by Katie Mack
If you’ve ever heard the rising and suddenly falling “vroo-oom!” sound of a race car passing by, or the shift in tone when a siren approaches and retreats, you’re familiar with the Doppler effect. A Doppler shift of the sort you normally experience is the phenomenon of a sound getting higher pitched as the object emitting it moves toward you, and dropping in pitch when it moves away. It has to do with the way the pressure waves in the air pile up on approach and stretch out on departure, changing the frequency of the sound that you hear. Frequency is, after all, just how quickly the waves hit you one after another. In sound, these are pressure waves, and higher frequency is higher pitch.
Figure 8: Illustration of a Doppler shift. When the source of the sound is stationary, the frequency heard by two stationary observers is the same. When the source of the sound is moving, the sound is stretched out to lower frequencies for the observer the source is receding from and compressed to higher frequencies for the observer the sound is approaching. The former hears a low note, while the latter hears a high one.
It turns out that light does something similar. A light moving toward you quickly will shift to a higher frequency, and one moving away will shift to a lower frequency. For light, frequency corresponds to color, so this shift looks like a color change. The electromagnetic spectrum extends far beyond the visible, but as a shorthand, when a Doppler shift of light occurs, a shift up is called a blueshift (because high-frequency visible light is on the blue end of the spectrum) and a shift down is called a redshift. Extremely blueshifted visible light could get all the way to gamma rays, while extremely redshifted light would show up as a radio signal. This phenomenon is one of the most important and versatile tools in astronomy, as it allows us to see, just from the color of a star or a galaxy, if it’s moving toward us or away.
Of course, in practice, it’s not quite that simple. (Astrophysics can be frustrating that way.) Some stars and galaxies are just redder, inherently, than others. How, then, to know if something is red because it’s just red, or red because it’s receding?II The key is that the light is never just one color, but a spread across frequencies—a spectrum. Characteristic patterns in the spectrum of a star are due to bits of the light being absorbed or emitted by different chemical elements in the star’s atmosphere. When you spread out the light through a prism, different colors appear at different intensities, and dark lines or gaps show up at those specific frequencies where atoms in the star’s atmosphere have absorbed the light—the light at those frequencies was removed by the gas before it could reach you. These features produce a kind of bar code unique to each element, with a pattern of lines that astronomers can recognize at a glance. So, for instance, light passing through a cloud of hydrogen will appear with a specific comblike pattern of dark lines when it’s spread out across all the frequencies. We know from laboratory tests where the lines should be, and what the pattern should look like, and we can repeat the process with the patterns from other elements as well. If a star has a recognizable comblike pattern in its spectrum, but the lines appear at the “wrong” frequencies, that indicates the light from the star has been shifted by the star’s motion. If each line is shifted in the same way to lower frequencies, that’s a redshift and the star is moving away. If each line is shifted high, that’s a blueshift and the star is approaching. And how far the lines have shifted tells us how quickly the star is moving.
Astronomers have gotten very good at this kind of measurement. Redshift or blueshift is now one of the most straightforward things to observe about any source of light in the universe, provided a spectrum is taken and it has any recognizable line patterns at all. We can use it to see how stars within our galaxy move relative to us, or to detect the tiny wobble of a star being pulled back and forth by the orbit of a planet around it.
And when it comes to distant galaxies, we can now use the redshift to measure not only how they’re moving relative to us—toward us or away, and how quickly or slowly—but how far from us they are while they’re doing it. How does that work? The expansion of the universe means that however a galaxy might be moving through its own space, the fact that the space between here and there is expanding means that it will also, in general, be moving away from us. And how quickly it’s moving away from us depends on how far away it is now.
In 1929, astronomer Edwin Hubble was looking at galaxy redshifts when he noticed a striking, conveniently simple pattern. Galaxies that are farther away have, on average, higher redshifts. This relationship has allowed us to both confirm the expansion of the cosmos and map out its evolution. Translating redshifts to speeds, the pattern Hubble detected meant that the more distant a galaxy, the faster it’s receding from us.
Figure 9: Cosmic expansion and redshift. As the universe expands, the light from distant galaxies is stretched out by cosmic expansion. This means that we will observe the light from a distant galaxy at a longer wavelength (redshifted) as the expansion of the universe proceeds. Because the expansion is happening everywhere, another observer watching a distant galaxy somewhere else in the cosmos will also see that galaxy’s light redshifted.
Imagine stretching a slinky between your hands. (Just stretching, not bouncing. This is for science.) As you move your hands apart, each curl of the slinky moves only a finger’s width away from the one next to it, but the two curls at opposite ends will end up a few feet apart in the same amount of time. If space is expanding uniformly in all directions, the same kind of relationship should hold, and that’s exactly what Hubble’s observations found. Mathematically, this gives us a conveniently simple rule of thumb: a galaxy’s apparent speed is directly proportional to its distance. Meaning, first, more distant things are moving away faster. And second: there’s a single number by which you can multiply any galaxy’s distance to get its speed. While it was Hubble’s data that ultimately proved the relationship and produced an estimate for that number, the proportionality was actually predicted theoretically a few years earlier by Belgian astronomer and priest Georges Lemaître. The relationship is accordingly known as the Hubble-Lemaître Law.III And the constant of proportionality (the number by which you multiply the distance) is called the Hubble Constant.
The crucial part for us here is the connection between redshift and distance. It means that we can look at a distant galaxy, measure the redshift, and determine from that exactly how distant the galaxy is. (With some technical caveats.)IV
But redshift is also connected to cosmic time. The expansion of the universe makes a lot of things weird in astronomy, and one of them is that we use what is essentially a color, written as a number, to denote speed, distance, and “the age the universe was at the time when the thing was shining.” Physics is wild.
Here’s how it works. If we measure a galaxy’s redshift, we know how quickly it’s receding from us, and we can use the Hubble-Lemaître Law to get its distance. But because light takes time to travel to us, and we know light’s speed, knowing the distance also tells us how long the light has been en route. That means that measuring a galaxy’s redshift tells us how long ago the light left the galaxy. And since we know how old the universe is now, that tells us how old the universe was when that galaxy sent out the light we see.
Taking this all into account, astronomers can use redshift to refer to earlier epochs of the universe. “High redshift” is long ago when the universe was young; “low redshift” is more recent. Redshift 0 is the local, present-day universe; redshift 1 is seven billion years ago. At the high end, redshift 6 is a universe only a billion or so years into its life, and the very beginning of the universe, if we could see it, would have a redshift of infinity.
So: a high-redshift galaxy is a distant galaxy that existed when the universe was young, and a low-redshift galaxy is a relatively nearby object living in what is basically the “modern” universe.
The distance-age-redshift relationship is incredibly useful in cosmology. But it relies on the fact that the recession speed always increases with distance in a known way. Wh
at if the expansion were to suddenly slow down? What if it were to stop, and reverse course? One consequence is that it would completely throw off our distance-measurement rules of thumb and upset a lot of astronomers. Another, nearly as important consequence, depending on who you ask: it would spell doom for the universe and everything in it.
WHAT GOES UP
For as long as we’ve known that (1) the universe started with a Big Bang and (2) it is currently expanding, the logical next question has been whether it will turn around and come back on itself, ending in a catastrophic Big Crunch. Starting with some very basic and reasonable physics assumptions, there appear to be only three possibilities for the future of an expanding universe, and they are all fairly direct analogs to what can happen to a ball thrown into the air.
You’re standing outside, on the planet Earth. You throw a baseball straight up. You have an inhumanly good arm, just for the sake of argument. (And air resistance isn’t a thing.) What will happen?
In the usual case, the ball goes up for a while, responding to that initial push you’ve given it, but it starts being slowed in its ascent by the gravitational pull of the Earth as soon as it leaves your hand.V Eventually it slows so much that it stops dead in the air and reverses course, falling back toward you and the planet you’re standing on. But if you were to throw the ball incredibly fast—specifically, 11.2 km/s, the escape velocity of the Earth—you could in principle give the ball so much of a push that it leaves the Earth entirely, slowing down slightly all the while, and only comes to rest infinitely far in the future (or, I suppose, when it hits something else). If you throw it even faster, it’ll be completely unbound from the Earth and just coast away forever.
The physics of an expanding universe follows very similar principles. There’s the initial push (the Big Bang) that set off the expansion, and from that point onward the gravity of all the stuff in the universe (galaxies, stars, black holes, etc.) works against the expansion, trying to slow it down and pull everything back together again. Gravity is a very weak force—the weakest of all the forces of nature—but it’s also infinite in range and always attractive, so even distant galaxies must pull toward each other. As in the baseball example, the question boils down to whether or not the initial push was enough to counteract all that gravity. We don’t even have to know what the initial push was; if we measure the expansion speed now, and also measure the amount of matter in the universe, we can determine whether its gravity will be enough to make the expansion eventually stop. Alternatively, if we can infer the expansion speed in the distant past, we can determine how the expansion is evolving over time by comparing that number against the expansion speed today.VI
If our universe were fated to someday suffer a Big Crunch, the first hint would be seen via just such an extrapolation. Before the collapse began, we’d be able to see that the expansion was faster in the past and had been slowing down, in a specific, doom-precipitating kind of way. Over time, with an increasing degree of certainty, we’d get signs of impending collapse many billions of years before it officially started.
But before we get into the data analysis, let’s stop to ask what the transition to a contracting universe and eventual apocalypse would look like, once it gets going. That’s really what you’re here for, after all.
Right now, the more distant an object, the faster it recedes, and therefore the higher its redshift (the Hubble-Lemaître Law). In a collapse-fated universe, this pattern will continue right up until the expansion stops completely—that top-of-the-roller-coaster moment. But since the speed of light prevents us from seeing the entire universe at once, we will still perceive distant objects receding long after they start turning around in actuality. Even though in some global sense the most distant objects are barreling toward us more quickly than nearby ones, at first we see the opposite behavior. Every galaxy nearby, out to just beyond our cosmic neighborhood, will appear to slowly come toward us. As with the Andromeda Galaxy, its light will be blueshifted. Just beyond those, there will be a distance at which everything seems to be standing still, while more distant things are redshifted, seeming to recede. Over time, the blueshifted nearby galaxies approach faster and faster, and the standstill radius moves out. Soon, we all stop worrying about what’s happening to distant objects as the rush of nearby galaxies into our region of space becomes impossible, or at least highly inadvisable, to ignore.
We might be slightly (if naively) reassured by the fact that we will have had some experience with such things by then: in this scenario, the first signs of collapse come long after our collision with Andromeda. Even with the most pessimistic estimates, any Big Crunch event can only occur many billions of years in the future—our universe has been around for 13.8 billion years and with respect to the possibility of future collapse, it is definitely no more than middle-aged.
As we already discussed, the Andromeda–Milky Way collision is unlikely to affect the Solar System directly. But the onset of universal collapse is another story entirely. At first, it might look fairly similar: galaxies colliding and rearranging, new stars and black holes igniting, some stellar systems flung off into space. Over time, though, it will become increasingly and terrifyingly clear that something very different is going on.
As galaxies get closer together and merge more frequently, galaxies across the sky will burst with the blue light of new stars, and giant jets of particles and radiation will rip through the intergalactic gas. New planets might be born along with those new stars, and perhaps some will have time to develop life, though the terrifying frequency of supernovae in this chaotic, collapsing universe might irradiate the new planets clean. The violence of the gravitational interactions between galaxies and between central supermassive black holes will increase, flinging stars out of their own galaxies to end up caught in the gravity of others. But even at this point, collisions of individual stars will be rare, and they will remain so until very late in the game. The destruction of stars comes about through another process, one that also ensures, with great finality, the destruction of any planetary life that might still be lingering on.
Here’s how.
The expansion of the universe as it is occurring today does more than just stretch out the light of distant galaxies. It also stretches out and dilutes the afterglow of the Big Bang itself. One of the strongest pieces of evidence for the Big Bang, discussed in the previous chapter, is the fact that we can actually see it, simply by looking far enough away. What we see, specifically, is a dim glow, coming from all directions, of light produced in the universe’s infancy. That dim glow is actually a direct view of parts of the universe that are so far away that, from our perspective, they are still on fire—they’re still experiencing the hot early stage of the universe’s existence, when every part of the cosmos was hot and dense and opaque with roiling plasma, like the inside of a star. The light from that long-burned-out fire has been traveling to us all this time, and, from sufficiently distant points, has just now arrived.
The reason we experience this as a low-energy, diffuse background (the cosmic microwave background) is that the expansion of the universe has stretched out and separated the individual photons to the point that they’re now merely a bit of faint static. And the fact that they show up as microwaves is due to extreme redshifting. The expansion of the universe can do a lot, including taking the heat of an unimaginable inferno and diluting and stretching it out until it’s just a faint microwave hum we might experience only as a tiny bit of static on an old-fashioned analog TV.
If the expansion of the universe reverses, this diffusion of radiation does too. Suddenly the cosmic microwave background, that innocuous low-energy buzz, is blueshifting, rapidly increasing in energy and intensity everywhere, and heading toward very uncomfortable levels.
But that’s still not what kills the stars.
It turns out that there is something that can create more high-energy radiation than concentrating the afterglow from space itself being on fire. As the universe has evol
ved over time, it has taken what was, at the very beginning of the cosmos, a fairly uniform collection of gas and plasma and used gravity to collect that gas into stars and black holes.VII Those stars have been shining for billions of years, sending their radiation out into the void to be dispersed, but not to disappear. Even the black holes have had their chance to shine, producing X-rays as the matter falling into them heats up and creates high-energy particle jets. The radiation produced by stars and black holes is even hotter than the final stages of the Big Bang, and when the universe recollapses, all that energy gets condensed too. So rather than being a nicely symmetric process of expansion and cooling followed by coalescence and heating, the collapse is actually much worse. If you’re ever asked to choose between being at a random point in space just after the Big Bang, or just before the Big Crunch, choose the former.VIII The collected radiation from stars and high-energy particle jets, when suddenly condensed and blueshifted to even higher energies by the collapse, will be so intense it will begin to ignite the surfaces of stars long before the stars themselves collide. Nuclear explosions tear through stellar atmospheres, ripping apart the stars and filling space with hot plasma.
At this point, things are really very bad. No planet that survived this long could possibly exist un-incinerated when stars themselves are exploded by background light. From here, the intensity of the universe’s radiation becomes so high that it can be compared to the central regions of active galactic nuclei, the places where high-energy particles and gamma rays shoot away from supermassive black holes with so much power they make jets of radiation a thousand light-years long. What happens to matter in an environment like that, after it’s reduced to its component particles, is uncertain. A collapsing universe will, in the final stages, reach densities and temperatures beyond what we can produce in a laboratory or describe with known particle theories. The interesting question becomes not “Will anything survive?” (because by this point it is very clear that the answer to that is a straightforward No), but “Can a collapsing universe bounce back and start again?”
