PDF Summary:Astrophysics for People in a Hurry, by Neil de Grasse Tyson

Since humans first looked up, we’ve wondered about our place in the cosmos, yet everything we’ve learned has shown that our ignorance still outstrips our knowledge. While to some that may seem disheartening, science educator Neil deGrasse Tyson finds it exciting to know there’s so much left in the universe to discover. The study of astrophysics, he says, gives us a valuable perspective on ourselves while revealing the nature of the universe in which we live.

Tyson explains in layman’s terms how physicists use the fundamentals of science and the full range of visible and invisible light to unravel the mysteries of the cosmos. He then goes on to cover the three biggest mysteries in astrophysics today—the origin of the universe, dark matter, and dark energy—while making a case for the benefits of understanding where we fit in the universal scheme. In this guide, we’ll also provide the historical context for astronomy’s groundbreaking discoveries and explain in more depth the current theories that are guiding astrophysics’ progress.

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The Big Bang

By observing the sky in all available wavelengths, astronomers have worked for the last century to answer one of humanity’s most basic questions: “Where do we come from?” The branch of astrophysics that tackles this subject is the science of cosmology, the study of the origin of the universe itself. Tyson tells the story of the birth of the cosmos, from the origins of the basic physical forces, the creation of matter and the first rays of light, to the formation of the stars and all the chemical elements that make up the world we know.

The Birth of the Big Bang Theory

Our conception of the Big Bang Theory began in 1926 with astronomer Edwin Hubble, who was the first to observe that the other galaxies are receding away from our own. The prevailing scientific belief at the time was that the universe existed in a “steady state” without any significant change. Hubble’s observations, however, suggested that in the past the universe was smaller and denser than it is now.

In 1931, Belgian astronomer and priest Georges Lemaȋtre proposed that the universe began as an infinitely dense “primordial atom” that rapidly expanded. The debate between Steady State and Big Bang proponents went on for decades, though the Big Bang model became widely accepted due to observations of the Cosmic Microwave Background.

According to the Big Bang theory of cosmology, the universe began as a microscopic point that contained all the matter and energy that would ever exist, as well as all the fundamental forces, such as gravity, that would govern the universe’s development. From that one tiny dot, the universe expanded into the vastness of space that we currently see, 13 billion years later. We have no way to observe anything that came before the initial starting point, but Tyson says that scientists are actually comfortable admitting a certain degree of ignorance. After all, without ignorance, there’d be nothing to explore.

(Shortform note: The question of what took place before the Big Bang may in fact be meaningless. In Brief Answers to the Big Questions, Stephen Hawking explains that according to the theory of relativity, time itself started with the Big Bang. Relativity shows that time slows down in regions with extreme mass and gravity. Therefore, in the universe’s initial state as a point where mass and gravity were infinitely dense, time did not exist. In an appearance on Tyson’s podcast StarTalk, Hawking suggested that asking what happened before the Big Bang is like asking what’s south of the South Pole.)

Understanding the first few moments of the cosmos is crucial to unlocking one of the biggest riddles science faces. The chief problem in physics today is that we have two working models of how the universe functions that are incompatible with each other. Einstein’s theory of general relativity is great at describing the universe at the macro level, but it doesn’t work at the level of atoms and electrons. Quantum mechanics effectively describes the realm of the very small, but it in no way relates to how physical objects move and interact at the scale that we can see with our eyes.

(Shortform note: The quest for a unified theory that would reconcile relativity with quantum mechanics has been ongoing for a century. In A Brief History of Time, Stephen Hawking suggests that the most promising approach to the problem is string theory, which models all fundamental particles as one-dimensional, vibrating “strings.” The issue with string theory is that to date there’s been no direct way to test it, leading some to call it unscientific. However, theorists working on microscopic gravity are searching for ways to match string theory’s predictions with observable reality.)

The importance of the Big Bang to unraveling this mystery is that in the universe’s first few instants, the realms of relativity and quantum mechanics overlapped. If we can discover how matter, energy, time, and space behaved during the initial moment of inception, Tyson argues that we should be able to resolve the discrepancies between the two theoretical systems. This is the research being done now at the Large Hadron Collider in Switzerland, where subatomic particles are smashed into each other at incredibly high pressure in order to simulate the conditions of the universe in its first few moments.

(Shortform note: When the Large Hadron Collider came online in 2008, some feared that replicating the conditions of the early universe might inadvertently create a dangerous black hole. Instead, the LHC has discovered a variety of subatomic particles that had only been hinted at in theory before, including the Higgs boson, the microscopic particle that gives other particles mass. Confirming the existence of the Higgs boson gives credence to our current understanding of particle physics. The Higgs boson is, in essence, the original form of matter in the universe that came into being in the moment of the Big Bang.)

The Forces of Nature

In order to unravel the riddles of the Big Bang, relativity, and quantum mechanics, we need to start with the fundamental forces that govern how matter and energy behave. As we currently understand them, there are four: Gravity is exerted by anything with mass. Electromagnetism governs the motion of particles with an electrical charge. The weak nuclear force holds protons and neutrons together in the nucleus of the atom, while the strong nuclear force glues quarks together (the subatomic particles that make up protons and neutrons).

According to Tyson, physicists believe that in the universe’s primordial state, all four forces were combined into one. Gravity untangled from the others first, taking the form described by Einstein and Newton. As the universe expanded, the strong nuclear force split apart next, leaving a third force, dubbed “electroweak,” that quickly divided into the electromagnetic and weak nuclear forces. All of this we believe took place within the universe’s first trillionth of a second.

The Standard Model

The theory that matter is made of subatomic particles that interact via the forces above is referred to as the Standard Model. Astrophysicists often assume the Standard Model to be true while at the same time looking for data that will either confirm it or show that it needs revision.

Particle physicists trying to reconcile relativity with quantum mechanics have often asked if the Standard Model’s four fundamental forces are really just aspects of the same basic force. The discovery of the electroweak force that encompasses two of the four is a step in that direction. However, further research has shown that there may be a fifth fundamental force, though the evidence is not yet conclusive.

Matter/Antimatter

At this point in the process of the Big Bang, the universe was boiling with subatomic particles—neutrinos, electrons, photons, and quarks. It was only under this extreme heat and pressure that quarks were even able to exist on their own. As the universe kept on expanding and cooling, all the quarks stuck together to form protons and neutrons, the building blocks of atoms. The quarks also formed into antiprotons and antineutrons that would annihilate the protons and neutrons, but Tyson writes that a slight imbalance in the early universe led to there being more matter than antimatter. By this point, a millionth of a second later, the universe had grown to the size of our solar system.

Whatever Happened to Antimatter?

Antimatter can be thought of as a mirror opposite to “normal” matter. For example, an electron is a subatomic particle with a negative electrical charge, whereas its antimatter counterpart, the positron, is an electron with a positive charge. When matter and antimatter particles collide, they annihilate each other, converting their mass into energy.

One mission of the Chandra X-Ray Observatory and the Compton Gamma Ray Observatory was to search for traces of antimatter left over from the Big Bang. Those missions found evidence that there is little antimatter in the universe at all, but more recent observations have discovered a halo of antimatter surrounding our galaxy that may be partly responsible for the creation of dark matter, covered later in this guide.

As the universe’s temperature dropped due to its expansion, so did the energy density needed to create more particles. The matter and antimatter already created continued to annihilate each other, but because of the slightly higher amount of “normal” matter, that’s what was left when this period was over. The few remaining free quarks fused together, and the universe (now one second old) was several light-years across. Tyson points out that if the amount of matter and antimatter had been perfectly balanced after the Big Bang, their mutual annihilation would have been complete, and the universe would be nothing but empty space.

(Shortform note: The most difficult aspect of the Big Bang to visualize is that it did not explode into empty space—the Big Bang’s expansion created space itself, stretching the universe in a process that still continues today. Therefore, there’s no “center” to the universe’s expansion. All points in space recede from every other point, carrying all the stars and galaxies with them. What drives this process remains unclear, but it now seems closely tied to the mysterious Dark Energy that we’ll cover later in this guide.)

Over the next two minutes, the first atoms condensed out of the primordial chaos. What formed were the two simplest elements—90% are hydrogen and 10% are helium. The universe’s temperature fell to a mere 1 billion degrees, and all the universe’s matter comprised a sea of hydrogen and helium gas so dense that any light it gave off was quickly reabsorbed. Tyson says that a universal dark age began that would last nearly 400,000 years.

(Shortform note: The early, dense universe was only able to produce hydrogen and helium because by the time those elements were able to form and stay together, the universe’s temperature and pressure dropped too low to fuse them into heavier elements. The rare instances of fusion between hydrogen and helium atoms that did occur produced unstable isotopes of beryllium that quickly broke apart back into hydrogen atoms.)

The Cosmic Microwave Background

During the dark age, the universe kept cooling. When at last it fell to 3,000 degrees, it was possible for photons of light to travel without being reabsorbed back into atoms. The universe began to glow, and it’s a glow we can still see today if we use the right instruments to detect it.

Tyson reminds us that when we look deep into the sky, we’re also looking backward in time. Therefore, from 13 billion light years away in every direction, we can detect the cosmic background radiation—a snapshot of the universe as it was when the clouds first broke after the Big Bang. Because those ancient photons of light have lost a lot of energy since the time they were emitted, they now come to us in the form of low-frequency microwaves. Hence, this glow from the early universe is commonly referred to as the Cosmic Microwave Background (CMB).

(Shortform note: Though the existence of the CMB was theorized in the 1940s and first detected in the ’60s, it was mapped by the Cosmic Background Explorer spacecraft in 1990. Much higher resolution images were produced in 2013 by NASA’s Wilkinson Microwave Anisotropy Probe and the European Space Agency’s Planck observation satellite. Together, these pictures reveal the microwave glow that covers the entire night sky, originating far beyond any stars and galaxies that we see.)

What’s striking about this snapshot of the past is that it shows the early universe was lumpy. There are hot and cold spots, dense areas and voids. This uneven distribution in the early cosmic medium explains the shape of the universe today, with matter lumped into huge galactic clusters with even larger deserts of empty space between them. Also, by examining the CMB, we can calculate how much energy and matter there is in the universe itself. Prior to the microwave background’s discovery, all our theories of the universe’s origin were guesses. Tyson argues that the CMB gives the science of cosmology a solid observational grounding.

(Shortform note: Using the CMB as a starting point, we can now make other observations about the very early universe. Astronomers at the University of Cologne have discovered a cloud of water vapor that existed only 880 million years after the Big Bang and allows them to measure the rate of the universe’s cooling at that time. Other astronomers are searching for gravitational waves caused by the early universe’s expansion and microwave polarization effects that would have happened when the primordial plasma became a stable gas.)

Stars and Everything Else

For the next billion years, the densest clumps of gas collapsed under their own gravitational pull until the pressure inside them grew high enough for nuclear fusion to ignite. The first stars began to shine, and the galaxies were born. Also, says Tyson, the birth of the stars added something to the universe we now take for granted—all the elements on the periodic table. The Big Bang gave birth to hydrogen and a fractional amount of helium, but all the other elements we know of today were created in the hearts of burning stars.

To understand how, you have to understand fusion. Unlike nuclear fission, in which relatively large and unstable atoms break apart, fusion creates energy by mashing small atoms together. If two hydrogen atoms slam into each other hard enough, they fuse to create one helium atom and release an amount of energy in the process. Tyson explains that stars first burn by fusing vast amounts of hydrogen, but as they age and build up excess helium, that helium starts to fuse into carbon. Carbon will eventually fuse into oxygen, and the process continues, creating new elements until at last producing iron.

The Lives of Stars

New stars are born out of collapsing clouds of interstellar dust and gas. The initial collapse may be triggered by the shockwave of an exploding giant star or simply by the galaxy’s rotation causing gas clouds to pile into each other. Once the gas is dense enough, gravity takes hold and pulls more matter in until the pressure inside the heart of the cloud is high enough for nuclear fusion to ignite. And thus, a star comes to life.

Throughout its existence, a star will burn as long as there’s enough hydrogen in its core. Ironically, the more massive a star is, the faster it will burn through its fuel. Our sun should have a 10-billion-year lifespan, while the largest stars only survive for a few million years. Red dwarf stars, the smallest and most common, have lives measured in trillions of years. Since the universe is only 13 billion years old, every red dwarf star ever born is still burning today.

Iron holds a special place in the cosmos—it’s the atom where nuclear fusion ends. Fusing iron into heavier elements takes more energy than it produces. Once a star builds up too much iron in its core, the process of nuclear fusion stops. The star dies, shedding most of its mass back into space, including all the new elements it created. This, says Tyson, is the most important link between ourselves and the stars. Every hydrogen atom in the water in our bodies was created by the Big Bang. Every other atom inside us came into being in the heart of a star.

(Shortform note: The elements heavier than iron are also produced by stellar fusion, not in the process of a star’s normal life, but during a supergiant star’s final moments. When the fusion process ends in a truly massive star, one at least eight times the size of our sun, the star’s great bulk collapses under its own weight, smashing atomic nuclei into heavier elements. The implosion then rebounds as a supernova explosion, spreading new elements far into space, where they’ll become part of the next generation of stars and planets.)

Dark Matter

In addition to expanding what we know about the universe, our deep space observations also reveal that there are major gaps in our understanding of the cosmos. The first of these is the “dark matter” problem—the fact that there appears to be far more gravity in the universe than all the matter we observe can account for. Dark matter’s effects have been known for nearly a century, but to date we can only tell what it is not, though our observations of the early universe give some tantalizing clues.

A Primer on Gravity

To understand the problem dark matter poses, it’s important to know the basics of gravity. It was first defined by Isaac Newton as the force by which any two objects with mass attract each other. For instance, just as Earth’s gravity exerts a pull on the mass of your body, your body also exerts a pull on the Earth. Newton couldn’t explain how this force works between objects that aren’t touching each other, such as the Earth and its moon, but he formulated a law that predicted exactly how much force objects exert on each other.

The next breakthrough was made by Albert Einstein, who was able to solve the problem Newton couldn’t. Einstein proposed that objects with mass bend space and time around them, and that gravitational attraction is the observable effect of the “dent” that objects with mass make in space. Einstein’s theory made several predictions that wouldn’t hold true under Newton’s theory, such as that gravity bends rays of light or that cataclysmic stellar events might create gravitational waves, as observed in 2015. However, because of dark matter, the light from distant galaxies is bent more than it would be by the visible objects in its path.

The discrepancy between gravity and observable mass isn’t trivial. Tyson asserts that Einstein’s expansion of Newton’s gravity equation has passed every practical and experimental test, yet 85% of the gravitational attraction we measure on the cosmic scale seems to come from invisible sources. This “missing mass” problem was first discovered in 1937 when astronomers plotting the motion of galaxies in clusters found them moving as if acted upon by a giant mass that wasn’t there. Since then, the same observations have been made even within individual galaxies.

(Shortform note: Unfortunately, it’s hard to put a galaxy on a scale to weigh it. Astronomers measure the mass of a galaxy by observing the motion of smaller galaxies and stars that orbit it. Using this method, we’ve been able to compute that our own Milky Way galaxy is 1-2 trillion times as massive as our sun. However, according to our best estimates, there are “only” about 200 billion stars in the Milky way, the majority of which are smaller than our sun, leaving 90% of the galaxy’s mass unaccounted for.)

To be clear, we don’t know what dark matter is, or even if it’s a form of matter at all. The name “dark matter” is merely a placeholder for something we only know is there because of its effects on what we see. It’s not a bunch of undiscovered black holes; we’d be able to detect them via x-ray emissions. It isn’t massive dark clouds in space; we’d see them blocking the light from behind them. It can’t be planets or asteroids; they’d have to take up six times more mass than stars, which Tyson says is exceedingly unlikely. What’s most confusing is that we only observe dark matter’s effects on the largest scale, but not on the motion of stars and planets. None of our space probes have ever gone off course because of the pull of a dark matter object.

(Shortform note: It may not be true for much longer that dark matter’s effects can’t be observed within the solar system. NASA scientist Jim Green and mathematician Edward Belbruno have calculated that the probes we’ve sent the farthest into space, such as Voyagers 1 and 2, may have been pulled minutely off course by the presence of dark matter in the solar system. They propose that a probe with highly sensitive equipment may be able to measure this divergence if sent out to 100 times the distance from the Earth to the sun.)

According to Tyson, our observations of the early universe give us some insight into the dark matter problem. We know that dark matter doesn’t fuse with hydrogen and helium—if it did, the proportion of those elements would be different from what we observe. However, it could be that dark matter is responsible for the “lumpy” nature of the early universe. If so, then without dark matter’s gravitational influence, the stars and galaxies may never have formed. Our best guess is that dark matter consists of an undiscovered type of particle that only interacts with others via gravity, ignoring the other three fundamental forces. Whatever it is, dark matter’s effects show that the so-called “normal” matter we know is only the tip of the universal iceberg.

(Shortform note: In the absence of direct observation, scientists create mathematical models to predict the hypothetical effects of dark matter, which astronomers can then search for in the universe. For example, one such model proposed that dark matter particles created in the Big Bang may have had an equivalent mass to protons and neutrons. However, the same model shows that the small dark matter particles would have interfered with the formation of light elements, such as hydrogen and helium. Models that predict a different universe than we actually observe are unlikely to be accepted as viable theories.)

Dark Energy

As mysterious as dark matter is, it isn’t the biggest puzzle in cosmology. In recent decades, astronomers have witnessed a mysterious force they’ve named “dark energy” that’s accelerating the universe’s expansion. Though the theoretical underpinnings of dark energy go back to Einstein’s equations of relativity, its existence wasn’t even guessed at until recent years. At the moment, we’re ignorant of its true nature, but Tyson reminds us that that’s the condition under which scientific exploration thrives.

Tyson traces dark energy’s theoretical roots back to a concept that Einstein thought was a mistake in his equations. When Einstein developed general relativity, no one knew that the universe was expanding. To explain why the universe wasn’t collapsing under its own gravity, Einstein included a value in his formulae that he called the “cosmological constant.” It was a placeholder for an unknown force that pushed against the pull of gravity, though Einstein didn’t know what it might represent. When the universe was discovered to be expanding as if from a giant explosion, Einstein threw away his cosmological constant, thinking he’d been in error.

(Shortform note: One way to envision Einstein’s cosmological constant is to imagine that empty space isn’t really empty. If empty space has an energy of its own, then the density of that energy is what the cosmological constant represents. The cosmological constant solves another problem in astronomy—without it, the universe’s rate of expansion would imply that it’s as little as 7 billion years old, yet we can see stars that are older than that. By factoring in the cosmological constant, Einstein’s theory of gravity corresponds with information from the Cosmic Microwave Background, dating the universe at 13.7 billion years.)

By the end of the 20th century, astronomers had established that the universe is expanding faster than it should be, at least according to previous assumptions that the rate of expansion was constant. Scientists had often wondered if the expansion would continue forever, or if the universe would turn back in upon itself in a Big Crunch to counter the Big Bang. Now it’s clear that not only will the expansion continue, but somehow it’s also accelerating.

No one understands where the energy comes from that drives the galaxies farther apart, but Tyson writes that there’s already a place for it in relativity’s equations—Einstein’s cosmological constant, which astrophysicists now refer to as “dark energy.” Half a century after Einstein’s death, it was shown that he’d been right all along.

The Shape of the Future of the Universe

Ever since it was discovered that the universe was expanding, astrophysicists have debated what form that expansion will take. All the evidence now points toward a scenario in which galaxies grow immeasurably far from each other, the gas clouds from which stars are born grow diffuse, and as a result, star formation ends. All the remaining stars that exist will eventually grow old and fade. The far, far future will be a very cold time.

The discovery of dark energy throws a weird twist on how the universe may end. The Big Rip hypothesis suggests that in approximately 22 billion years, the accelerating expansion of the universe could reach a point where the force of dark energy overcomes the force of gravity, tearing apart galaxies, stars, and even atoms—a very different, more apocalyptic demise than astronomers had previously imagined.

Even more so than dark matter, Tyson reiterates that we don’t have a clue what dark energy is. However, current estimates suggest that dark energy makes up almost 70% of the universe, with dark matter taking up a little more than 25%, and the regular matter and energy we know of comprising a mere 5% of the observable cosmos. Clearly, there’s a lot that we don’t understand.

(Shortform note: Just as we estimate the amount of dark matter by observing its gravitational effects on objects that we can see, we can estimate the amount of dark energy by calculating how much is required to explain the expansion of the observable universe. Some research on the Cosmic Microwave Background hints that dark energy may be a strange medium called “quintessence” that permeates the universe. Others suggest that Einstein’s theory of gravity might simply be wrong when it comes to how matter behaves on the grand cosmic scale.)

The Current Frontier

Do the existence of dark matter and dark energy mean that our current theories of physics need an overhaul? Perhaps. Tyson posits some current theories: Dark matter could be the gravitational effect of objects in some other dimension. Dark energy could be the result of the spontaneous creation and destruction of subatomic particles in empty space. Whatever the answers turn out to be, not knowing is a place where we’ve found ourselves before.

There was a time before we understood fusion, when scientists couldn’t explain how the sun produced light. There was a time before we understood light, and scientists believed it traveled through an invisible medium called “aether.” There was a time when we thought we were the center of creation, and the stars in the sky were just pretty, twinkling lights. Tyson reminds us that climbing out of ignorance is an adventure, and that there will always be something new to discover.

An Age of Discovery

Though it’s unlikely we’ll ever explore the stars in the same way as on Star Trek, the modern age has been a gold rush of space exploration. In addition to the discoveries already covered in this guide, there have been many other exciting advances in what we’ve learned about the cosmos.

We’ve discovered ice on the surface of the Moon and taken samples of organic material from a comet. We’ve landed a probe on Saturn’s moon Titan and flown a spacecraft past distant, icy Pluto. The Kepler Space Telescope has identified more than 2,600 planets outside our solar system, while astronomers using the Event Horizon Telescope took the first picture of a black hole.

At present, there are plans to return to the moon, bring back soil samples from Mars, and take the first direct photographs of planets outside our solar system. However, as Isaac Asimov once said, the most exciting phrase in science isn’t “Eureka!” but “That’s funny.” While it’s guaranteed that we’ll keep making discoveries about the universe, the ones that will truly overturn our ways of thinking are the ones we’ll never see coming.