If stars don't cut it, what can? Whatever this standard candle could be, it has to be (a) common, so we can get a lot of measurements, (b) very bright, so we can see it reliably from far away, and (c) actually standard, so we can compare the measured to the true brightness and get work done.
In almost all circumstances, nature is sneakily cruel to us, offering only tantalizing hints of the mysteries of the cosmos, usually not enough to slake our thirst. But when it comes to cosmological questions, we have caught a couple of lucky breaks (it's still up for debate if nature willingly intended this or if they were just lucky accidents that slipped by her usually vigilant gaze). One case was the cosmic microwave background, which confirmed the big bang picture and unlocked the modern age of cosmology, and the other has to do with supernovae.
Stars are born, stars live, and stars die. Most stars, like our sun, simply exhaust themselves and extinguish their nuclear flames with only a modest flare or two to signify the ends of their lives. Some stars, the big ones, mark the end of their short, furious lives with explosions that literally light up entire galaxies—the supernovae.
Astronomers had been sporadically spotting new stars for millennia, and it was Tycho Brahe who named them novae when he extensively wrote about one that appeared in his sky one evening. By the late 1800s, it was realized that some novae were much brighter than others. Superlatively so, and the word supernova came into fashion.
As is the usual custom in science, the more a phenomenon like migration patterns or beetle larvae is studied, the more divisions, classifications, and subgroups get attached. It's no different with (super)novae, which, thanks in part to the work of our dear friend Fritz Zwicky, are helpfully organized into several different classifications with unhelpful naming schemes.4
Honestly, we don't need to care about the naming schemes and their origins and definitions. I know three of you are now going to throw away this book in disgust because I'm not going into detail on the subject, but for the rest of us, we're just going to pay attention to one particular kind of supernova: the type Ia. We're going to care about it the most because that's the one that cosmologists care about the most, and this is a book on cosmology.
In the 1990s astronomers noted, probably because they were looking for something exactly like this, that type Ia supernovae have a few desirable properties. While these kinds of supernovae are relatively rare, popping up a handful of times per century per galaxy, there are so many galaxies in the universe that they're basically happening all the time. They are exceedingly bright too: for a few weeks, a single supernova detonation will release more energy than an entire galaxy. That's a few hundred billion stars, for those keeping score. This means that we can see type Ia supernovae all over the place, even from incredibly distant galaxies.
So they're common and they're bright. Two out of three criteria met—but what about the third? You know, the most important one: do they have the same brightness? No, they don't. Well, almost, kind of.
When a supernova goes off, it quickly reaches a peak brightness, then slowly fades down over the course of a few days or weeks. Different type Ia's have different peak brightnesses, and they also have different cooldown times, which at first blush isn't surprising: supernovae are going to do whatever they dang well feel like. At first this might look hopeless as a standard candle, but the ever-ingenious astronomers noticed a pattern: the brighter a supernova reached at its peak, the longer it took to cool off. And not in a general, vague, hand-wavey sort of way; there was a very clear mathematical relationship between those two quantities.
That was the key that opened the door to the distant universe. Type Ia supernovae aren't standard candles, but they are standardizable candles. With a little bit of finagling,5 you can capture a random supernova in some remote stretch of the far-flung cosmos and calculate its true, inherent brightness, the same brightness you would measure if you were right there in front of it, having your face melted by the blast. You can then compare the true brightness to the brightness you measure safely on Earth, and calculate a distance.
Even at a distance of fifty million light-years, this type Ia supernova still manages to dazzle, briefly expending more energy than its entire host galaxy. (Image courtesy of NASA / ESA.)
But wait, there's more! The magnificent explosions are cacophonies of light, featuring the usual assortment of absorption and emission spectral lines that astronomers of the nineteenth century learned to love. A reliable spectral line means you can get a redshift, which means a velocity (and if we can't use the spectrum from the supernova itself, we can always use the one from its host galaxy). This is the exact same technique that was unlocked more than a hundred years prior, was utilized to great effect to discover the motion of stars, and decades later revealed the expanding universe to Hubble's amazed eyes. And now it was brought to its ultimate conclusion, its final form: studying distant dying stars to prize out one last datum of cosmological significance from the fading embers of those cataclysmic explosions. Their stellar sacrifice is our gain, thanks to a practice developed in the steam age.
Two key pieces of info for the price of one (difficult) measurement: an unlikely gift from nature. If a type Ia supernova goes off in a distant galaxy, you can capture not only its distance but its speed. So not only do you get to extend the cosmic distance ladder to new celestial heights, you can also get a handle on the expansion rate between us and a distant point. And by figuring out the expansion rate at different points in deep time, you can tease out the matter and energy contents of the cosmos and figure out why all the other astronomers are hating on each other.
Of course, there's way more to the distance ladder—it's more than three rungs. There are more steps in between Cepheids and supernovae with fantastic names like RR Lyrae variables and the Tully-Fisher relation. Those aren't as useful for cosmological purposes directly, but they do serve as important cross-checks: each rung on the ladder isn't entirely separate, but overlaps with others, so we have multiple independent ways of measuring and calibrating distances. For the craziness I'm about to share with you, it's important to mention that part, because what cosmologists have recently revealed about our universe is easy to dismiss (that's how weird it is) if you don't know how robust the measurement really is.
I'm also purposely keeping you in the dark (for now) about the actual physics of type Ia supernovae, because as I said with Cepheids, it doesn't matter. What matters for the ultimate goal—measuring the expansion history of the universe—is that we can identify a standard candle. Who cares how the candle is made? All we need to know is that we can estimate its true brightness to a certain level of accuracy. Let's not make our lives messier than need be. Words of wisdom from the cosmological community indeed.
So here's the game plan: measure a bunch of type Ia supernovae (only type Ia behave in this friendly way, so unfortunately we have to stick to them). Measure a bunch more. Toss a few more in for good statistics. Calculate their distance and speed. Estimate the expansion history of the universe over the past few billion years. Use general relativity to calculate what the matter and energy contents have been over those past few billion years. Resolve dispute among astronomers and cosmologists. Sit back and relax.
Two independent teams in the 1990s went about exactly that, and their goal was to measure the deceleration, however slight, of the universe's expansion. That's because any amount of matter, however slight, would eventually pull in the reins on our current expansionary phase. It may not be enough to fully stop it—that requires a lot more matter than we could ever find—but the presence of matter in the universe slows down expansion over the course of billions of years. So by carefully pinning down that deceleration, they could tell everybody else what's out there in the cosmos.
The two teams, working independently, managed to measure a deceleration—but with a minus sign in front. The distant supernovae that they collected were too dim. They were farther away than they ought to be. It was 1998, and after checking their work over
and over again, and a few furtive phone calls between the two groups (“Are—are you seeing what we're seeing?”), the world came to know that the expansion of our universe is accelerating.6
The collaborations also quietly scribbled out the word “deceleration” from their project subtitles.
We live in an expanding universe; it's getting bigger and bigger every day. We've had a century to become grudgingly comfortable with the concept. But now, at this very moment, the expansion is accelerating. It's getting bigger and bigger faster and faster every day. To really drive the point home, the universe is expanding faster than if it were totally empty of all matter. Like finding your favorite comfort food at the buffet, there's no going back.
In a replay of the Shapley-Curtis debates from generations past, the supernova results were the ultimate compromise. They satisfied everybody by satisfying nobody. Everyone was right because they were all wrong. The answers clicked into place in a way that nobody enjoyed. You go into the doctor with a broken finger and a nosebleed, and you come out with your amputated finger sewn onto your nostril. You're technically healed, but not in the most pleasant or straightforward way.
The universe is indeed flat—parallel lines stay parallel, and triangles add up to an agreeable 180 degrees. The inflation theorists and cosmic microwave background observers were vindicated. And the universe is only about 30 percent matter. The astronomers had done a fine job accounting for all the mass after all. What made up the difference, what filled up the universe from edge to edge, was a previously unknown substance (or, at least, an effect). This substance is causing the expansion of the universe to accelerate unbridled.
We had a good thing going with dark matter, sounding all cool and mysterious and a little sassy, so why not extend the concept? We have absolutely no clue what's behind this accelerated expansion, so let's call it—drumroll, please—dark energy.
Welcome to the modern universe. It doesn't make any sense.
What is dark energy? There isn't much to say on the subject, because if you didn't catch it the first time, we have no idea what it is. “Dark energy” is a sweet name for an observed phenomenon—the accelerated expansion of the universe—but that doesn't really illuminate (snicker) the cause of that expansion. It's worse than our current problems with dark matter; at least there we have some ways of navigating the theories and putting experiments online. With dark energy, even twenty-plus years after the initial detection, we're still in the groping-around phase.
But let's paint the picture, hazy as it is, as we've got it right now. We can start with the reason we call it “energy,” and it's not because “matter” was already taken and there are only two kinds of things in the universe (and really, if you want to get pedantic—and who doesn't?—matter and energy are two sides of the same coin, but that's another book).
It's your birthday and I get you a present, because I'm pretending to be your friend for the purposes of this explanation. You undo the intricate bow and carefully unwrap the package, opening the box to find…nothing. Completely empty. A pure vacuum, in fact. No matter, no radiation. Not a single particle, not a single photon. Absolutely pure nothing.
You politely mask your disappointment and thank me, internally rolling your eyes that I'm using a special event like your birthday for another stupid science thought experiment. You sigh, preparing yourself for the inevitable monologue. Here it comes.
Your empty box is anything but—it's actually full of dark energy. Dark energy fills the cosmos to the brim. It's a property of the vacuum of space itself. Have a small empty box? You have a little bit of dark energy. Big empty box? A lot more dark energy. Since we live in an expanding universe, we're getting more and more vacuum, more empty space, and hence more dark energy every day.
Now it just so happens that when you take a material with this property—that you get more of it when you expand the volume of its container—the math of general relativity produces a surprising result, and you can probably guess what it is: you get accelerated expansion. The very fact that dark energy is a property of the vacuum means that it drives the continued creation of itself. The universe has dark energy. It expands. It has more dark energy. This pushes the expansion a little bit faster. Even bigger universe. Even more dark energy. Even more accelerated expansion. Rinse and repeat if desired.
And before you interrupt me, I'll interrupt myself: you may be familiar with the concept of conservation of energy (usually incanted in some fashion like “Energy cannot be created or destroyed, it can only change forms”), and you're probably asking yourself why this first pass at explaining dark energy doesn't violate rule 1 of How Things Work. The answer is very simple and very annoying: energy isn't always conserved. It's conserved in a few special systems, especially systems found in homework problems of physics textbooks, but nature is much more subtle than that. In a changing universe (like, say, one that's expanding), energy can be added at will. It's just not a big deal, and we're all going to have to get comfortable with that if we're going to make any progress, OK?
Anyway, this accelerated expansion business didn't get started in earnest until about five billion years ago. It's not like dark energy just poofed into existence by some poorly understood process. Dark energy has always been here. It's a property of the vacuum of space-time. It's there, right in front of your face and inside your very bones. It's been with the universe since the earliest moments of the big bang itself. But it's been hidden, in the background, unimportant.
It's a game of densities: the same game that's been played for billions of years in our expanding universe. Matter finally won out over radiation, eventually causing the release of the cosmic microwave background, because the expansion diluted the radiation and dropped its density well below thresholds where anybody would notice or care. And while matter had its eventual—and inevitable—triumph, its reign was not forever. There's only a fixed amount of particles—dark or otherwise—available in the universe. Day by day, cubic meter by cubic meter, the density of matter has been dropping.
But dark energy's distinguishing feature is constant density. The bigger your universe, the more total dark energy there is. While matter is dominating, its gravitational attraction overwhelms any inclination dark energy might have to accelerate the cosmos. Indeed, the expansion of the universe slowed down over the course of the first few billion years. But like too little butter spread over too much bread, matter lost its grip on the fate of the cosmos. It's simply not a player anymore. Starting five billion years ago, the density of matter slipped below that of dark energy and continued falling. Dark energy dominates the modern-day universe. It's the single largest component. It's in the driver's seat now, and it doesn't even know where the brake pedal is.
I think it's important to remind you that you can sleep soundly tonight—dark energy only makes itself noticeable on large scales. Clusters are still gravitationally bound together. Galaxies safely keep their stars within their embrace. The planets still twirl in their waltzes around their suns. Dark energy is persistent but weak—any place where another force is stronger, the accelerated expansion can't take hold. Just as an expanding universe doesn't mean the Earth is expanding, an accelerated expansion only takes place in cosmological settings.
Unfortunately for the cosmic web, it can't sleep soundly tonight. Beyond the scale of already-formed clusters, there's not enough attractional oomph to overwhelm the tearing tendency of dark energy. Like a swimmer caught in the riptide, getting farther away from the shores no matter how hard she fights.
It's happening in the voids first. They're already empty of most matter, and it's there that dark energy is most dominant, expanding and inflating them, pushing their boundaries ever larger. The tenuous cosmic web exists now only as gossamer strands and almost-transparent walls between the indomitable nothingness of the voids. Eventually those grand structures will dissolve. As the cosmic web becomes unspun, only the isolated clusters, mere pockets of gravitational attraction strong enough to stand fast,
will remain, becoming remote islands separated by vast black wastelands ruled by one thing and one thing only: dark energy.
Pretty dark stuff (pun most definitely intended). I'll get to the future fate of the cosmos in more gruesome detail later, but for now I want to clear up a few loose ends. First off, this business of energy. Hmmm, what else fills up the vacuum, permeating all of space-time within the entire universe? Good question—I'm glad you asked. Quantum fields do! The buzzing and vibrating substrate has all the needed properties to explain dark energy. And it is an energy too—a vacuum energy. At the ground state, the bare minimum allowable configuration, the quantum fields that constitute our reality have some base energy, and they are perhaps ultimately responsible for the accelerated expansion of the universe itself.
We've broken open the quantum field In Case of Emergencies box before, to explain the driving force behind inflation and to seed the growth of the largest structures in the universe, so at first blush, this seems like a natural extension of those strategies. Have a mystery in the universe? Just blame quantum fields—look, look, they're practically oozing guilt on their faces. It's easy. Everyone will believe you.
One small, tiny, niggling caveat: it doesn't work. I have a confession to make. When physicists go to calculate the actual amount of vacuum energy—like, actually produce a number instead of just talking about it vaguely—they get infinity. That's right: infinity. It turns out that all the wiggling and jiggling those quantum fields do at a subatomic level keeps adding and adding to itself without end. The fundamental (ha!) problem is that the wiggles can be as small as they want. It's like trying to listen to an orchestra with an infinite number of instruments that can play at every frequency imaginable. All the sounds add up to an infinite amount of energy and your ears bleed and your head explodes.
That's…kind of a problem, which we saw earlier in the saga of physicists first trying to extend the work of Dirac and marry special relativity to quantum mechanics. Fortunately for most calculations of importance, the absolute value of this vacuum energy doesn't matter. I know it's weird and nonintuitive to think about, and I'm truly sorry about that, but that's physics for you. You can do your particle science on the first floor or the tenth floor or the infiniteenth floor; it doesn't matter.
Your Place in the Universe Page 21