Astrophysics for People in a Hurry (Astrophysics for People in a Hurry Series)

by Neil deGrasse Tyson

In other words, if telescopes observed mass rather than light, then our cherished galaxies in clusters would appear as insignificant blips amid a giant spherical blob of gravitational forces.

As already noted, looking out into the cosmos is analogous to a geologist looking across sedimentary strata, where the history of rock formation is laid out in full view. Cosmic distances are so vast that the travel time for light to reach us can be millions or even billions of years. When the universe was one half its current age, a very blue and very faint species of intermediate-sized galaxy thrived. We see them. They hail from a long time ago, representing galaxies far, far away.

Quasars are super-luminous galaxy cores whose light has typically been traveling for billions of years across space before reaching our telescopes.

Every known quasar reveals these hydrogen features, so we conclude that the hydrogen clouds are everywhere in the universe. And,

either case, where there is mass there is gravity. And where there is gravity there is curved space, according to Einstein’s general theory of relativity. And where space is curved it can mimic the curvature of an ordinary glass lens and alter the pathways of light that pass through. Indeed, distant quasars and whole galaxies have been “lensed” by objects that happen to fall along the line of sight to Earth’s telescopes.

One of the most distant (known) objects in the universe is not a quasar but an ordinary galaxy, whose feeble light has been magnified significantly by the action of an intervening gravitational lens. We may henceforth need to rely upon these “intergalactic” telescopes to peer where (and when) ordinary telescopes cannot reach, and thus reveal the future holders of the cosmic distance record.

From the department of exotic happenings, intergalactic space is regularly pierced by super-duper high-energy, fast-moving, charged, subatomic particles. We call them cosmic rays. The highest-energy particles among them have a hundred million times the energy that can be generated in the world’s largest particle accelerators. Their origin continues to be a mystery, but most of these charged particles are protons, the nuclei of hydrogen atoms, and are moving at 99.9999999999999999999 percent of the speed of light.

Perhaps the most exotic happenings between (and among) the galaxies in the vacuum of space and time is the seething ocean of virtual particles—undetectable matter and antimatter pairs, popping in and out of existence. This peculiar prediction of quantum physics has been dubbed the “vacuum energy,” which manifests as an outward pressure, acting counter to gravity, that thrives in the total absence of matter. The accelerating universe, dark energy incarnate,

It took the mind of the millennium’s most brilliant and influential person, Isaac Newton, to realize that gravity’s mysterious “action-at-a-distance” arises from the natural effects of every bit of matter, and that the attractive force between any two objects can be described by a simple algebraic equation. It took the mind of the last century’s most brilliant and influential person, Albert Einstein, to show that we can more accurately describe gravity’s action-at-a-distance as a warp in the fabric of space-time, produced by any combination of matter and energy.

Einstein demonstrated that Newton’s theory requires some modification to describe gravity accurately—to predict, for example, how much light rays will bend when they pass by a massive object. Although Einstein’s equations are fancier than Newton’s, they nicely accommodate the matter that we have come to know and love. Matter that we can see, touch, feel, smell, and occasionally taste.

We don’t know who’s next in the genius sequence, but we’ve now been waiting nearly a century for somebody to tell us why the bulk of all the gravitational force that we’ve measured in the universe—about eighty-five percent of it—arises from subs...

The Coma cluster, as we call it, is an isolated and richly populated ensemble of galaxies about 300 million light-years from Earth. Its thousand galaxies orbit the cluster’s center, moving in all directions like bees swarming a beehive.

planet must have to maintain a stable orbit at any distance from the Sun, lest it descend back toward the Sun or ascend to a farther orbit. Turns out, if we could boost Earth’s orbital speed to more than the square root of two (1.4142 . . .) times its current value, our planet would achieve “escape velocity,” and leave the solar system entirely. We

When we examine the Coma cluster, as Zwicky did during the 1930s, we find that its member galaxies are all moving more rapidly than the escape velocity for the cluster. The cluster should swiftly fly apart, leaving barely a trace of its beehive existence after just a few hundred million years had passed. But the cluster is more than ten billion years old, which is nearly as old as the universe itself. And so was born what remains the longest-standing unsolved mystery in astrophysics.

Perhaps the “missing mass” needed to bind the Coma cluster’s galaxies does exist, but in some unknown, invisible form. Today, we’ve settled on the moniker “dark matter,” which makes no assertion that anything is missing, yet nonetheless implies that some new kind of matter must exist, waiting to be discovered.

These largely empty volumes of space—the far-rural regions of each galaxy—contain too little visible matter to explain the anomalously high orbital speeds of the tracers. Rubin correctly reasoned that some form of dark matter must lie in these far-out regions, well beyond the visible edge of each spiral galaxy. Thanks to Rubin’s work, we now call these mysterious zones “dark matter haloes.”

From galaxy to galaxy and from cluster to cluster, the discrepancy between the mass tallied from visible objects and the objects’ mass estimated from total gravity ranges from a factor of a few up to (in some cases) a factor of many hundreds.

our own solar system, for example, everything that is not the Sun adds up to less than one fifth of one percent of the Sun’s mass.

From this we conclude that most of the dark matter—hence, most of the mass in the universe—does not participate in nuclear fusion, which disqualifies it as “ordinary” matter, whose essence lies in a willingness to participate in the atomic and nuclear forces that shape matter as we know it. Detailed observations of the cosmic microwave background, which allow a separate test of this conclusion, verify the result: Dark matter and nuclear fusion don’t mix.

Dark matter exerts gravity according to the same rules that ordinary matter follows, but it does little else that might allow us to detect it. Of course, we are hamstrung in this analysis by not knowing what the dark matter is in the first place. If all mass has gravity, does all gravity have mass? We don’t know. Maybe there’s nothing wrong with the matter, and it’s the gravity we don’t understand.

Dark matter also has no bearing on the Moon’s orbit around Earth, nor on the movements of the planets around the Sun—but as we’ve already seen, we do need it to explain the motions of stars around the center of the galaxy.

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 the cosmic cake, modest buoys afloat in a vast cosmic ocean of something that looks like nothing.

At odds in the universe were two competing effects: gravity wants to make stuff coagulate, but the expansion wants to dilute it. If you do the math, you rapidly deduce that the gravity from ordinary matter could not win this battle by itself. It needed the help of dark matter, without which we would be living—actually not living—in a universe with no structures: no clusters, no galaxies, no stars, no planets, no people.

So dark matter is our frenemy. We have no clue what it is. It’s kind of annoying. But we desperately need it in our calculations to arrive at an accurate description of the universe.

As a wave, light was thought to require a medium through which to propagate its energy, much as sound requires air or some other substance to transmit its waves. But light turns out to be quite happy traveling through the vacuum of space, devoid of any medium to carry it. Unlike sound waves, which consist of air vibrations, light waves were found to be self-propagating packets of energy requiring no assistance at all.

the existence of dark matter derives not from mere presumption but from the observed effects of its gravity on visible matter. We’re not inventing dark matter out of thin space; instead, we deduce its existence from observational facts.

Dark matter is just as real as the many exoplanets discovered in orbit around stars other than the Sun, discovered solely through their gravitational influence on their host stars and not from direct measurement of their light.

Other unrelenting skeptics might declare that “seeing is believing”—an approach to life that works well in many endeavors, including mechanical engineering, fishing, and perhaps dating. It’s also good, apparently, for residents of Missouri. But it doesn’t make for good science. Science is not just about seeing, it’s about measuring, preferably with something that’s not your own eyes, which are inextricably conjoined with the baggage of your brain. That baggage is more often than not a satchel of preconceived ideas, post-conceived notions, and outright bias.

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Particle physicists are confident that dark matter consists of a ghostly class of undiscovered particles that interact with matter via gravity, but otherwise interact with matter or light only weakly or not at all. If you like gambling on physics, this option is a good bet. The world’s largest particle accelerators are trying to manufacture dark matter particles amid the detritus of particle collisions.

The copious flux of neutrinos from the Sun—two neutrinos for every helium nucleus fused from hydrogen in the Sun’s thermonuclear core—exit the Sun unfazed by the Sun itself, travel through the vacuum of space at nearly the speed of light, then pass through Earth as though it does not exist. The tally: night and day, a hundred billion neutrinos from the Sun pass through every thumbnail square of your body, every second, without a trace of interaction with your body’s atoms. In spite of this elusivity, neutrinos are nonetheless stoppable under special circumstances. And if you can stop a particle at all, you’ve detected it.

forces other than the strong nuclear force, weak nuclear force, and electromagnetism. These three, plus gravity, complete the fab four forces of the universe, mediating all interactions between and among all known particles.

So, dark matter’s effects are real. We just don’t know what it is. Dark matter seems not to interact through the strong nuclear force, so it cannot make nuclei. It hasn’t been found to interact through the weak nuclear force, something even elusive neutrinos do. It doesn’t seem to interact with the electromagnetic force, so it doesn’t make molecules and concentrate into dense balls of dark matter. Nor does it absorb or emit or reflect or scatter light. As we’ve known from the beginning, dark matter does, indeed, exert gravity, to which ordinary matter responds. But that’s it. After all these years, we haven’t discovered it doing anything else.

For now, we must remain content to carry dark matter along as a strange, invisible friend, invoking it where and when the universe requires it of us.

For the most mind-warping ideas of twentieth-century physics, just blame Einstein. Albert Einstein hardly ever set foot in the laboratory; he didn’t test phenomena or use elaborate equipment. He was a theorist who perfected the “thought experiment,” in which you engage nature through your imagination, by inventing a situation or model and then working out the consequences of some physical principle.

One of the most powerful and far-reaching theoretical models ever devised, already introduced in these pages, is Einstein’s general theory of relativity—but you can call it GR after you get to know it better. Published in 1916, GR outlines the relevant mathematical details of how everything in the universe moves under the influence of gravity. Every few years, lab scientists devise ever more precise experiments to test the theory, only to further extend the envelope of the theory’s accuracy.

Among the mammals, a sub-branch would evolve frontal lobes and complex thought to accompany them. We call them primates. A single branch of these primates would develop a genetic mutation that allowed speech, and that branch—Homo sapiens—would invent agriculture and civilization and philosophy and art and science. All in the last ten thousand years. Ultimately, one of its twentieth-century scientists would invent relativity out of his head, and predict the existence of gravitational waves. A century later, technology capable of seeing these waves would finally catch up with the prediction, just days before that gravity wave, which had been traveling for 1.3 billion years, washed over Earth and was detected. Yes, Einstein was a badass.

When first proposed, most scientific models are only half-baked, leaving wiggle room to adjust parameters for a better fit to the known universe.

Copernicus’s basic idea was correct, and that’s what mattered most. It simply required some tweaking to make it more accurate. Yet, in the case of Einstein’s relativity, the founding principles of the entire theory require that everything must happen exactly as predicted. Einstein had, in effect, built what looks on the outside like a house of cards, with only two or three simple postulates holding up the entire structure.