Astrophysics for Young People in a Hurry Read online

  So, in a word, far.

  When Superman first travels to Earth, his ship carries him faster than the speed of light. Yes, this is impossible, as we discussed in the last chapter. But since they are actually intelligent aliens, maybe they figured out how to create wormholes and travel through them. This gets you wherever you want in the universe by taking a shortcut.

  Wormholes

  One of Albert Einstein’s big ideas was that gravity could actually change the shape of space, turning straight lines into curves. But if you stretch this idea to its extreme, then it becomes possible to bend whole sections of the universe and bring distant locations close together. Pretend our universe is reduced to a simple piece of paper. If you were to draw a picture of Earth in one corner, then add a circle representing Krypton in the opposite corner, the shortest distance between the two would be a straight line, right? Normally, yes. But if gravity were to bend this flattened universe, and you folded the paper over so that the two planets were close to touching, then the shortest distance changes. A wormhole—Einstein referred to it as a bridge—is a kind of tunnel through space that connects these distant points. We don’t know if they actually exist, or if you’d be able to travel through one safely in a spaceship without every atom in your body being torn apart, but science fiction writers certainly do love them.

  Superman arrives on Earth, but when his sun explodes, the light from that event has to travel across space at the usual speed. While Superman is growing up on Earth, learning about farming, memorizing his state capitals, and discovering his powers, the light beams from that exploding star are still on their way across the cosmos.

  When he becomes an adult, moves to Metropolis, which is really just a variation of my hometown, New York City, and transforms into the famed Man of Steel, the light beams are still traveling.

  When he falls in love with Lois Lane, those beams of light still haven’t arrived yet.

  When he heads to Mars to fight off the invading aliens, the photons are finally getting close. Since the star is twenty-seven light-years from Earth, and his sun explodes right after his birth, Superman is twenty-seven years old when the light from that supernova finally reaches our telescopes.

  That’s when the Man of Steel rushes to the Hayden Planetarium to visit yours truly. In the story, the comic version of me has arranged it so that all of Earth’s most powerful telescopes are aimed toward Corvus, to capture as much light as possible—visible and invisible.

  This a terribly sad moment for the big guy. He finally sees for certain that his home planet has been toasted by a supernova. But it’s a perfect illustration of one of the strangest things about astrophysics—or even nature itself. We’ve been over this idea already, but it’s worth revisiting. Light needs time to travel from its source to our telescopes. So, whenever we look at something, whenever the light from an object strikes our eyes, we are really seeing that thing as it was in the past, when the photons first started on their journey. The farther out in space we look, the longer light has to travel, and the farther back in time we see.

  Looking back in time twenty-seven years, as Superman and I do in the comic book, is normal for astrophysicists. Today, our telescopes and detectors allow us to gaze billions of years into the past. We can almost see back to the very beginning of the universe. For this, we can thank a pair of scientists named Arno Penzias and Robert Wilson, who made one of the greatest astrophysical discoveries of the twentieth century by accident.

  In 1964, Penzias and Wilson were working for Bell Telephone Laboratories, the research branch of AT&T (American Telephone and Telegraph), the same company that provides wireless and smartphone service today. We’ll cover this in more detail in chapter 9, but the sky is filled with different kinds of light energy. Some, like the familiar colors of the rainbow, are visible. Others are invisible. But they are all waves, and one of the main differences between these forms of light is their wavelengths, or the distance from one peak in a wave to the next. AT&T had built a giant, horn-shaped antenna to send and receive radio waves.

  Scientists at Bell Telephone Laboratories used this antenna to learn about the birth of the universe.

  Penzias and Wilson pointed their giant antenna at the sky, but wherever they aimed the device, it picked up another form of light, microwaves. Today, most American kitchens have microwave ovens, which cook or reheat food by flooding it with these long, invisible, low-energy waves. But why were the scientists finding all these microwaves?

  Penzias and Wilson were stumped.

  They searched for potential sources, both on Earth and in space. In almost every case, they could explain where the light was coming from. But one microwave signal remained a mystery. No matter where they pointed the antenna, the scientists found this signal. Naturally they wondered if something was wrong with their detector. The two scientists looked inside the antenna and found pigeons nesting there. The antenna was also covered with a white substance.

  Pigeon poop.

  The pigeon droppings covered most of the dish. So the mysterious microwaves could have just been a dirty antenna. Penzias and Wilson cleaned away the substance, encouraged the pigeons to find a new place to live, and tested the instrument again.

  The signal dropped slightly, but not completely. So it wasn’t all the pigeons’ fault. Yet the scientists still did not have an explanation for the mysterious light.

  Meanwhile, a team of physicists at Princeton University, led by Robert Dicke, heard about their work. But unlike Penzias and Wilson, they knew exactly where the strange light was coming from.

  Penzias and Wilson didn’t have a pigeon poop problem.

  They had discovered light from the early universe.

  After the big bang, the universe expanded rapidly.

  The cosmos has many mysterious rules, as we have discussed, and one of these says that energy cannot be created or destroyed. This is known as the law of conservation of energy, and you can’t break it. Really. All the energy in our universe today was around at the time of the big bang. As the cosmos grew, all that energy was stretched out over a larger and larger space. With each passing moment the universe got a little bit bigger, a little bit cooler, and a little bit dimmer.

  For 380,000 years, things carried on that way.

  In this early period, if your mission had been to see across the universe, you couldn’t. You would need to see photons that had made this cosmic trip, but back then, photons couldn’t travel very far. Have you ever tried to leave your house only to have a parent stop you at the door and remind you of an unfinished chore or some neglected homework? Such was the life of a photon. Electrons were constantly stopping them before they’d even started on their journey. Since the photons couldn’t get anywhere, there was nothing to see. The universe was a glowing fog in every direction.

  As the temperature dropped, however, the particles moved more and more slowly. Eventually the electrons slowed just enough to be captured by passing protons. Once electrons and protons had joined together, we had atoms.

  So what does this have to do with pigeon poop?

  Now that the protons were grabbing those electrons, nothing was stopping the photons anymore. They were set free to travel on uninterrupted paths across the universe.

  As they raced across the cosmos, the universe continued expanding and cooling. The photons became weaker and weaker. At first, they were energetic enough to be visible—the kind of photons our eyes capture when we’re staring at the printed or electronic page of a book. After traveling for millions and then billions of years, these photons cooled. They were stretched out, transforming into long, low-energy microwaves. Together, all these long-traveling photons make up what we call the “cosmic microwave background.”

  Don’t let that somewhat fancy scientific name confuse you. And please resist the temptation to imagine a giant microwave oven floating somewhere out in space. The cosmic microwave background is the leftover light from a dazzling, sizzling early universe.

  And it is
the same light that Penzias and Wilson caught with their dish.

  George Gamow

  In addition to being an influential cosmologist, George Gamow was a successful teacher. One of his students, Vera Rubin, would make important discoveries about dark matter, the mysterious substance holding together distant galaxies. Gamow even wrote books for children. One series features a character named Mr. Tompkins who embarks on all kinds of strange scientific adventures. At one point Mr. Tompkins becomes an electron, and, like the particles in our early universe, is annihilated when he meets his antimatter twin, the positron. Tough ending.

  The scientists were looking at the universe as it was nearly fourteen billion years ago.

  The existence of the cosmic microwave background was predicted decades earlier by a Russian-born American physicist, George Gamow. When Dicke and his colleagues at Princeton learned of the strange signal discovered by Penzias and Wilson, they knew what it really meant. They’d been looking for evidence of the cosmic microwave background themselves. Everything fit, including that the signal came from every direction in the sky.

  More than a decade later, in 1978, the discovery of the cosmic microwave background earned Penzias and Wilson the highest honor in science, the Nobel Prize.

  Was It Fair?

  Robert Dicke, the scientist who helped Penzias and Wilson understand what they were seeing with their telescope, did not win the award. This might seem unfair. But the Nobel Prize typically goes to a discovery. If the theorist, the person who explains what is being observed, participates in the discovery or tells the others what to look for, then he or she might share the prize. But in this case, Penzias and Wilson found the cosmic microwave background first, so they earned the award.

  How do we know we’re right about the cosmic microwave background?

  Consider the alien point of view. Remember, light takes time to reach us from distant places in the universe. If we look out in deep space, we actually see far back in time. So if the intelligent inhabitants of a galaxy far, far away were to measure the temperature of the cosmic microwave background right before those photons start traveling to our telescopes, their result should be slightly higher than our own measurement, because they would be living in a younger, smaller, hotter universe.

  You can actually test this idea.

  The molecule cyanogen gets excited when exposed to microwaves. By “excited” I mean that its electrons jump to a different level as they orbit the nucleus, but if you want to picture them dancing, that’s okay, too. Warmer microwaves excite cyanogen a little more than colder ones do. Astrophysicists have compared the cyanogen we see in our own Milky Way galaxy to the cyanogen in distant, younger galaxies. Since those galaxies are younger, the cyanogen is bathed in warmer microwaves, so it should be more excited. And that’s exactly what we observe.

  You can’t make this up.

  Why should any of this be interesting? Because it creates a rich picture of how the universe formed. Since Penzias and Wilson, astrophysicists have used increasingly sensitive tools to create a detailed map of the cosmic microwave background. This map is not completely smooth. It’s got spots that are slightly hotter and slightly cooler than average. By studying these temperature differences, these bumps in the map, we can figure out what the early universe looked like, and where the matter started to clump together.

  We can see where and when the first galaxies started to form.

  The cosmic microwave background tells us that we understand how the universe behaved and expanded. But the cosmic microwave background also reveals that most of the universe is made up of stuff about which we are clueless. These two mysteries are the subject of chapters 5 and 6.

  Beware, reader. Our story will soon grow dark.

  4.

  Between the Galaxies

  The summer after finishing ninth grade, I climbed into a van with a bunch of other kids and we were driven fifty-three straight hours from New York City to the Mojave Desert of Southern California. Our destination was Camp Uraniborg, a monthlong getaway for science-minded young people named after the observatory of the Danish astronomer Tycho Brahe, pronounced “Tee-you-ko Brah,” a brilliant observer with a brass nose. We’ll meet him later.

  I’d observed the sky before. As I said before, on clear nights I would climb up to the rooftop of our Bronx apartment building to study the stars and planets. This wasn’t easy work. Often I had to recruit my little sister to help me carry up the parts of my telescope. On several occasions, our neighbors called the police, thinking a burglar was sneaking around on the rooftop.

  Saturn to the Rescue

  As part of my effort to prove to these police officers that I was a young astronomer, and not a criminal, I’d offer them a view of the night sky. Saturn always proved popular. It is a stunningly beautiful planet, and on those occasions, it helped prevent me from being wrongly arrested.

  Really, how could you not love Saturn? Look at those rings! It’s a solar system wonder like no other.

  We could see stars in the city sky. A few dozen on average. Maybe a hundred.

  The Mojave Desert revealed a far more crowded universe. The entire sky was filled with stars. This was like my first planetarium show, only it was real. Over the next month I recorded images of moons, planets, star systems, galaxies, and more. But I still didn’t see everything. The observable universe, or the parts of the cosmos we can see, may contain a hundred billion galaxies. Bright and beautiful and packed with stars, galaxies decorate the night sky. Because they’re in your face, it would be easy to believe that nothing else out there is important. But the universe contains hard-to-detect things between the galaxies. Those things may be more interesting than the galaxies themselves.

  These dark stretches between the galaxies make up what we call intergalactic (“inter” means “between”) space. Pretend for a moment that you were suddenly transported there. Let’s ignore the fact that you would slowly freeze to death or that your blood cells would burst while you suffocated. Never mind that you would pass out and start to puff up like a kid having a massive allergic reaction.

  These are ordinary dangers.

  Why We Call It the Milky Way

  When we study the cosmos, we tend to focus on the galaxies because they are so eye catching. Our own galaxy, the Milky Way, has a spiral shape, and is named for its spilled-milk appearance across Earth’s nighttime sky. Indeed, the very word “galaxy” derives from the Greek galaxias, “milky.”

  When viewed through proper telescopes, our Milky Way galaxy appears as a thick smear across the sky. Not exactly milky, but close.

  You might also be struck by super-duper high-energy, fast-moving, charged subatomic particles called cosmic rays. We don’t know where they come from or what launches them on their journey. We do know most of them are protons, and that they travel almost as fast as the speed of light. A single cosmic ray particle carries enough energy to knock a golf ball from anywhere on a putting green into a cup. NASA is so worried about what cosmic rays might do to astronauts that the agency designs its spacecraft with special shielding to block those rays.

  Yes, intergalactic space is, and will forever be, where the action is.

  If scientists didn’t have advanced telescopes, we might still declare the space between the galaxies to be empty. The bright stars and splotchy, milky galaxies dominate the night sky, and hold enough secrets to keep astrophysicists busy for centuries.

  But as we discussed earlier, light comes in many forms. We are all familiar with visible light, but it can also be invisible. The X-rays that doctors and hospitals use to peer through your skin and see whether you’ve broken any bones after an accident are a form of light. So are the microwaves drifting in from the distant universe, offering clues to the birth of the cosmos. Even the radio waves that give us Wi-Fi are distant, low-energy cousins of the visible light that floods and colors the world all around us.

  Modern detectors and probes can study these invisible forms of light. They can tell us a
bout cosmic events and happenings that we cannot see with our eyes alone. Using these detectors, we have probed our cosmic countryside and revealed all manner of fantastic space oddities.

  Allow me to introduce you to a few of my favorites.

  Dwarf Galaxies

  In a given area of space, there will be ten small galaxies—known as dwarf galaxies—for every large one. Our own Milky Way has dozens of nearby dwarf galaxies. While a normal, large galaxy might have hundreds of billions of stars, dwarf galaxies can have as few as a million. That might still seem impressive. But because dwarf galaxies have fewer stars, they are much, much dimmer in the sky, which makes them harder to find.

  In the dwarf galaxy at the center of this image, stars are still forming in the bright, whitish patches.

  We are discovering new ones all the time.

  You will find most (known) dwarf galaxies hanging out near bigger galaxies, orbiting around them like spaceships. Ultimately, the dwarfs are ripped apart, and then eaten, by the main galaxy.

  The Milky Way engaged in at least one act of cannibalism in the last billion years, when it swallowed a dwarf galaxy. The shredded remains of this galaxy can be seen as a stream of stars orbiting the center of the Milky Way. These cosmic leftovers are known as the Sagittarius Dwarf system. But given that they were so rudely eaten, we should have named them Lunch.

  Runaway Stars

  Galaxies are grouped together into clusters, in the same way nearby towns and cities are lumped together into counties. But our cities and towns tend to stay put. New York doesn’t spin up the coast and crash into Boston. Large galaxies, on the other hand, routinely collide, and when they do, they leave behind an enormous mess. After one of these crashes, hundreds of millions of stars, normally held in place by gravity, escape its pull. These stars scatter, ending up spread across the sky. Some stars reassemble to form blobs that could be called dwarf galaxies.