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

by Neil deGrasse Tyson

Einstein’s general theory of relativity, put forth in 1916, gives us our modern understanding of gravity, in which the presence of matter and energy curves the fabric of space and time surrounding

set physicists off on a race to blend the theory of the small with the theory of the large into a single coherent theory of quantum gravity.

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.

One thing quarks do have going for them: all their names are simple—something chemists, biologists, and especially geologists seem incapable of achieving when naming their own stuff.

Without the billion-and-one to a billion imbalance between matter and antimatter, all mass in the universe would have self-annihilated, leaving a cosmos made of photons and nothing else—the ultimate let-there-be-light scenario.

But this freedom comes to an abrupt end when the temperature of the universe falls below 3,000 degrees Kelvin (about half the temperature of the Sun’s surface), and all the free electrons combine with nuclei. The marriage leaves behind a ubiquitous bath of visible light, forever imprinting the sky with a record of where all the matter was in that moment, and completing the formation of particles and atoms in the primordial universe.

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.

The one we call Earth formed in a kind of Goldilocks zone around the Sun, where oceans remain largely in liquid form. Had Earth been much closer to the Sun, the oceans would have evaporated. Had Earth been much farther away, the oceans would have frozen. In either case, life as we know it would not have evolved.

organic molecules transitioned to self-replicating life. Dominant in this primordial soup were simple anaerobic bacteria—life that thrives in oxygen-empty environments but excretes chemically potent oxygen as one of its by-products. These early, single-celled organisms unwittingly transformed Earth’s carbon dioxide-rich atmosphere into one with sufficient oxygen to allow aerobic organisms to emerge and dominate the oceans and land.

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.

But what if the universe was always there, in a state or condition we have yet to identify—a multiverse, for instance, that continually births universes? Or what if the universe just popped into existence from nothing? Or

What we do know, and what we can assert without further hesitation, is that the universe had a beginning. The universe continues to evolve. And yes, every one of our body’s atoms is traceable to the big bang and to the thermonuclear furnaces within high-mass stars that exploded more than five billion years ago. We are stardust brought to life, then empowered by the universe to figure itself out—and we have only just begun.

This universality of physical laws drives scientific discovery like nothing else.

Spectra are not only beautiful, but contain oodles of information about the light-emitting object, including its temperature and composition. Chemical elements reveal themselves by their unique patterns of light or dark bands that cut across the spectrum. To people’s delight and amazement, the chemical signatures on the Sun were identical to those in the laboratory.

Further analysis of the Sun’s spectrum revealed the signature of an element that had no known counterpart on Earth. Being of the Sun, the new substance was given a name derived from the Greek word helios (“the Sun”), and was only later discovered in the lab. Thus, helium became the first and only element in the chemist’s Periodic Table to be discovered someplace other than Earth.

And, like the geologist’s stratified sediments, which serve as a timeline of earthly events, the farther away we look in space, the further back in time we see. Spectra from the most distant objects in the universe show the same chemical signatures that we see nearby in space and in time.

You’ve probably never walked through a cloud of glowing million-degree plasma, and I’d bet you’ve never greeted a black hole on the street. What matters is the universality of the physical laws that describe them.

This universality of physical laws tells us that if we land on another planet with a thriving alien civilization, they will be running on the same laws that we have discovered and tested here on Earth—even if the aliens harbor different social and political beliefs. Furthermore, if you wanted to talk to the aliens, you can bet they don’t speak English or French or even Mandarin. Nor would you know whether shaking their hands—if indeed their outstretched appendage is a hand—would be considered an act of war or of peace. Your best hope is to find a way to communicate using the language of science.

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

If you do the math, you can determine that a star’s luminosity is steeply dependent on big G. In other words, if big G had been even slightly different in the past, then the energy output of the Sun would have been far more variable than anything the biological, climatological, or geological records indicate. Such is the uniformity of our universe.

Unlike getting caught speeding on Earth roads, the good thing about the laws of physics is that they require no law enforcement agencies to maintain them, although I did once own a geeky T-shirt that proclaimed, “OBEY GRAVITY.” 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.

Jupiter’s Great Red Spot, a raging anticyclone that has been going strong for at least 350 years, is driven by identical physical processes that generate storms on Earth and elsewhere in the solar system.

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. These laws are in evidence on Earth, and everywhere we have thought to look—from the domain of particle physics to the large-scale structure of the universe.

It happens that we cannot see, touch, or taste the source of eighty-five percent of the gravity we measure in the universe. This mysterious dark matter, which remains undetected except for its gravitational pull on matter we see, may be compo...

Einstein’s 1916 general theory of relativity expanded on the principles of Newton’s gravity in a way that also applied to objects of extremely high mass. Newton’s law of gravity breaks down in this expanded realm, which was unknown to him. The lesson here is that our confidence flows through the range of conditions over which a law has been tested and verified. The broader that range, the more potent and powerful the law becomes in describing the cosmos. For ordinary household gravity, Newton’s law works just fine. It got us to the Moon and returned us safely to Earth in 1969. For black holes and the large-scale structure of the universe, we need general relativity. And if you insert low mass and low speeds into Einstein’s equations they literally (or, rather, mathematically) become Newton’s equations—all good reasons to develop confidence in our understanding of all we claim to understand.

The power and beauty of physical laws is that they apply everywhere, whether or not you choose to believe in them. In other words, after the laws of physics, everything else is opinion.

Knowledge of physical laws can, in some cases, give you the confidence to confront surly people. A few years ago I was having a hot-cocoa nightcap at a dessert shop in Pasadena, California. Ordered it with whipped cream, of course. When it arrived at the table, I saw no trace of the stuff. After I told the waiter that my cocoa had no whipped cream, he asserted I couldn’t see it because it sank to the bottom. But whipped cream has low density, and floats on all liquids that humans consume. So I offered the waiter two possible explanations: either somebody forgot to add the whipped cream to my hot cocoa or the universal laws of physics were different in his restaurant. Unconvinced, he defiantly brought over a dollop of whipped cream to demonstrate his claim. After bobbing once or twice the whipped cream rose to the top, safely afloat. What better proof do you need of the universality of physical law?

After the big bang, the main agenda of the cosmos was expansion, ever diluting the concentration of energy that filled space. With each passing moment the universe got a little bit bigger, a little bit cooler, and a little bit dimmer.

Since that’s the largest distance that information can travel before reaching your eyes, the entire universe was simply a glowing opaque fog in every direction you

As the cosmos continued to cool, the photons that had been born in the visible part of the spectrum lost energy to the expanding universe and eventually slid down the spectrum, morphing into infrared photons. Although the visible light photons had become weaker and weaker, they never stopped being photons.

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. They’re now microwaves, which is where we derive the modern moniker “cosmic microwave background,” or CMB for short.

When something glows from being heated, it emits light in all parts of the spectrum, but will always peak somewhere. For household lamps that still use glowing metal filaments, the bulbs all peak in the infrared, which is the single greatest contributor to their inefficiency as a source of visible light. Our senses detect infrared only in the form of warmth on our skin. The LED revolution in advanced lighting technology creates pure visible light without wasting wattage on invisible parts of the spectrum. That’s how you can get crazy-sounding sentences like: “7 Watts LED replaces 60 Watts Incandescent” on the packaging.

In this case, besides peaking in microwaves, the CMB also gives off some radio waves and a vanishingly small number of photons of higher energy.

But it was American physicists Ralph Alpher and Robert Herman who, in 1948, first estimated what the temperature of the cosmic background ought to be. They based their calculations on three pillars: 1) Einstein’s 1916 general theory of relativity; 2) Edwin Hubble’s 1929 discovery that the universe is expanding; and 3) atomic physics developed in laboratories before and during the Manhattan Project that built the atomic bombs of World War II.

The first direct observation of the cosmic microwave background was made inadvertently in 1964 by American physicists Arno Penzias and Robert Wilson of Bell Telephone Laboratories, the research branch of AT&T. In the 1960s everyone knew about microwaves, but almost no one had the technology to detect them. Bell Labs, a pioneer in the communications industry, developed a beefy, horn-shaped antenna for just that purpose.

After cleaning out the dielectric substance, the interference dropped a little bit, but a leftover signal remained. The paper they published in 1965 was all about this unaccountable “excess antenna temperature.”†† Meanwhile, a team of physicists at Princeton, led by Robert Dicke, was building a detector specifically to find the CMB. But they didn’t have the resources of Bell Labs, so their work went a little slower. And the moment Dicke and his colleagues heard about Penzias and Wilson’s work, the Princeton team knew exactly what the observed excess antenna temperature was. Everything fit: especially the temperature itself, and that the signal came from every direction in the sky. In 1978, Penzias and Wilson won the Nobel Prize for their discovery. And

Because light takes time to reach us from distant places in the universe, if we look out in deep space we actually see eons back in time. So if the intelligent inhabitants of a galaxy far, far away were to measure the temperature of the cosmic background radiation at the moment captured by our gaze, they should get a reading higher than 2.7 degrees, because they are living in a younger, smaller, hotter universe than we are. Turns out you can actually test this hypothesis. The molecule cyanogen CN (once used on convicted murderers as the active component of the gas administered by their executioners) gets excited by exposure to microwaves. If the microwaves are warmer than the ones in our CMB, they excite the molecule a little more. In the big bang model, the cyanogen in distant, younger galaxies gets bathed in a warmer cosmic background than the cyanogen in our own Milky Way galaxy. And that’s exactly what we observe. You can’t make this stuff up.

The universe was opaque until 380,000 years after the big bang, so you could not have witnessed matter taking shape even if you’d been sitting front-row center. You couldn’t have seen where the galaxy clusters and voids were starting to form. Before anybody could have seen anything worth seeing, photons had to travel, unimpeded, across the universe, as carriers of this information.

That surface is where all the atoms in the universe were born: an electron joins an atomic nucleus, and a little pulse of energy in the form of a photon soars away into the wild red yonder.

By studying these temperature variations in the CMB—that is to say, by studying patterns in the surface of last scatter—we can infer what the structure and content of the matter was in the early universe.

To figure out how galaxies and clusters and superclusters arose, we use our best probe, the CMB—a potent time capsule that empowers astrophysicists to reconstruct cosmic history in reverse. Studying its patterns is like performing some sort of cosmic phrenology, as we analyze the skull bumps of the infant universe.

When constrained by other observations of the contemporary and distant universe, the CMB enables you to decode all sorts of fundamental cosmic properties. Compare the distribution of sizes and temperatures of the warm and cool areas and you can infer how strong the force of gravity was at the time and how quickly matter accumulated, allowing you to then deduce how much ordinary matter, dark matter, and dark energy there is...

What our phrenological exam says is that we understand how the universe behaved, but that most of the universe is made of stuff about which we are clueless.

Our profound areas of ignorance notwithstanding, today, as never before, cosmology has an anchor, because the CMB reveals the portal through which we all walked. It’s a point where interesting physics happened, and where we learned about the universe before and after its light was set free. The simple discovery of the cosmic microwave background turned cosmology into something more than mythology. But it was the accurate and detailed map of the cosmic microwave background that turned cosmology into a modern science.

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.

This spiral galaxy, historically dubbed the Great Nebula in Andromeda, is a somewhat more massive and luminous twin of the Milky Way.

without the benefit of telescopes operating in multiple bands of light we might still declare the space between the galaxies to be empty. Aided

With a list like that, one could argue that all the fun in the universe happens between the galaxies rather than within them.

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. No wonder they are still being discovered in front of our noses.

The Milky Way engaged in at least one act of cannibalism in the last billion years, when it consumed a dwarf galaxy whose flayed remains can be seen as a stream of stars orbiting the galactic center, beyond the stars of the constellation Sagittarius. The system is called the Sagittarius Dwarf, but should probably have been named Lunch.