by Katie Mack
The next step in the distance ladder is a crucial one, but it’s also where things get really messy, in every sense of the term. In the previous chapter, we mentioned that a certain kind of supernova can be used to measure distances. This kind of explosion, a Type Ia supernova, is what happens when a white dwarf star somehow picks up some mass from another, equally unfortunate star and spectacularly rips itself apart. Because all white dwarf stars are fairly simple objects,IX and because the explosion is governed by physics that we feel we have a somewhat decent handle on, Type Ia’s were considered for a time to be good standard candles—the explosions all looked pretty similar. But it was later found that they’re better described as standardizable, in the same way Cepheid variables are. If you can measure how the explosion peaks and dims, you can get a good sense for the total amount of energy put out by the explosion, and thus an idea of how bright it really is.
Figure 16: Cosmic distance ladder. For bodies within the Solar System, we can use lasers or radar (in addition to relationships between orbital times and distances) to measure distances. Distances to nearby stars can be measured with parallax, and Cepheid variable stars can help us determine distances within the Milky Way and some nearby galaxies. For more distant sources, we can use Type Ia supernovae.
STAR LIGHT THERMONUCLEAR BRIGHT
But this book is about destruction, and I would be remiss if I glossed over Type Ia supernovae as just “a kind of exploding star.” A white dwarf star, the kind of star our Sun is fated to eventually become, is itself a marvel of stellar evolution. And when one explodes, it does so by undergoing an all-out full-body thermonuclear detonation that outshines its entire galaxy.
If you are any kind of star, no matter what stage you are at in your life cycle, your existence depends on a careful balancing act between the pressure produced in your core and the gravity of the material you’re made of. (We call this “hydrostatic equilibrium” but it really just comes down to the idea that the push out has to be equal to the pull in for a star to neither blow up nor collapse.) Most of the time, a star creates outward pressure by doing fusion reactions in its core—pressing nuclei together so tightly that they fuse and become a heavier kind of atom. For all the lightest elements, fusing them together produces radiation, and that radiation is the pressure that holds the star up against collapse.
For a star like the Sun, the outward pressure is provided by the fusion of hydrogen into helium. Most stars are, in fact, just giant helium factories, taking the abundant hydrogen in the universe and sticking it together, countless billions of times per second.
Let’s consider the Sun, in particular, for sentimental reasons.
Right now, the Sun is happily burning hydrogen, creating a surplus of helium in its core and causing the temperature and pressure to change over time as the hydrogen-helium balance tips. Because the efficiency of the factory depends on both temperature and pressure, the energy output and size of the Sun will change over time—most noticeably, the Sun will get a little bit more radiant and a little bit biggerX over the next few million years.
At somewhere around a billion years, we get to the part where we’re all fried. But even after the Earth is well on its way to becoming a charred lifeless rock, the Sun has a long way to go yet. As that increased heat is incinerating the inner planets (Mercury and Venus) and evaporating all the oceans off the Earth, so much hydrogen will be burned off that there will only be a shell of hydrogen burning around the central helium-filled core. The core then gets hot enough to start fusing helium into oxygen and carbon and turns the Sun into a huge bloated red giant star. When the Sun eventually runs out of all hydrogen to fuse, a few billion years into its red giant phase, it’ll begin its death throes in earnest. The core will start filling with oxygen, then carbon, the production fueled by the squeezing of the core by the gravity of the rest of the star. In the end, though, after the Sun has swelled to engulf the orbit of Venus and the Earth is a smoking ruin, the Sun’s gravity won’t be enough to maintain the temperatures needed for any further fusion. The outer atmosphere of the star will slough off, and the core will begin to contract.
You might think this would be the end of the Sun—depleted, transformed, and planet-devouring, left with no fusion reactions available that are strong enough to hold it up. Fortunately, there’s a kind of pressure even stronger than fusion reactions that can keep the post-red-giant Sun and other stars like it from collapsing entirely, allowing it instead to live out its convalescence as a white dwarf star. And this pressure comes directly from quantum mechanics.
A QUANTUM HEAP
The first thing you need to know is that most of the subatomic particles you know and love—electrons, protons, neutrons, neutrinos, quarks—are fermions, which, in this context, means they are fiercely independent, in a particle physics kind of way. Specifically, they obey the Pauli exclusion principle, which says that they won’t abide being in the same place and the same energy state at the same time. This is why, if you recall high school chemistry lessons, electrons attached to atoms end up in different kinds of “orbitals,” which are really just energy states.
Anyway, in the core of a burned-out, collapsing star, there are so many atoms, pressed so tightly together, their electrons start to get antsy. At those kinds of pressures, the electrons aren’t bound to specific atoms, but rather are packed in together in a big atomic mess so crowded that they have to jump to higher and higher energy states to keep from all being in the same one. This creates a kind of pressure, called electron degeneracy pressure, which is strong enough to halt the collapse of the star and create an entirely new kind of object: a white dwarf.
A white dwarf is a kind of star that isn’t burning at all. It has no fusion. It is a solid object held up entirely by the quantum mechanical principle that electrons just don’t like each other that much. And it can persist, silently smoldering, for billions and billions of years, until it slowly fades and cools and darkens, and is disintegrated in the Heat Death of the universe, ignited in the Big Crunch, or torn apart by phantom dark energy in the Big Rip, along with everything else.
Unless it gets just a little bit more mass.
Electron degeneracy pressure can do a lot. It can support an ENTIRE STAR. But only up to a point. If something happens to push the white dwarf past this point—it pulls in material from a companion star, or collides with another white dwarf—it will have too much mass for the degeneracy pressure to balance further collapse. Once that balance is tipped, a number of things happen in rapid succession.
The central core temperature of the star increases. Carbon begins burning. The material of the star starts to roil and churn, dragging more material in and out of the central flames. A deflagration tears through the star, creating a thermonuclear explosion so powerful that it rips the star apart, spectacularly and completely.
The explosion of a white dwarf star is so bright that it can briefly outshine its entire galaxy, and is visible to observers with telescopes billions of light-years away. Supernovae in distant parts of the Milky Way and nearby galaxies have even been seen without instruments, in ancient times, by the naked eye, in the daytime.XI
It’s a matter of some frustration in the astronomical community that, aside from this broad-brush-stroke picture, we still don’t know exactly how Type Ia supernovae happen. There are ongoing debates about whether they’re primarily caused by material falling onto the white dwarf from companion stars or by white dwarf collisions. Simulating the explosion ripping through the star is also computationally extremely difficult. Most simulations result in incredibly impressive visualizations of bubbling, churning stellar material without actually quite getting to the exploding part. But they’re working on it. (Stars, it turns out, are complicated. Especially when quantum mechanics and thermonuclear explosions are both important.)
The thing that makes us think we can learn anything useful from Type Ia supernova observations is the fact that we can reasonably expect that white dwarfsXII are pretty much alwa
ys at the same mass when they go off. In 1930, a twenty-year-old prodigy physicist from India named Subrahmanyan Chandrasekhar was on a ship traveling to England to begin his studies at Cambridge when he casually revolutionized stellar evolution in his free time. By improving on existing calculations and adding in important effects from relativity, he discovered a hard limit on the mass of any star held up by electron degeneracy pressure. That limit, about 1.4 times the mass of the Sun, became known, appropriately, as the Chandrasekhar Limit. Any white dwarf that gains enough mass to exceed that limit is immediately doomed to explode spectacularly as a supernova. And now that we know that the physics of the explosion is always the same, we know how bright a Type Ia supernova is intrinsically, and can therefore figure out its distance.
When Chandrasekhar’s ship finally reached the shore, his breakthrough tore through the scientific establishment like a detonation front of knowledge, forever changing our view of these weird and wonderful explosive stellar objects. (Though not everyone was convinced. Apparently, celebrated big-shot astronomer Sir Arthur Eddington,XIII whose work Chandrasekhar had refined, was not pleased about being outshone by this upstart, and made the young physicist’s life miserable for years before eventually giving in to superior calculational excellence.)
COSMIC POPCORN
The idea that all white dwarf stars explode when they gather enough mass to exceed the Chandrasekhar Limit gives astronomers hope that we can use these stars as distance benchmarks, with some tweaks to account for slight differences in stellar circumstances.
Exactly how well we can do this is still a matter of incredibly intense debate in the astrophysics community. Which is understandable, as the stakes couldn’t be higher. Type Ia supernovae are the gold standardXIV for distance measurements across vast expanses of the cosmos. They’re what allowed astronomers in the late 1990s to detect the accelerated expansion of the universe, and they’re what astronomers now use as their best handle on the nature of dark energy.
(It might sound odd to use massive stellar explosions as distance benchmarks, because, of course, we can’t predict exactly when or where one will go off. But it turns out that the stellar explosion rate is high enough—a good rule of thumb is one supernova per galaxy per century—and there are so many galaxies, that if we just take pictures of lots of galaxies every night, we’re likely pretty often to see a blip in one that wasn’t there the night before, and then we can follow it up with more detailed observations.)
The precision with which we can now calibrate galaxy distances with supernovae is impressive, with accuracies pushing toward the 1 percent level. This makes it possible to measure the expansion rate of the universe, by determining how distant the galaxies are and how fast they’re moving away. As discussed in Chapter 3, we talk about the expansion rate in terms of the Hubble Constant—the number that relates distance and recession speed. As of this writing, supernova measurements allow us to measure the Hubble Constant to an accuracy of 2.4 percent.
Which is weird, because the number we get totally disagrees with the value of the exact same number we derive from looking at the cosmic microwave background.
EXPANDING CONFUSION
For the last several years, measurements of the Hubble Constant from supernovae have been giving us a number around 74 km/s/Mpc—that means that a galaxy one megaparsec away (that’s around 3.2 million light-years) is receding from us at around 74 km/s. One twice as far away is moving, relative to us, about twice as fast. But we can also measure the Hubble Constant indirectly, by carefully studying the geometry of the hot and cold spots in the cosmic microwave background. When we measure it that way, the number we get is closer to 67 km/s/Mpc. Even though these observations are looking at very different epochs of cosmic history, each of them can tell us the expansion rate today. In a universe made of what we think it’s made of, both methods of determining the Hubble Constant really ought to give us the same number. And they don’t.
This hasn’t always been considered to be that big a problem, since no one thought either measurement was so incredibly precise as to settle the question. Until recently, the state of play was that the cosmic microwave background folk assumed that there was some distance ladder mis-estimate that would be sorted out eventually, dropping the number down a tad, and the supernova folk figured that the CMB measurements, which derive ultimately from attempting to measure the shape of space itself, were so complicated that surely something would show that the number was really just a little bit higher. This isn’t an unreasonable assumption, given the number of calculations and conversions that go into looking at a baby picture of the universe and converting that into a present-day expansion rate. And the distance ladder, likewise, really is fantastically complicated. Before even getting into all the possible biases that might creep in if you don’t account for every relevant property of the supernovae themselves, calibrating variable stars is not easy, and even distances to relatively nearby galaxies sometimes come with huge uncertainties. Part of this is due to how the populations of Cepheid variables we can see nearby are different from those far away, and… well, I could go on. Let me just say there are debates.
While the assumptions from each side that the other has done something wrong haven’t quite gone away, the situation is getting increasingly uncomfortable due to the fact that both sides are improving their methods, knocking out all known sources of measurement bias, and still finding numbers that ever more precisely do not agree with each other.
It’s unclear what the solution to this problem will end up being. Maybe it really does come down to systematic errors in the data, or some problem with the measurements themselves. Maybe it’s just a statistical fluke, as unlikely as that looks on the surface. Some of the most intriguing explanations involve dark energy that is not your garden-variety cosmological constant, but is instead something rather more ominous—something that could perhaps lead to a Big Rip. There’s one hypothesis that would go a reasonable way toward fixing the discrepancy between the measurements: dark energy getting more powerful over time, in just the way you might expect from the early stages of a phantom-dark-energy-dominated cosmos.
We probably shouldn’t panic just yet. As discussed, the data still aren’t that clear. Most measurements of w give a value that is fully consistent with -1, and though it’s true that values less than -1 are sometimes very slightly preferred, that preference isn’t really statistically meaningful. As for the Hubble Constant disagreement, even if all the measurements are correct, nonapocalyptic explanations for the discrepancy—involving weird models of dark matter, or altered conditions in the early universe—are very much in the running. In fact, even tweaking dark energy wouldn’t be enough to totally solve the problem, so it’s not unreasonable to assume that the solution might lie elsewhere. And even if there has been a sharp upturn in the effects of dark energy in recent cosmic history, suggesting something like phantom dark energy, we still have a LOT of time before a Big Rip could possibly occur.
In fact, the one thing that all the universe-ending scenarios we’ve already discussed have in common is that they definitely aren’t coming around anytime soon. As far as we can tell from our best understanding of physics, we have at least tens of billions of years before even the most extreme version of a sudden Big Crunch reversal could occur, and no Big Rip could be less than a hundred billion years off. A Heat Death, considered by most to be even more likely, would be so far into the cosmic depths of the future that we hardly have terms to describe it.
There is one possibility, though, that is decidedly more menacing than all the rest. It presents the prospect of a doomsday brought upon us by, in essence, a manufacturer’s defect in the fabric of the cosmos itself. It’s plausible, well described, and supported by the very latest results from the most precise fundamental physics experiments ever performed. And it could happen literally at any moment.
I. He has to be wrong about something.
II. In explaining the adoption of the term “phantom”
in the first paper to touch on this idea in 1999, Caldwell wrote, “A phantom is something which is apparent to the sight or other senses but has no corporeal existence—an appropriate description for a form of energy necessarily described by unorthodox physics.”
III. The newest one, called Gaia, is producing spectacularly detailed maps of the stars in our galaxy and is already giving us incredible insights into our cosmic history. What it will tell us about our fate is to be determined.
IV. When the danger is space itself, you want to be in a structure that has as little space in it as possible.
V. If you ask them, my colleagues will claim that their real motivation is understanding the nature of dark energy because of what it tells us about fundamental physics and our cosmological model. But I know it’s really the dread.
VI. Yes, we do this. It’s called laser ranging, and the only reason we CAN do it is because the Apollo astronauts left a mirror up there. It’s a handy tool for both seeing how far away the Moon is (fun fact: it’s drifting away from the Earth at almost four centimeters per year) but also for testing how gravity works, by watching the orbit very, very carefully.
VII. She wasn’t referred to as an astronomer at the time. She was one of a group of women called “computers,” who were hired to examine astronomical plates as cheap labor and who ended up doing a huge number of foundational calculations in astrophysics. Edwin Hubble, who used her discovery to measure the size and expansion of the universe, later said she deserved a Nobel Prize. Unfortunately, beyond being known and respected by her immediate colleagues, she was almost entirely unacknowledged in her lifetime.
VIII. I like to think of the bright Cepheids like giant lazy Saint Bernard dogs, while the dim ones are excitable jumpy Chihuahuas.