Astrophysics for Young People in a Hurry Read online

  This massive runaway star is traveling so fast that it creates a shock wave out in front of itself—visible here as a curved red streak.

  Other stars remain adrift. Our observations suggest that there may be as many homeless stars as there are stars within the galaxies themselves.

  Exploding Runaway Stars

  Some of astrophysicists’ favorite cosmic events are supernovas, stars that have blown themselves to smithereens and, in the process, glow a billion times brighter for a period of several weeks. With advanced telescopes, we can see supernovas across the universe. Most of them happen within galaxies, but scientists have found more than a dozen supernovas that exploded far away from any galactic neighborhood. Normally, for every star that goes supernova, a hundred thousand to a million nearby stars do not. So those dozen exploding stars out in the middle of nowhere may be clues to the existence of many more stars that we cannot see.

  What could possibly be more awesome than a runaway star? An exploding runaway star! The one shown here is ejecting gas and dust.

  Some of those undiscovered, unexploded stars could be similar to our own Sun.

  Planets could be orbiting those stars, and maybe even supporting intelligent life.

  Million-Degree Gas

  Matter, the stuff from which everything in the universe is built, generally comes in three forms or phases: solid, liquid, and gas. The easiest example is water, which is ice in its solid form, clear and drinkable in its liquid state, and transforms into vapor when it becomes a gas.

  Some telescopes have revealed a gas that stretches across the spaces between galaxies, glowing at tens of millions of degrees. Even though it’s not clumped together, this gas is still made of matter. It’s also very, very hot.

  When galaxies move through this superheated gas, it strips them of all their excess matter, like the lunchroom bully who grabs the chocolate chip cookie off your tray as you walk past. The superheated gas doesn’t just ruin a galaxy’s day, though. By removing all that extra matter, it prevents the galaxy from making new stars.

  Faint Blue Galaxies

  Outside the major clusters, there is a population of galaxies that thrived long ago. As we already discussed, looking out into the cosmos is like looking back in time. The travel time for light to reach us from distant galaxies can be millions or even billions of years.

  When the universe was one-half of its current age, a very blue and very faint type of midsized galaxy dominated. We see them today. They are difficult to detect not only because they are far away but because they contained so few bright stars. These faint blue galaxies no longer exist. What happened to them is a cosmic mystery. Did all their stars burn out? Have they become invisible corpses spread across the universe? Did they change into the dwarf galaxies we see today? Or were they all eaten by larger galaxies?

  Did all of them become lunch?

  We don’t know.

  Vacuum Energy

  Even empty space isn’t really empty. We refer to these regions as vacuums—not the noisy household cleaning machines, but areas that contain no matter or energy at all. But in these supposedly empty regions, oceans of virtual particles are constantly popping in and out of existence. When they meet, they often destroy each other and release energy. These miniature collisions create what scientists call “vacuum energy”—an outward pressure that acts against gravity and may help drive the expansion of the universe.

  Why We Hate Vacuums

  There is an old saying in science that nature abhors, or hates, a vacuum. It is an established fact that children detest vacuums, too. So do dogs. But these truths refer to the cleaning appliance. How would you feel about the intergalactic version of the vacuum? I suspect you wouldn’t be overly fond of it, either. As detailed earlier in the chapter, it wouldn’t be a very nice place to hang out. Why nature hates vacuums, and insists on filling them with strange activity, we do not know. It just does.

  With all this stuff between the big galaxies, some of it might block our view of what lies beyond. This could be a problem for the most distant objects in the universe, such as quasars, pronounced kway-zarz. Quasars are the incredibly bright centers of galaxies—in scientific terms, superluminous galaxy cores. Their light has typically been traveling for billions of years before reaching our telescopes.

  The quasar in this artist’s illustration is shining a beam of energy across the cosmos.

  Quasar light changes slightly as it races through gas clouds and other space junk, and astrophysicists can study this light to reveal what happened along that billion-plus-year journey. For example, we can tell if the quasar light passed through multiple gas clouds. Every known quasar, no matter where on the sky it’s found, shows features from dozens of different clouds scattered across time and space.

  So even though these clouds aren’t visible, we know they are there.

  The combination of hungry galaxies, runaway stars, and superheated gas clouds certainly makes intergalactic space an interesting place. Add in those super-duper high-energy charged particles and the mysterious vacuum energy, and one could argue that all the fun in the universe happens between the galaxies rather than within them.

  But I wouldn’t suggest vacationing there. Your trip might at first be interesting, but it would end very, very badly.

  5.

  Dark Matter

  Years ago, when my daughter was a toddler, she performed a fascinating experiment from her booster seat. As I watched, she carefully dropped nearly two dozen overcooked peas from her dinner plate. She let them go one at a time, and not a single pea disobeyed the universal law of gravity. Each one fell directly to the floor.

  Gravity is a marvelous force, but a troubling one.

  Newton and Einstein explained how gravity affects the matter in the cosmos. Overcooked peas, ripe apples, people, planets, giant stars—their ideas apply to all the matter that we can see, touch, feel, smell, and occasionally taste. And according to Newton and Einstein, most of the matter in the universe appears to be missing. I don’t mean “missing” in the sense of a lost sock.

  By watching certain stars and galaxies, astrophysicists can measure the power of gravity in distant parts of the cosmos. Typically, if gravity is strong, we see a large object or objects nearby. The effect around a giant star or a black hole will be enormous, for example. The gravity around a tiny space rock drifting through the cosmos? Not so much.

  For years, astrophysicists have been tracking incredibly powerful gravitational fields without enough visible mass to create that power. Something has to be there, generating all that gravity. But we don’t see the stuff. Whatever is there doesn’t interact with “our” matter or energy. We’ve now been waiting nearly a century for someone to tell us why most of the gravity we’ve measured in the universe—about eighty-five percent of it—is tied to some kind of material we can’t detect.

  We are essentially clueless.

  This is a major scientific riddle, and we find ourselves no closer to an answer today than when this “missing mass” problem was first uncovered in 1937. At the time, the Swiss-American astrophysicist Fritz Zwicky was studying the movement of galaxies within a huge region called the Coma cluster. This cosmic neighborhood lies very far from Earth. A beam of light leaving the Coma cluster has to speed across the universe for 300 million years to reach our telescopes.

  The Coma cluster appears delightfully crowded from a distance. A thousand galaxies orbit around its center, moving in all directions like bees swarming in a beehive. Gravity holds the cluster together, preventing anything inside from drifting away. Zwicky measured the strength of this gravitational field by watching a few dozen of the galaxies inside.

  Astrophysicist Fritz Zwicky first found evidence of the mysterious dark matter in this group of galaxies known as the Coma cluster.

  But something wasn’t right.

  There was just too much gravity. So he added up the mass of all the galaxies inside. Even though Coma ranks among the largest and most massive cluste
rs in the universe, the sum total was not enough to crank out the kind of gravity that would hold all these galaxies in place.

  Something else was there.

  Something he could not see.

  After Zwicky, astrophysicists have discovered other galaxy clusters with the same problem. This “missing mass” remains the longest-standing unsolved mystery in astrophysics.

  Today, we’ve settled on a term for the stuff: “dark matter.”

  As a kid, I lived in one of two matching apartment houses. My close friend, a classmate in elementary school, lived in the other building. Thanks to him, I learned how to play chess and poker and the board games Risk and Monopoly. More importantly, he taught me how to really use binoculars, to point them at the Moon and the stars. As I switched from binoculars to telescopes and moved from the rooftop of my building to the clear views from the desert or the open sea, I fell in love with all the amazing sights scattered across the night sky.

  Yet astrophysics isn’t just about what we see. It also deals with what we don’t see.

  Fritz Zwicky found evidence for matter he could not see within clusters or groups of galaxies. Years later, in 1976, Vera Rubin, an astrophysicist at the Carnegie Institution for Science in Washington, discovered missing mass hiding within the galaxies themselves. She was studying spiral galaxies: flat, disk-shaped collections of stars with a bright bulge in the center and several star-packed arms twisting outward. Rubin tracked how fast the stars race around the spiral galaxy centers. At first, she found what she expected. The stars farther from the center, held tight by gravity, moved at greater speeds than stars close in.

  Now you understand why we call them spiral galaxies, right? This one may contain a trillion stars.

  Yet Rubin also watched the areas beyond this disk. A few bright stars and lonely gas clouds lingered out there. Since there was little visible matter between these objects and the edges of the disk, nothing was holding them tight to the rest of the rotating galaxy. Their speed should have been falling with increasing distance out there in Nowheresville. But for some reason, their speeds in fact remained high.

  Rubin correctly reasoned that a form of dark matter must lie in these far-out regions, holding onto those distant objects, hiding well beyond the visible edge of each spiral galaxy. Thanks to Rubin’s work, we now call these mysterious zones “dark matter haloes.”

  This halo problem exists under our noses, right here in the Milky Way. From galaxy to galaxy and cluster to cluster, the difference between the combined mass of the stuff we see and the amount of mass that should be there based on gravity’s strength is enormous. Cosmic dark matter has about six times the total gravity of all the visible matter. Or, to put it another way, there is six times as much dark matter as normal matter.

  Studying dark matter haloes like this one allowed astrophysicist Vera Rubin to find more evidence of missing mass in the cosmos.

  Dark Matter Detective

  As a kid, Vera Rubin watched the stars from her bedroom window, then built her first telescope out of a cardboard tube. She caught the bug early. After college, she applied to Princeton University to earn an advanced degree in astrophysics, but the school told her that the program did not accept women. That didn’t stop Vera Rubin. She went on to earn her degree from another university and use her studies of spiral galaxies to prove that dark matter really does exist. Many people believe Rubin should have been awarded the Nobel Prize for her work. After all, the greatest award in science goes to discoveries, and what could be more worthy than the discovery of dark matter, the mysterious substance that glues together galaxies?

  So what is this dark matter?

  We know it cannot be made of ordinary matter like protons, neutrons, and the rest. We’ve also ruled out black holes and other cosmic oddities. Could the dark matter just be asteroids or comets? Planets wandering through space, untied to a solar system? They all have mass, but none of them produce any light of their own. They would appear dark to our detectors. In that sense, they fit. But there would not be enough of them, so we have to rule out wandering planets.

  We also know that dark matter can’t be made of the same particles as planets or humans or hamburgers because it doesn’t seem to follow the same rules. The forces that bind the particles in our world together don’t apply to dark matter. The only rule dark matter seems to follow is gravity.

  Maybe there’s nothing the matter with the matter, and it’s the gravity we don’t understand. Maybe Newton was wrong. Einstein, too. Maybe you, reader, will eventually discover, while cruising past an apple orchard in your self-driving robotic car, how gravity really works. In the meantime, we have to work with the facts we have now. And as best we can figure, dark matter isn’t just matter that happens to be dark.

  Instead, it’s something else altogether.

  Don’t worry. You’re not going to hit your head on a clump of dark matter while tiptoeing to the bathroom at night. You won’t trip over a pile of it on your way from one class to the next in the crowded halls of your middle school, although you are more than welcome to use that as an excuse should one of your less scientifically minded classmates mock your accidental stumble. Dark matter lives in galaxies and galaxy clusters. For the smallest objects, such as moons and planets, we see no effect. The gravity on Earth can be explained entirely by the stuff that’s under our feet. Down here, at least, Newton got it right.

  So what is dark matter made of? What do we know about it? Normal matter clumps together into molecules and objects of all sizes, from tiny grains of sand to giant space rocks. Dark matter does not. If it did, we would find chunks of dark matter dotting the universe.

  We’d have dark matter comets.

  Dark matter planets.

  Dark matter galaxies.

  As far as we can tell, though, that’s not the way things are. What we know is that the matter we have come to love in the universe—the stuff of stars, planets, and life—is only a light frosting on a much larger, darker cosmic cake.

  We don’t know what it is. But we do know that we need dark matter. We always have.

  During the first half million years after the big bang, a mere eyeblink in the fourteen-billion-year sweep of cosmic history, matter in the universe had already begun to come together into loose blobs. These blobs would become clusters and superclusters of galaxies. But the cosmos would also double in size during its next half million years, and continue growing after that. During this growth period, two effects were competing with each other.

  Gravity was working to bring everything together.

  The expanding universe was working to spread everything out.

  The gravity from ordinary matter could not win this battle by itself. We needed the added strength of the gravity from dark matter. Without it, we would be living in a universe with no structures.

  No clusters.

  No galaxies.

  No stars.

  No planets.

  No people.

  Without dark matter, we would not be here at all.

  So dark matter is our frenemy. We have no clue what it is, and in that sense, it’s kind of annoying. But we desperately need it. Scientists are generally uncomfortable whenever we must rely on ideas we don’t understand, but we’ll do it if we have to. And dark matter is not the first time we scientists have had to depend on something mysterious.

  In the nineteenth century, for example, scientists measured the energy output of our Sun and showed its effect on our seasons and climate. They knew the Sun warmed us and provided some of the energy needed for life. But they had no idea how the Sun actually worked until a woman named Margaret Burbidge and her colleagues figured it out. Before Burbidge, the Sun was just as mysterious to scientists as dark matter. Some scientists proposed that it was really a burning lump of coal.

  Why the Sun Shines

  Stars like our Sun began as giant gas clouds. Gravity collapses these clouds, shrinking them smaller and smaller, and making them hotter and hotter. Some gas clou
ds will stop collapsing and settle as a giant, glowing mass. But others, like the one that formed our Sun, are so large that they trigger a process called thermonuclear fusion. Hydrogen molecules in the core slam into each other and combine—or fuse—and then release energy. The energy from all these little collisions pushes out against gravity, preventing the cloud from collapsing further, and provides enough energy to let the Sun shine.

  Dark matter is a strange idea, but it is grounded in facts. We assume it is there because of the work of Vera Rubin and Fritz Zwicky and what we still observe today. Dark matter is just as real as the distant planets astronomers have discovered in recent years. Scientists have never seen or touched or felt these exoplanets, planets that exist outside our solar system. But science is not just about seeing. It’s about measuring unseen effects, too, preferably with an instrument that’s more powerful and sensitive than your eyes. We know these exoplanets are real because we use our amazing instruments to study the stars they orbit. In examining those stars, we uncover solid clues of the planets’ existence.

  The worst that can happen is we discover that dark matter does not consist of matter at all, but of something else. Could we be seeing the effects of forces from another dimension?* Are we feeling the ordinary gravity of ordinary matter that exists in a phantom universe next to ours? If so, this could be just one of an infinite assortment of universes in a larger multiverse. There could be infinite versions of Earth. An unlimited number of versions of you.