by Katie Mack
Cyclic universes that go from Bang to Crunch and back again forever have a certain appeal in their tidiness. (And we’ll explore these in more detail in Chapter 7.) Rather than a beginning from nothing and catastrophic, final end, a cycling universe can in principle bounce along in time arbitrarily far in each direction, with endless recycling and no waste.
Of course, like everything in the universe, it turns out to be significantly more complicated. Based purely on Einstein’s theory of gravity, general relativity, any universe with a sufficient amount of matter has a set trajectory. It starts with a singularity (an infinitely dense state of spacetime) and ends with a singularity. There isn’t really a mechanism in general relativity to transition from an end-singularity to a beginning one, however. And there is reason to believe that none of our physical theories, general relativity included, can describe the conditions of anything close to that kind of density. We have a pretty good understanding of how gravity works on large scales, and for relatively (ha!) weak gravitational fields, but we have no idea how it works on extremely small scales. And the kinds of field strengths you’d encounter when the entire observable universe is collapsing into a subatomic dot are all kinds of incalculable. We can be fairly confident that for that particular situation, quantum mechanics should become important and do something to make a mess of things, but we honestly don’t know what.
Another problem with a bouncing Crunch-Bang universe is the question of what makes it through the bounce. Does anything survive from one cycle to another? The asymmetry I mentioned between an expanding young universe and a collapsing old one, in terms of the radiation field, is actually potentially very problematic here, as it implies that the universe gets (in a precise, physically meaningful sense) messier with every cycle. That makes the cyclic universe less appealing from the standpoint of some very important physical principles that we’ll discuss in later chapters, and it’s certainly more difficult to fit into a nice neat reduce-reuse-recycle ecology.
THE ALLURE OF THE INVISIBLE
Bounce or no bounce, a universe with too much matter and not enough expansion is destined for a crunch, so checking where we’re at in terms of that balance seems like a good idea. Unfortunately, measuring the matter content of the universe is complicated by the fact that not all matter is easy to see, and determining how much a galaxy weighs when all you have is a picture of it can be challenging at best. As far back as the 1930s, it was clear that just counting up galaxies and stars meant missing something important. Astronomer Fritz Zwicky studied the motions of galaxies moving around in galaxy clusters and noticed they seemed to be moving too quickly, and should by rights be flying off into empty space, like children on a merry-go-round that’s spinning too fast. He suggested that perhaps there was some unseen “dark matter” holding everything together. That idea floated around the astronomical community as an unsettling possibility until sometime in the 1970s, when Vera Rubin came along and demonstrated once and for all that whole heaps of spiral galaxies really didn’t make any sense without some extra invisible stuff.
Since Rubin’s time, the evidence for dark matter has only strengthened, partly because we now know how important it was in the early universe, but it has remained stubbornly hard to directly detect, on account of being apparently uninterested in interacting with our particle detectors. The leading idea is that dark matter is some kind of as yet undiscovered fundamental particle that has mass (and therefore gravity) but doesn’t have anything to do with electromagnetism or the strong nuclear force. Theories suggest it might be able to interact with other particles via the weak nuclear force, opening up some possibilities for detection, but the signal would be hard to find and we haven’t yet seen it. What we have seen is a massive amount of evidence for its gravitational impact on stars and galaxies, and on the ability for stars and galaxies to form out of the primordial soup in the first place. And even better than that, we can see evidence for dark matter’s existence in the shape of space itself.
One of Einstein’s big insights (among many) was that gravity isn’t best understood as a force between objects, but rather as the bending of space around anything that has mass. Imagine rolling a tennis ball across the surface of a trampoline. Now put a bowling ball in the center. The way the tennis ball falls toward the bowling ball, or curves as it goes past it, is a pretty good analogy for how objects move through space in the presence of large masses. The shape of the space itself is causing the object’s trajectory to curve. But it’s not just the paths of massive objects that are affected by the bending of space—even light responds to the shape of the space it’s moving through. Just like a curved fiber-optic cable can make the light inside it turn a corner, a massive object bending space can cause light to curve around it. Galaxies and clusters of galaxies become distorting magnifying glasses for the objects behind them. Some of our most compelling evidence for dark matter comes from finding that this “gravitational lensing” effect is stronger than can be accounted for by the mass of the stuff we can actually see—some of the mass is due to something invisible. It turns out there’s a lot of dark matter out there. The first attempts to weigh up the matter in the universe by looking at just the visible stuff gave us a wildly inaccurate accounting. Not long after Vera Rubin’s studies, it became clear that the vast majority of the matter in the universe is dark.
But even when dark matter was properly accounted for, it was difficult to determine whether the density of matter in space was on one side or the other of the “critical density” that defined the border between a recollapsing universe and an eternally expanding one. Determining the contents of the universe was only one part of the problem; the other part was figuring out exactly how fast the universe is expanding, or, alternatively, how the expansion has changed over cosmic time. This, it turns out, is no mean feat.
In order to get a good measurement of the cosmic expansion rate for a reasonable fraction of the history of the universe, you need to survey a huge number of galaxies, at a range of distances. Then, for each galaxy, you need to work out two things: its speed and its actual physical distance from us. Astronomers worked out the local expansion rate with the Hubble-Lemaître Law back in 1929 (though the exact number for the proportionality was argued about for decades after, and is still a point of some controversy). But to answer the Big Crunch question, we need to know the expansion rate across a huge swath of cosmic time, which means a huge distance in space. That’s not so much of a problem for the galaxy speed part of the equation, since this can be determined with redshift measurements, which are, generally, reasonably straightforward. Measuring distance accurately over billions of light-years, however, is a lot harder.
Studying the distances and speeds of galaxies using images from photographic plates in the late 1960s led astronomers to state with increasing confidence, despite quite a lot of remaining uncertainty, that we were, in fact, fated to collapse. This prompted a few astronomers to write some very exciting papers delving into exactly what that was going to be like. It was a heady time.
In the late 1990s, however, astronomers perfected a more precise method for measuring the expansion of the universe, involving stringing together several methods for measuring cosmic distance and applying them to extremely distant exploding stars. Finally, they could take the true measurement of the universe and determine, once and for all, its eventual fate. What they found shocked pretty much everyone, earned three of the team leads a Nobel Prize, and made a complete mess of our understanding of the fundamental workings of physics.
The fact that the discovery indicates that we are almost certainly safe from a fiery death in a Big Crunch has turned out to be cold comfort.IX The alternative to recollapse is eternal expansion, which, like immortality, only sounds good until you really think about it. On the bright side, we’re not doomed to perish in an apocalyptic cosmic inferno. On the, well, dark side, the most likely fate for our universe turns out to be, in its own way, much more upsetting.
I. Being the cente
r of your own universe might sound appealing at first, until you factor in that the observational evidence for this is that everything is trying to get away from you as fast as possible.
II. We sometimes have a similar problem with “small” versus “far away.”
III. It is frequently called just Hubble’s Law by those in the astronomy community, but in 2018 the International Astronomical Union voted to officially recognize Lemaître for his contribution by adding him into the name. As a theorist myself, I approve.
IV. In the “nearby” universe, where the recession speeds are small, this is just a simple division problem: velocity divided by Hubble Constant equals distance. For more distant sources, it is complicated by the fact that the Hubble Constant isn’t actually constant over all of cosmic time, and the proportionality isn’t a strict proportionality when the speeds are very high. It is safe to assume in general that if anything in cosmology sounds extremely simple, it’s an approximation, a special case, or the ultimate Theory of Everything we’ve all been searching for all our lives. (I wouldn’t bet on Option 3.)
V. Technically, the ball and the Earth are both pulling on each other, because gravity is a two-way street, but the amount of motion the Earth experiences due to the baseball’s gravitational tug is… not much.
VI. You might be wondering if we can just measure the expansion now and ten years from now, and see how it’s changed. Unfortunately, our current technology doesn’t allow for measurements this precise, but in the coming decades we might be able to make this comparison.
VII. And other minor things like planets and people, but for the purpose of this discussion we can ignore those.
VIII. To quote the legendary D:Ream, “things can only get better.”
IX. Recollapse isn’t impossible, from our current understanding. If dark energy, which we’ll discuss in the next chapter, has especially weird and unexpected properties, it could reverse our expansion. But the evidence so far doesn’t seem to point us in that direction.
CHAPTER 4: Heat Death
VALENTINE: The heat goes into the mix.
(He gestures to indicate the air in the room, in the universe.)
THOMASINA: Yes, we must hurry if we are going to dance.
Tom Stoppard, Arcadia
One of my earliest astronomy memories is of a 1995 Discover Magazine cover story proclaiming a “Crisis in the Cosmos.” Something impossible was showing up in the data: the universe appeared to be younger than some of its own stars.
All the careful calculations of the age of the universe, based on extrapolating the current expansion back to the Big Bang, suggested the universe was somewhere in the vicinity of 10 or 12 billion years old, whereas measurements of the oldest stars in nearby ancient clusters gave a number closer to 15. Of course, estimating the ages of stars is not always an exact science, so there was a chance that better data might show that the stars were a bit younger than they looked, shaving maybe a billion or two years off the discrepancy. But extending the age of the universe to finish solving that problem would create an even bigger one. Making the universe older would have required scrapping the theory of cosmic inflation—one of the most important breakthroughs in the study of the early universe since the discovery of the Big Bang itself.
It would take another three years of combing through data, revising theories, and creating entirely new ways of measuring the cosmos before astronomers would find a solution that didn’t break the early universe. It just broke everything else. In the end, the answer came down to a new kind of physics woven into the very fabric of the cosmos—one that would fundamentally change our view of the universe and completely rewrite its future.
MAPPING THE VIOLENT SKY
The scientists who discovered the solution to the cosmic age crisis in the late 1990s weren’t trying to revolutionize physics. They were trying to answer a seemingly straightforward question: how quickly is the expansion of the universe slowing down? It was common knowledge at the time that the expansion of the cosmos was set off by the Big Bang, and that the gravity of everything inside it has been slowing it down ever since. Measuring one number—the so-called deceleration parameter—would tell us the balance between the outward momentum from the Big Bang and the inward pull of the gravity of everything the universe is made of. The higher the deceleration parameter, the harder gravity is pushing the brakes on cosmic expansion. A high number would indicate the universe is fated for a Big Crunch; a low one would suggest that even though the expansion is slowing, it will never completely stop.
Of course, to measure deceleration, you have to find a way to measure how quickly the universe was expanding in the past, and compare that to how quickly it’s expanding now. Fortunately, that whole thing where we can see the past directly by looking at distant things, coupled with the bit where the expansion of the universe makes everything look like it’s moving away from us, means that this is totally possible. All we have to do is look at something nearby, and something really far away, see how quickly they’re each moving away from us, and apply a little math. Simple!
Okay, in practice it’s not simple at all, because you have to know the distances as well as the redshifts, and distances are hard to measure across deep space. But suffice to say, the measurement is possible, if very, very difficult. Fortunately, astronomers have a vast and varied toolkit for measuring things in the cosmos, and in this case it turns out that cataclysmic thermonuclear explosions of distant stars do just the trick!
The short explanation is that certain types of supernovae make explosions whose properties are so predictable we can use them as mile-markers for the universe. They involve the violent deaths of white dwarf stars, which are, when they’re not busy exploding, the kind of slowly cooling stellar remnant that our Sun will eventually become after it gets through its planet-murdering red giant phase. When a white dwarf grows to a certain critical mass (either by pulling matter off a companion star or by colliding with another white dwarf),I it detonates. This is called a Type Ia supernova, and it produces a kind of characteristic rising and falling of brightness and a telltale spectrum of light that we can pretty reliably distinguish from other cosmic conflagrations. In principle, if you understand the physics of this kind of explosion really well, you know how bright it would be up close, and factoring in how bright it looks from all the way out here, you can deduce how far the light has traveled. (We call this the “standard candle” method because it’s like you have a light bulb where you know the exact wattage. Once you have that information, you can always deduce the distance using the fact that the bulb will look dimmer when it’s far away by a factor of the distance squared. Only we say “candle” instead of “light bulb” because it sounds more poetic that way.)
Once you have a measure of the distance, you need to know how fast the supernova is receding. For that, you can use the redshifting of the light from the galaxy the star exploded in, which tells you how quickly cosmic expansion is happening at that point. Use the distance and the speed of light to work out how long ago this whole thing went down, and you have a measurement of the expansion rate in the past.
In 1998, just a few years after that Discover Magazine article raising the alarm about the age of the cosmos, two independent research groups collecting observations of distant supernovae came to the same utterly unreasonable conclusion. That deceleration parameter—the one measuring how quickly the expansion rate was slowing down—was negative. The expansion wasn’t slowing down at all. It was speeding up.
THE SHAPE OF THE COSMOS
If the cosmos were behaving itself, the basic physics involved in the expansion of the universe should be about as simple as throwing a ball up into the air, as we discussed in the previous chapter. Throw it too slowly, it goes up for a bit, slows down, stops, and falls down again: that’s like a universe where there’s enough matter (or a weak enough initial Big Bang expansion) that gravity wins and recollapses the universe. Throw the ball incredibly inhumanly fast and it might just escape th
e Earth’s pull and drift out in space forever, always slowing: a universe perfectly balanced between expansion and gravity. Throwing it even faster than that means it’ll escape and just coast forever, approaching a constant speed as the gravity of Earth becomes less and less of an influence: that’s like a universe that keeps expanding forever, not having anywhere near enough matter in it to turn around the expansion or even slow down it very much.
Each of these possible universe types has a name and a particular geometry. The geometry isn’t the external shape of the universe in the sense of it being a sphere or a cube or something. It’s an internal property—something that can tell you how giant laser beams would behave while shooting across the cosmos on immense scales. (Because if you’re going to measure a property of space, you may as well do it with giant laser beams.) A Big Crunch–fated universe is called a “closed” universe, because two parallel laser-cannon beams would eventually bend toward each other—it’s the same kind of thing that happens to lines of longitude on a globe. What’s happening in the cosmic case is that there’s so much matter in a closed universe that all of space is curved inward. A perfectly balanced universe is “flat” because the beams would just stay parallel forever, in much the same way two parallel lines would stay parallel on a flat sheet of paper. A universe with way more expansion than gravity is called an “open” universe, and in that case, as you may have already guessed, the two laser beams would diverge from each other over time. The 2D-surface analog here is a saddle shape: try drawing parallel lines on a saddle (or, if you don’t have a saddle handy, you can use a Pringles chip) and they’ll get farther apart as they go. What these shapes represent is the “large-scale curvature” of the universe—the amount that space on the whole is distorted (or not) by the matter and energy within it.