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

by Neil deGrasse Tyson

The German physicist Max Planck, after whom these unimaginably small quantities are named, introduced the idea of quantized energy in 1900 and is generally credited as the father of quantum mechanics.

with the weak force controlling radioactive decay, the strong force binding the atomic nucleus, the electromagnetic force binding molecules, and gravity binding bulk matter.

All the while, the interplay of matter in the form of subatomic particles, and energy in the form of photons (massless vessels of light energy that are as much waves as they are particles) was incessant. The universe was hot enough for these photons to spontaneously convert their energy into matter-antimatter particle pairs, which immediately thereafter annihilate, returning their energy back to photons.

These transmogrifications are entirely prescribed by Einstein’s most famous equation: E = mc2, which is a two-way recipe for how much matter your energy is worth, and how much energy your matter is worth. The c2 is the speed of light squared—a huge number which, when multiplied by the mass, reminds us how much energy you actually get in this exercise.

Strong theoretical evidence suggests that an episode in the very early universe, perhaps during one of the force splits, endowed the universe with a remarkable asymmetry, in which particles of matter barely outnumbered particles of antimatter: by a billion-and-one to a billion. That small difference in population would hardly get noticed by anybody amid the continuous creation, annihilation, and re-creation of quarks and antiquarks, electrons and antielectrons (better known as positrons), and neutrinos and antineutrinos. The odd man out had oodles of opportunities to find somebody to annihilate with, and so did everybody else.

But high-mass stars fortuitously explode, scattering their chemically enriched guts throughout the galaxy. After nine billion years of such enrichment, in an undistinguished part of the universe (the outskirts of the Virgo Supercluster) in an undistinguished galaxy (the Milky Way) in an undistinguished region (the Orion Arm), an undistinguished star (the Sun) was born.

Within the chemically rich liquid oceans, by a mechanism yet to be discovered, organic molecules transitioned to self-replicating life.

We owe the remarkable diversity of life on Earth, and we presume elsewhere in the universe, to the cosmic abundance of carbon and the countless number of simple and complex molecules that contain it. There’s no doubt about it: more varieties of carbon-based molecules exist than all other kinds of molecules combined.

People who believe they are ignorant of nothing have neither looked for, nor stumbled upon, the boundary between what is known and unknown in the universe.

We are stardust brought to life, then empowered by the universe to figure itself out—and we have only just begun.

Nebulium was simply the signature of ordinary oxygen doing extraordinary things.

Science thrives not only on the universality of physical laws but also on the existence and persistence of physical constants.

No matter how fast you go, you will never overtake a beam of light.

All measurements suggest that the known fundamental constants, and the physical laws that reference them, are neither time-dependent nor location-dependent. They’re truly constant and universal.

Another class of universal truths is the conservation laws, where the amount of some measured quantity remains unchanged no matter what. The three most important are the conservation of mass and energy, the conservation of linear and angular momentum, and the conservation of electric charge.

after the laws of physics, everything else is opinion.

Today, the universe has expanded by a factor of 1,000 from the time photons were set free, and so the cosmic background has, in turn, cooled by a factor of 1,000. All the visible light photons from that epoch have become 1/1,000th as energetic.

Ordinary matter is what we are all made of. It has gravity and interacts with light. Dark matter is a mysterious substance that has gravity but does not interact with light in any known way. Dark energy is a mysterious pressure in the vacuum of space that acts in the opposite direction of gravity, forcing the universe to expand faster than it otherwise would.

In the grand tally of cosmic constituents, galaxies are what typically get counted. Latest estimates show that the observable universe may contain a hundred billion of them.

Our pair of nearest-neighbor galaxies, 180,000 light-years distant, are both small and irregularly shaped. Ferdinand Magellan’s ship’s log identified these cosmic objects during his famous round-the-world voyage of 1519. In his honor, we call them the Large and Small Magellanic Clouds, and they are visible primarily from the Southern Hemisphere as a pair of cloudlike splotches on the sky, parked beyond the stars.

The nearest galaxy larger than our own is two million light-years away, beyond the stars that trace the constellation Andromeda. This spiral galaxy, historically dubbed the Great Nebula in Andromeda, is a somewhat more massive and luminous twin of the Milky Way.

Aided by modern detectors, and modern theories, we have probed our cosmic countryside and revealed all manner of hard-to-detect things: dwarf galaxies, runaway stars, runaway stars that explode, million-degree X-ray-emitting gas, dark matter, faint blue galaxies, ubiquitous gas clouds, super-duper high-energy charged particles, and the mysterious quantum vacuum energy.

While full-blooded galaxies contain hundreds of billions of stars, dwarf galaxies can have as few as a million, which renders them a hundred thousand times harder to detect.

In ordinary galaxies, for every star that explodes in this way, a hundred thousand to a million do not, so isolated supernovas may betray entire populations of undetected

There’s more to clusters than their constituent galaxies and their wayward stars. Measurements made with X-ray-sensitive telescopes reveal a space-filling, intra-cluster gas at tens of millions of degrees. The gas is so hot that it glows strongly in the X-ray part of the spectrum. The very movement of gas-rich galaxies through this medium eventually strips them of their own gas, forcing them to forfeit their capacity to make new stars. That could explain it. But when you calculate the total mass present in this heated gas, for most clusters it exceeds the mass of all galaxies in the cluster by as much as a factor of ten. Worse yet, clusters are overrun by dark matter, which happens to contain up to another factor of ten times the mass of everything else. 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.

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

Sure enough, when you separate quasar light into its component colors, revealing a spectrum, it’s riddled with the absorbing presence of intervening gas clouds. Every known quasar, no matter where on the sky it’s found, shows features from dozens of isolated hydrogen clouds scattered across time and space.

Every known quasar reveals these hydrogen features, so we conclude that the hydrogen clouds are everywhere in the universe. And, as expected, the farther the quasar, the more clouds are present in the spectrum.

In 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.

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, may be driven by the action of this vacuum energy.

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.”

cosmic dark matter has about six times the total gravity of all the visible matter.

More direct evidence for the strange nature of dark matter comes from the relative amount of hydrogen and helium in the universe.

To a close approximation, nuclear fusion during the first few minutes after the big bang left behind one helium nucleus for every ten hydrogen nuclei (which are, themselves, simply protons). Calculations show that if most of the dark matter had involved itself in nuclear fusion, there would be much more helium relative to hydrogen in the universe. 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.

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.

Does a different kind of gravitational physics operate on the galactic scale? Probably not. More likely, dark matter consists of matter whose nature we have yet to divine, and which gathers more diffusely than ordinary matter does. Otherwise, we would detect the gravity of concentrated chunks of dark matter dotting the universe—dark matter comets, dark matter planets, dark matter galaxies. As far as we can tell, that’s not the way things are.

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.

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.

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.

Dark matter particles may reveal themselves through similarly rare interactions, or, more amazingly, they might manifest via 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.

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.

The gravitational waves of the first detection were generated by a collision of black holes in a galaxy 1.3 billion light-years away, and at a time when Earth was teeming with simple, single-celled organisms. While the ripple moved through space in all directions, Earth would, after another 800 million years, evolve complex life, including flowers and dinosaurs and flying creatures, as well as a branch of vertebrates called mammals. 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.

The Russian physicist Alexander Friedmann would subsequently show mathematically that Einstein’s universe, though balanced, was in an unstable state.

Einstein’s universe was precariously perched between a state of expansion and total collapse.

Einstein knew that lambda, as a negative gravity force of nature, had no known counterpart in the physical universe.

GR regards gravity as the response of a mass to the local curvature of space and time caused by some other mass or field of energy. In other words, concentrations of mass cause distortions—dimples, really—in the fabric of space and time. These distortions guide the moving masses along straight-line geodesics,††† though they look to us like the curved trajectories we call orbits.

The twentieth-century American theoretical physicist John Archibald Wheeler said it best, summing up Einstein’s concept as, “Matter tells space how to curve; space tells matter how to move.”

At the end of the day, general relativity described two kinds of gravity. One is the familiar kind, like the attraction between Earth and a ball thrown into the air, or between the Sun and the planets. It also predicted another variety—a mysterious, anti-gravity pressure associated with the vacuum of space-time itself.

Thirteen years later, in 1929, the American astrophysicist Edwin P. Hubble discovered that the universe is not static. He had found and assembled convincing evidence that the more distant a galaxy, the faster the galaxy recedes from the Milky Way. In other words, the universe is expanding.