as a hidden force began to wrest control of cosmological fate from the hands of gravity.

  In the 1990s on Earth, the geopolitical cold war had finally ended with the collapse of the Soviet Union, but growing tensions within the astronomical community were now running hot. The stakes were never so high. The ultimate future of the universe hung in the balance. Well, to be fair, the universe is going to do whatever the universe was going to do anyway, but we'd like to understand it before it actually, you know, happens.

  Cosmology, the science of the universe, is a game of taking almost stupidly simple questions and asking them to the whole entire cosmos as a single physical object. But, as we've seen, simple doesn't mean easy, and straightforward questions like “Hey, how much does that galaxy over there weigh?” can turn out to have some frustratingly surprising answers. So astronomers, in fine academic form, instead of taking the difficulties of measuring the mass of objects like galaxies and clusters as a proper warning from nature to back off, ramped up the observations and attempted the same feat on cosmological scales. Because science.

  And immediately they ran into problems.

  To explain the problems, I need to talk about geometry. It turns out Kepler was sort of right hundreds of years ago—the universe is ruled by geometry that can predict the future of the universe, just not divine geometry that can predict who my true love is. So, half right. Not so bad.

  The language of the cosmos at the very largest scales is general relativity, and as I'm totally sure you remember from a few chapters ago, general relativity connects stuff (matter, energy, etc.) to geometry (the bending, warping, and flexing of space-time). This language can be applied to small systems like planets orbiting stars and black holes, and it can be applied to large systems, like the whole flipping universe. Seriously, like Einstein himself did shortly after inventing the concept. The equations of general relativity aren't picky (insert “general” pun here); to solve cosmological problems, the recipe is simple.

  Tally up all the matter and energy contents of the universe. Be sure to include any nonvisible matter, radiation, and neutrinos. Everything. Stuff that into one side of Einstein's equations. Solve said equations (this is the messy part, but lucky for you, we get to skip over it in this narrative). General relativity then tells you how the whole entire universe bends, warps, and flexes.

  Now what? Well, again with your handy general relativity tool kit, you take that knowledge of bending, warping, and flexing, and you can predict how it will eventually evolve. Voilà: the future fate of the universe in your hot little hands. We know that the universe is expanding, thanks to Hubble, but just how fast is it expanding? Is it speeding up or slowing down? Will it stop and turn around? Will it coast to a stop? If we fast-forward a hundred billion years from now, what will the universe look like?

  The answers to these questions depend on how much of what kind of stuff the universe contains. Heaps of matter will drive a different expansion rate than heaps of radiation, and both of those will certainly result in a different fate than just a thin sprinkling of matter. So to get some clarity on the issue, it's absolutely essential to get a complete and total census of the material population of the cosmos.1

  But wait, there's more. You can flip the equations of general relativity around. If you know—or can measure—the geometry of the universe, you turn the relativistic crank and learn the contents. All it takes is being able to measure the geometry of the universe, which, as you might imagine, could be a little challenging. Go ahead and make a bet now on which is harder: measuring the contents or measuring the geometry.

  When I say geometry, I don't just mean circles and triangles; the geometry you learned in high school and promptly forgot until you had to paint your living room is only a subset of the full picture. That's the realm of what's called Euclidean geometry, which as you might have guessed was developed by Euclid himself, more than two thousand years ago. And that picture is all about flatness: circles, triangles, angles, lines, and everything all living in a plain, flat universe.

  But general relativity is all about curvature—so, step number one, how do you define “curvature”? You have an intuition that the Earth's surface is curved, but your own backyard is relatively flat. How can we define curvature in such a way that we can apply it to any system that we care about, and specifically to the system that we care about the most: the universe?

  Here's one way: parallel lines. If you draw two parallel lines on a piece of paper, and extend those lines to infinity, the lines will stay parallel. That's kind of the definition of “parallel,” so I hope that's not a big issue for you. But try drawing some parallel lines on a globe. Start at the equator with two tiny lines, perfectly parallel. Advance each line northward in a perfectly straightforward way, never turning left or right. Soon enough, despite your best efforts, the lines will intersect—at the north pole.

  You guessed it: the globe, and the Earth, is curved.

  Another method is triangles. If you draw a triangle on a flat piece of paper, the interior angles will add up to 180 degrees. That's also the definition of a triangle, so we should be good. Now draw a triangle on a globe. Add up the interior angles. You'll get a
number larger than 180, I guarantee it. Don't trust me? Do it yourself, right now. I dare you.

  If you happen to have a horse saddle lying around (I won't judge), you can play the same games, but you'll find the opposite result: initially, parallel lines will spread farther apart, and triangle angles will fall short of 180 degrees.

  The beauty of these definitions of curvature is that they can be applied to any number of dimensions—most important for us, three: the spatial dimensions of our universe.

  That's great! Easy-peasy, we just need to bust out the markers and see how triangles and parallel lines on billion-light-year scales behave. Thankfully, nature provided the measuring tools already: the cosmic microwave background. We know, based on our knowledge of the nuclear realm, about the sizes and scales of the minute bumps and wiggles in that afterglow light pattern—we know how big they were when they were formed. And we know how big they are now, eons later, because they're right there, projected onto our sky.

  Beams of light make for excellent cosmic Sharpies. Two beams of light, initially parallel and given enough distance, will eventually trace out the geometry of the universe on the grandest of scales. Sure, little things like galaxies (and yes, in a cosmological sense entire galaxies are considered “little”) will distort their paths, but we want to paint a much bigger picture. In the intervening billions of years between generation and acquisition of those light beams, they will either remain parallel, spread apart, or converge. By comparing what we know about the typical sizes of patches in the cosmic microwave background to what we actually measure, we can literally measure the geometry of the entire universe.

  It's flat.

  Like a pancake. Like Kansas. Like a board. The universe is flat. Parallel lines stay parallel over the course of billions of years and billions of light-years. Our universe appears totally geometrically flat to a ridiculously small margin of error—we're talking one part in a million here, folks.2

  When this answer—that we live in a flat universe—first started to become apparent in the 1990s with the first high-resolution maps of the cosmic microwave background, it was totally fine for the theorists. Why, they had been crowing about a flat universe for decades now, and they were glad the observers were finally catching up. Their reasoning came from inflation, that poorly understood but seemingly necessary process in the very early universe, when the entire cosmos blew up many orders of magnitude in the blink of an eye.

  The process of inflation sent the true scale of the universe far, far, beyond our relatively pathetically small observable bubble. The universe could have any curvature it liked, like the surface of the Earth or a saddle, but it wouldn't matter. Our observable patch is so small that we're essentially guaranteed to measure a flat universe. Just like the Earth is round, but your backyard is so small it appears flat (insert usual caveats about small-scale deviations like groundhog holes not mattering for our measurements of the bigger picture).

  Inflation went one better. To measure a certain degree of flatness today, with a giant universe, meant that the universe had to be even more flatter when it was smaller: the dilution of matter should have driven us far, far away from perfect flatness long ago. Inflation solves that problem by making the true universe so gigantic that we can't help but measure flatness. Nice.

  If the inflation story is correct—and even though we don't know the characters, we understand the general plotline—then we must live in a (locally) flat universe. We don't have any other choice. So it's no surprise that the cosmic microwave background reveals precisely that answer.

  But nobody else, most especially the astronomers, bought into that argument. Hence the mild disagreements of the 1990s.

  The problem was that the measurement of flatness was simultaneously also a measurement of the total contents of the universe, due to all that general relativity stuff you just read about a minute ago. And the astronomers were already busy running around weighing all the galaxies and clusters they could train their telescopes on, and they were falling far short of the predicted number.

  Like, less than a third. The total mass of everything in the universe, including even dark matter, was less than 30 percent of the matter necessary to match up with the observations of a flat cosmos.3

  Hence, argument. The theorists accused the observers of being lazy and not working hard enough in their measurements, because surely they were missing something. The observers countered that theorists should take a break from the chalkboards for once and actually look at the universe around them that was abundantly not agreeing with their predictions—perhaps they forgot to carry the two in one of their calculations? And the cosmic background is pretty far away, even for astronomy. You sure you got it right?

  You know, the usual stuff.

  In comes a third approach, also tied to general relativity: the expansion history. Matter and energy tell space-time how to bend, and the bending of space-time tells matter how to move. So you can measure geometry, contents, or behavior to get at the underlying physics. And the behavior in this case is, of course, the expansion of the universe.

  What Hubble managed to measure back in the early twentieth century was the local expansion rate, based on a relatively small sample of nearby galaxies. And in cosmology, “local” is synonymous with “today.” The light from those galaxies didn't take that long to get here—it might as well have been from just last week, cosmologically speaking. So Hubble's measurement, and any other measurement based on close galaxies, is the current expansion rate of the universe right now at this very moment.

  But the universe could have had different expansion rates in the past. Indeed, the entire concept of inflation depends on that ludicrous-speed expansion in its early moments. And with radiation and then matter taking center stage at different times with different densities, that can affect how quickly (or, for that matter, slowly) the universe expands.

  To get these measurements we have to go deep into the universe, pushing further with surveys and observations than we ever have before. Redshift by redshift, galaxy by galaxy, we need to reconstruct the expansion history of the universe. Of course there will be limitations—the dark ages won't have much to offer us, at least not yet—but galaxies arrived on the scene pretty early in cosmic history. Surely there's some method to capture their distance and velocity, and to combine that with the data from as many other galaxies as possible.

  That's the key; that will resolve the tension, one way or another, between the camps of scientists who have spent decades refining their techniques and sharpening their arguments but haven't come to a solution. It's the Kepler-Brahe and Shapley-Curtis debates all over again, centuries later.

  To reach tall heights you need a ladder, and to pierce deeper into the celestial realm than our ancestors would have ever dreamed possible, you need…

  …a cosmic ladder. No, that's not a joke. I mean, I totally set it up as a joke, but “cosmic distance ladder” is a real-life phrase used by real-life scientists to refer to a real-life concept. The challenge with measuring truly astronomical distances is that objects can be very far away even for an astronomer.

  We've already met parallax, which after centuries of frustration finally led to an accurate distance to another star. Less than a century later, Edwin Hubble used Cepheid variable stars to confirm the remoteness of Andromeda. If you recall (which, by the way, is an academic code phrase meaning “you should have recalled”), Hubble couldn't use parallax for his groundbreaking work because parallax just wasn't good enough: the distances were too great, and the back-and-forth wiggles were too small to measure.

  But even Cepheids, as useful as they are, can only be used to capture distances to nearby galaxies. While that technique swaps out a very difficult measurement (a distance) for a relatively easier one (brightness variations), you still need enough raw light to get the job done. If the Cepheid is too far away, you simply don't have enough photons to work with, and you're back to being hopeless.

  So we need something else, som
e other way to hook into the distance of an object without actually having to measure the distance. And preferably we need some of these to overlap with at least a few known Cepheids. We could use Cepheids to reach farther than parallax because we had a few of those variable stars near enough that we could practically break out the astronomical measuring tape and pin them down. Once the method was safely confirmed, we could extend the Cepheids into the unknown depths without losing too much sleep over the issue.

  So parallax and Cepheids are the first two rungs of a ladder, where we (hopefully) use a series of different objects and techniques to (hopefully) take us deep into the cosmic depths, each more distant method overlapping with a closer, vetted technique. A ladder that lets us measure cosmic distances. A cosmic distance ladder. See, I told you it was a real thing in real life.

  Say you have two light bulbs of identical make, manufacture, wattage, color, and so on. Keep one bulb next to you, and throw the other one as hard as you can—or better yet, to continue with the experiment, walk it across the room and gently place it down. Turn on both lights. Break out your brightness-o-meter (I know you have one). Measure the brightness of the bulb next to you. Measure the brightness of the bulb far away from you.

  Hopefully the distant bulb will be dimmer, and if you think a little bit about the relationship between dimness and distance, you could reasonably argue that if we had a bunch of such light bulbs scattered around the universe, we could use their relative dimnesses to calculate distances.

  Unfortunately, flying around placing light bulbs at strategic cosmic locations would render the whole distance-measurement game moot, so we need nature to manufacture some for us. Many folks back in the day, like Newton and Galileo, had hoped/assumed that stars were of equal true brightness, allowing them to be used like our light bulb analogy, but sadly, that didn't work out well for anybody.