early universe

Physics describes the behavior of matter, energy, space, and time, and the interplay among them in the universe. From what scientists have been able to determine, all biological and chemical phenomena are ruled by what those four characters in our cosmic drama do to one another. And so everything fundamental and familiar to us earthlings begins with the laws of physics.

In almost any area of scientific inquiry, but especially in physics, the frontier of discovery lives at the extremes of measurement. At the extremes of matter, such as the neighborhood of a black hole, you find gravity badly warping the surrounding space-time continuum. At the extremes of energy, you sustain thermonuclear fusion in the ten-million degree cores of stars. And at every extreme imaginable, you get the outrageously hot, outrageously dense conditions that prevailed during the first few moments of the universe.

Daily life, we're happy to report, is wholly devoid of extreme physics. On a normal morning, you get out of bed, wander around the house, eat something, dash out the front door. And, by day's end your loved ones fully expect you to look no different than you did when you left, and to return home in one piece. But imagine arriving at the office, walking into an overheated conference room for an important 10:00 a.m. meeting, and suddenly losing all your electrons—or worse yet, having every atom of your body fly apart. Or suppose you're sitting in your office trying to get some work done by the light of your desk lamp, and somebody flicks on the overhead light, causing your body to bounce randomly from wall to wall until you're jack-in-the-boxed out the window. Or what if you went to a sumo wrestling match after work and saw the two spherical gentlemen collide, disappear, then spontaneously become two beams of light?

If those scenes played out daily, then modern physics wouldn't look so bizarre, knowledge of its foundations would flow naturally from our life experience, and our loved ones probably would never let us go to work. Back in the early minutes of the universe, though, that stuff happened all the time. To envision it, and understand it, one has no choice but to establish a new form of common sense, an altered intuition about how physical laws apply to extremes of temperature, density, and pressure.

Enter the world of E = mc².

Albert Einstein first published a version of this famous equation in 1905 in a seminal research paper titled On the Electrodynamics of Moving Bodies. Better known as the special theory of relativity, the concepts advanced in that paper forever changed our notions of space and time. Einstein, then just twenty-six years old, offered further details about his tidy equation in a separate, remarkably short paper published later the same year: Does the Inertia of a Body Depend on Its Energy Content? To save you the effort of digging up the original article, designing an experiment, and testing the theory, the answer is Yes. As Einstein wrote,

If a body gives off the energy E in the form of radiation, its mass diminishes by E/c2 . . . . The mass of a body is a measure of its energy-content; if the energy changes by E, the mass changes in the same sense

Uncertain as to the truth of his statement, he then suggested,

It is not impossible that with bodies whose energy-content is variable to a high degree (e.g. with radium salts) the theory may be successfully put to the test.

There it is. The algebraic recipe for all occasions when you want to convert matter into energy or energy into matter. In those simple sentences, Einstein unwittingly gave astrophysicists a computational tool, E = mc², that extends their reach from the universe as it now is, all the way back to infinitesimal fractions of a second after its birth.

The most familiar form of energy is the photon, a massless, irreducible particle of light. You are forever bathed in photons: from the Sun, the Moon, and the stars to your stove, your chandelier, and your night light. So why don't you experience E = mc² every day? The energy of visible light photons falls far below that of the least massive subatomic particles. There is nothing else those photons can become, and so they live happy, relatively uneventful lives.

Want to see some action? Start hanging around gamma-ray photons that have some real energy—at least 200,000 times more than that of visible photons. You'll quickly get sick and die of cancer, but before that happens you'll see pairs of electrons—one matter, the other antimatter; one of many dynamic duos in the particle universe—pop into existence where photons once roamed. As you watch, you will also see matter-antimatter pairs of electrons collide, annihilating each other and creating gamma-ray photons once again. Increase the light's energy by a factor of another 2,000, and you now have gamma rays with enough energy to turn susceptible people into the Hulk. But pairs of these photons now have enough energy to spontaneously create the more massive neutrons, protons, and their antimatter partners. High-energy photons don't hang out just anywhere. But the place needn't be imaginary. For gamma rays, almost any environment hotter than a few billion degrees will do just fine.

The cosmological significance of particles and energy packets transmuting into each other is staggering. Currently the temperature of our expanding universe, calculated from measurements of the microwave bath of light that pervades all of space, is a mere 2.73 degrees Kelvin. (On the Kelvin scale, zero is the temperature at which molecules have the lowest possible energy, room temperature is about 295 degrees, and water boils at 373 degrees.) Like the photons of visible light, microwave photons are too cool to have any realistic ambitions to become a particle via E = mc²; in fact, there are no known particles they can spontaneously become. Yesterday, however, the universe was a little bit smaller and a little bit hotter. The day before, it was smaller and hotter still. Roll the clocks backward some more—say, 13.7 billion years—and you land squarely in the primordial soup of the big bang, a time when the temperature of the cosmos was high enough to be astrophysically interesting.

The way space, time, matter, and energy behaved as the universe expanded and cooled from the beginning is one of the greatest stories ever told. But to explain what went on in that cosmic crucible, you must find a way to merge the four forces of nature into one, and find a way to reconcile two incompatible branches of physics: quantum mechanics (the science of the small) and general relativity (the science of the large). Spurred by the successful marriage of quantum mechanics and electromagnetism in the mid twentieth century, physicists set off on a race to blend quantum mechanics and general relativity (into a theory of quantum gravity). Although we haven't yet reached the finish line, we know exactly where the high hurdles are: during the Planck era. That's the phase up to 10^-43 seconds (one ten-million-trillion-trillion-trillionths of a second) after the beginning, and before the universe grew to 10^-35 meters (one hundred billion trillion-trillionths of a meter) across. 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 with being the father of quantum mechanics.

Not to worry, though. The clash between gravity and quantum mechanics poses no practical problem for the contemporary universe. Astrophysicists apply the tenets and tools of general relativity and quantum mechanics to very different classes of problems. But in the beginning, during the Planck era, the large was small, and there must have been a kind of shotgun wedding between the two. Alas, the vows exchanged during that ceremony continue to elude us, and so no (known) laws of physics describe with any confidence the behavior of the universe during the brief interregnum.

At the end of the Planck era, however, gravity wriggled loose from the other, still-unified forces of nature, achieving an independent identity nicely described by our current theories. As the universe aged through 10^-35 seconds it continued to expand and cool, and what remained of the unified forces split into the electroweak and the strong nuclear forces. Later still, the electroweak force split into the electromagnetic and the weak nuclear forces, laying bare the four distinct forces we have come to know and love—with the weak force controlling radioactive decay, the strong force binding the nucleus, the electromagnetic force binding molecules, and gravity binding bulk matter.

By now, the universe was a mere trillionth of a second old. Yet its transmogrified forces and other critical episodes had already imbued our universe with fundamental properties each worthy of its own book.

While the universe dragged on for its first trillionth of a second, the interplay of matter and energy was incessant. Shortly before, during, and after the strong and electroweak forces parted company, the universe was a seething ocean of quarks, leptons, and their antimatter siblings, along with bosons, the particles that enable their interactions. None of these particle families is thought to be divisible into anything smaller or more basic. Fundamental though they are, each come in several species. The ordinary visible-light photon is a member of the boson family. The leptons most familiar to the nonphysicist are the electron and perhaps the neutrino; and the most familiar quarks are. . . . well, there are no familiar quarks. Each species has been assigned an abstract name that serves no real philological, philosophical, or pedagogical purpose except to distinguish it from the others: up and down, strange and charmed, and top and bottom.

Bosons, by the way, are simply named after the Indian scientist Satyendranath Bose. The word lepton derives from the Greek leptos, meaning light or small. Quark, however, has a literary and far more imaginative origin. The physicist Murray Gell-Mann, who in 1964 proposed the existence of quarks, and who at the time thought the quark family had only three members, drew the name from a characteristically elusive line in James Joyce's Finnegans Wake: Three quarks for Muster Mark! One thing quarks do have going for them: all their names are simple—something chemists, biologists, and geologists seem incapable of achieving when naming their own stuff.

Quarks are quirky beasts. Unlike protons, each with an electric charge of +1, and electrons, with a charge of –1, quarks have fractional charges that come in thirds. And you'll never catch a quark all by itself; it will always be clutching on to other quarks nearby. In fact, the force that keeps two (or more) of them together actually grows stronger the more you separate them—as if they were attached by some sort of subnuclear rubber band. Separate the quarks enough, the rubber band snaps and the stored energy summons E = mc² to create a new quark at each end, leaving you back where you started.

But during the quark-lepton era the universe was dense enough for the average separation between unattached quarks to rival the separation between attached quarks. Under those conditions, allegiance between adjacent quarks could not be unambiguously established, and they moved freely among themselves, in spite of being collectively bound to each other. The discovery of this state of matter, a kind of quark soup, was reported for the first time in 2002 by a team of physicists at the Brookhaven National Laboratories.

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 hardly got noticed 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 plenty of opportunities to find someone to annihilate with, and so did everybody else. But not for much longer. As the cosmos continued to expand and cool, it became the size of the solar system, with a temperature dropping rapidly past a trillion degrees Kelvin. A millionth of a second had passed since the beginning.

This tepid universe was no longer hot enough or dense enough to cook quarks, and so they all grabbed dance partners, creating a permanent new family of heavy particles called hadrons (from the Greek hadros, meaning thick). That quark-to-hadron transition soon resulted in the emergence of protons and neutrons as well as other, less familiar heavy particles, all composed of various combinations of quark species. The slight matter-antimatter asymmetry afflicting the quark-lepton soup now passed to the hadrons, but with extraordinary consequences.

As the universe cooled, the amount of energy available for the spontaneous creation of basic particles dropped. During the hadron era, ambient photons could no longer invoke E = mc² to manufacture quark-antiquark pairs. Not only that, the photons that emerged from all the remaining annihilations lost energy to the ever-expanding universe and dropped below the threshold required to create hadron-antihadron pairs. For every billion annihilations—leaving a billion photons in their wake—a single hadron survived. Those loners would ultimately get to have all the fun: serving as the source of galaxies, stars, planets, and people.

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

By now, one second of time has passed.

The universe has grown to a few light-years across, about the distance from the Sun to its closest neighboring stars. At a billion degrees, it's still plenty hot—and still able to cook electrons, which, along with their positron counterparts, continue to pop in and out of existence. But in the ever-expanding, ever-cooling universe, their days (seconds, really) are numbered. What was true for hadrons is true for electrons: eventually only one electron in a billion survives. The rest get annihilated, together with their antimatter sidekicks the positrons, in a sea of photons.

Right about now, one electron for every proton has been frozen into existence. As the cosmos continues to cool—dropping below a hundred million degrees—protons fuse with protons as well as with neutrons, forming atomic nuclei and hatching a universe in which 90 percent of these nuclei are hydrogen and 10 percent are helium, along with trace amounts of deuterium, tritium, and lithium.

Two minutes have now passed since the beginning.

Not for another 380,000 years does much happen to our particle soup. Throughout these millennia the temperature remains hot enough for electrons to roam free among the photons, batting them to and fro. But all 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 electrons combine with free nuclei. The marriage leaves behind a ubiquitous bath of visible-light photons, completing the formation of particles and atoms in the primordial universe.

As the universe continues to expand, its photons continue to lose energy, dropping from visible light to infrared to microwaves.

Today, everywhere astrophysicists look we find an indelible fingerprint of 2.73 degree microwave photons, whose pattern on the sky retains a memory of the distribution of matter just before atoms formed. From this we can deduce many things, including the age and shape of the universe. And although atoms are now part of daily life, Einstein's equilibrious equation still has plenty of work to do—in particle accelerators, where matter-antimatter particle pairs are created routinely from energy fields; in the core of the Sun, where 4.4 million tons of matter are converted into energy every second; and in the cores of every other star.

It also manages to occupy itself near black holes, just outside their event horizons, where particle-antiparticle pairs can pop into existence at the expense of the black hole's formidable gravitational energy. Stephen Hawking first described that process in 1975, showing that the mass of a black hole can slowly evaporate by this mechanism. In other words, black holes are not entirely black. Today the phenomenon is known as Hawking radiation, and is a reminder of the continued fertility of E = mc².

But what happened before all this? What happened before the beginning?

Astrophysicists have no idea. Or, rather, our most creative ideas have little or no grounding in experimental science. Yet certain type of religious person tends to assert, with a tinge of smugness, that something must have started it all: a force greater than all others, a source from which everything issues. A prime-mover. In the mind of such a person, that something is, of course, God.

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

Such replies usually satisfy nobody. Nonetheless, they remind us that ignorance is the natural state of mind for a research scientist on the ever-shifting frontier. 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 cosmos. And therein lies a fascinating dichotomy. The universe always was goes unrecognized as a legitimate answer to What was around before the beginning? But for many religious people, the answer God always was is the obvious and pleasing answer to What was around before God?

No matter who you are, engaging in the quest to discover where and how things began tends to induce emotional fervor—as if knowing the beginning bestows upon you some form of fellowship with, or perhaps governance over, all that comes later. So what is true for life itself is no less true for the universe: knowing where you came from is no less important than knowing where you are going.

Back at the beginning of everything, when the universe was a fraction of a second old, a ferocious trillion degrees hot, and glowing with an unimaginable brilliance, its main agenda was expansion. With every passing moment the universe got a little bit bigger. But it also got a little bit cooler and a little bit dimmer. Meanwhile, matter and energy cohabited in a kind of opaque soup, in which free electrons continually scattered light to and fro. For 380,000 years things went on that way, until the temperature dropped below 3,000 degrees. Right about then, electrons slowed down enough to be captured by passing protons, thus bringing atoms into the world, and a cosmic background of visible light was set free.

The cosmic background is the incarnation of the leftover light from a dazzling, sizzling early universe. It's a ubiquitous bath of photons—massless vehicles of light energy that are as much waves as they are particles. 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 became weaker and weaker, they never stopped being photons.

What's next on the spectrum? Today, more than 14-billion years after the beginning, the photons of the cosmic background have cooled to microwaves, giving us the modern moniker cosmic microwave background, or CMB for short. Continue to expand and cool the universe, and astrophysicists some 50 billion years from now will be writing about the cosmic radio wave background.

In the 3,000 degree youthful universe, photons didn't travel far before scattering every which way off an electron. Back then, if your mission had been to see across the universe, you couldn't have. Any photons you saw would, just nano- and pico-seconds earlier, have bounced off an electron right in front of your face, and so you would have seen only a glowing fog everywhere you looked. The entire luminous, translucent, reddish-white sky would have been nearly as bright as the surface of the Sun.

Every photon travels at the speed of light until it slams into an electron. Then it bounces off and travels in another direction at the speed of light, slams into another electron, bounces off, and so on. Only when photons are no longer slamming into electrons can they freely traverse the universe. And electrons didn't cease their obstructionism until they made some more or less permanent commitments to atomic nuclei. That's when the the universe became transparent, and that's when the cosmic background was formed.

The temperature of the universe is directly related to the size of the universe. It's a physical thing. If a photon lost half its original energy, it's because the universe grew to twice its original size. As the universe grows, the photon's wavelength gets longer, stretching with the expanding fabric of space and time.

A photon's wavelength is the simple separation between one crest and the next, a distance you can measure with a ruler. A photon's energy is measured by its frequency, or the number of wiggles it makes during a given interval of time. Photons of lower frequencies carry less energy than photons of higher frequencies.

When an object glows from being heated, it emits radiation (light) in all parts of the spectrum, but will always peak somewhere. For household lamps, the light bulbs all peak in the infrared, a part of the spectrum we detect only in the form of warmth on our skin. But bulbs also, of course, emit plenty of visible light. That's why when you put your hand near your bedside lamp, you feel the light as well as see it. Currently the peak wavelength of the cosmic background is about 1 millimeter, which is smack dab in the microwave part of the spectrum. If you open a channel on your walkie-talkie where there isn't a strong signal, you hear static. That static is ambient background microwaves, a few percent of which are the CMB; the rest comes from the Sun, cell phones, police radar guns, leaky microwave ovens, and so on.

Being the remnant of something that was once brilliantly aglow, the CMB has the wavelength profile we expect of a radiant but cooling object: it peaks in one part of the spectrum but radiates in other parts of the spectrum as well. 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.

The existence of the CMB was predicted by the Russian-born U.S. physicist George Gamow and colleagues during the 1940s, culminating in a 1948 paper that laid out the physics of the early universe. The foundation of these ideas came from the 1927 work of the Belgian physicist and Jesuit priest Georges Lemaître, who is the generally recognized father of big bang cosmology. But it was U.S. physicists Ralph Alpher and Robert Herman, in 1948, who estimated what the temperature of the cosmic background ought to be. They based their calculations on a of Einstein's 1916 general theory of relativity, Edwin Hubble's 1929 observations that the universe is expanding, and the particle and nuclear physics developed before, and during the Manhattan Project of WWII.

With hindsight, the argument is relatively simple. The fabric of space-time would have been smaller yesterday than it is today, and if it was smaller basic physics requires that it must have been hotter. So they turned the clock backward, and imagined a time when the universe was so hot that all the atoms would have been completely ionized, laying bare with all their electrons roaming free. And if that was the case, light would not always have arced, uninterrupted, across the universe. The photons' free ride couldn't have begun before a certain stage in the cooling of the cosmos.Not before the electrons would have combined with atoms and stopped interfering with the light. This transition occurs at a few thousand degrees.

While it was Gamow who suggested that the universe was once hotter, and that you could know the physics of the early universe, it was Herman and Alpher, who proposed a temperature of 5 degrees. Yes, they got the wrong temperature—the CMB is 2.7 degrees Kelvin—but together, those three physicists made an extrapolation unlike any other in the history of science. To take some basic atomic physics from a slab in the lab, and then deduce the largest-scale phenomenon ever measured, was nothing short of extraordinary. Writing about this feat, the U.S. physicist J. Richard Gott says, Predicting that the radiation existed and then getting its temperature correct to within a factor of 2 was a remarkable accomplishment—rather like predicting that a flying saucer 50 feet in width would land on the White House lawn and then watching one 27 feet in width actually show up.

At the time Gamow, Herman, and Alpher were coming up with their prediction, physicists were still undecided about the beginning of the universe. The year 1948 also saw the publication of a rival steady state theory of the universe by the English physicist Hermann Bondi and the American Astronomer Thomas Gold. The CMB proved to be the turning point. It indicated clearly that the universe had once been different—smaller, and hotter—whereas the rival, steady state model implied that the universe, while expanding had always looked the same. The universe could not be hotter yesterday And they provided for matter to pop into our universe from someplace else, at just the right rate, to leave the expanding universe with the same average density. And. The first direct observation of the CMB ware the nails in the coffin of the steady-state theory, and that observation was made inadvertently in 1964 by Arno Penzias and Robert Wilson of Bell Laboratories. In 1978, they won the Nobel Prize for it.

In the 1960s everyone knew about microwaves, but almost no one had the technology to detect them. Back then most wireless communication was with radio waves, which are longer, and so the existing receivers and detectors and transmitters weren't useful. You needed a shorter-wavelength detector and a suitable antenna to capture them. Bell Labs, pioneer in the communications industry, had a beefy horn-shaped antenna that could focus the microwaves down to the detectors.

If you're going to send or receive a signal, you don't want too many things contaminating it. Penzias and Wilson wanted to get a measure of the background interference—from the Sun, from the center of the galaxy, from terrestrial sources, from whatever. So they made an innocent measurement. They weren't cosmologists; they were physicist-technologists looking for microwaves, unaware of the Gamow, Herman, and Alpher predictions. What they were decidedly not looking for was the cosmic microwave background; they were just trying to open up a new channel of communication for Bell Laboratories, and before doing that, they needed to characterize all the things that would contaminate a signal.

So Penzias and Wilson run their experiment, and correct their data for all the sources of interference they know about, but there's one part of the signal that doesn't go away, and they just can't figure out how to eliminate it. Finally they look inside the dish. They see pigeons nesting inside. And so they're worried that a white dielectric substance (pigeon poop) might be responsible for the signal, because the signal comes from every direction, and it doesn't change, and the only thing that's all over their fancy horn-shaped antenna and doesn't change is the pigeon poop. So they clean it up, the signal drops a little bit, but there's still something left. The paper they publish in 1965 in the Astrophysical Journal talks about the unaccountable excess antenna temperature.

But at the same time Penzias and Wilson were scrubbing bird droppings off their fancy antenna, a team of physicists at Princeton, led by Robert Dicke, were building a detector specifically to find the CMB. They, however, didn't have the resources of Bell Labs, so their work went a little slower. as the moment Dicke and his colleagues heard about Penzias and Wilson's work, they knew they'd been scooped. The Princeton team knew exactly what the observed excess antenna temperature was. Everything fit: the temperature, the fact that the signal came from every direction, and that it wasn't linked in time with Earth's rotation or position around the Sun.

Because light takes long stretches of time to us from distant places in the universe, if we look out in space we are actually look back in time. So if the intelligent inhabitants of a galaxy far, far away were measuring the temperature of the cosmic background radiation at the moment captured by our gaze, they would get a temperature higher than 2.7 degrees, because they are living in a younger, smaller, hotter universe than we are.

Can such an assertion be tested? It turns out that the molecule cyanogen (CN)—best known to 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 molecule a little more. The cyanogen in distant, and thus younger, galaxies is exposed to a warmer cosmic background than the cyanogen in our galaxy. Indeed, their cyanogens lives at a higher exited state than ours. Although CN is a powerful cosmic marker, its behavior was all learned in the lab

You can't make this stuff up.

But why should any of this be interesting? 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.

The spot where each photon began its cross-cosmos journey is where it smacked into the last electron that would ever stand in its way—the point of last scatter. As more and more photons escape unsmacked, they create an three-dimensional, expanding surface of last scatter, some 120,000 years deep. 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 then, some regions of the universe had already begun to coalesce by the gravitational attraction of its parts. Photons that last scattered off electrons in these regions developed a different, slightly cooler profile than those scattering off the less sociable electrons sitting in the middle of nowhere. Where matter accumulated, the strength of gravity grew, enabling more and more matter to gather. These regions seeded the formation of galaxy superclusters while other regions were left relatively empty.

When you map the CMB in detail [see Sharper Focus, by Charles Liu, Natural History, May 2003], you find that it's not completely smooth. It does have spots that are slightly hotter and slightly cooler spots than average. And by studying these temperature variations in the CMB—that is to say, by studying the structure of the surface of last scatter—we can infer what the structure and content of the matter was in the early universe. We know what the structure of matter is today, because we see galaxies and clusters and superclusters. To figure out how those systems arose, we use out best probe, the cosmic microwave background. The CMB is a remarkable time capsule a way to reconstruct history in reverse. Studying its patterns is like performing some sort of cosmic phrenology. We're looking at the skull bumps of the grown-up universe and inferring its behavior as an infant.

When constrained by other observations of the contemporary and distant universe, the CMB enables you to find out 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 (4%), dark matter (23%), and dark energy (73%) there is in the universe. From here, it's then easy to tell whether or not the universe will expand forever.

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 that acts in the opposite direction of gravity, forcing the universe to expand faster than it otherwise would. What our phrenological exam says is that we understand how the universe behaved, but that most of the universe is made of stuff for which we have no clue what it is.

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: the surface of last scatter. It's a point where interesting physics happened, and where we learned about the universe before and after it's 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 an experimental science. Cosmologists have plenty of ego—how can a person not be ego-driven when it's your job to deduce what brought the universe into existence? But without data, their explanations were just tall tales. In this modern era of cosmology, each new observation, each morsel of data wields a two-edged sword: it enables cosmology to thrive on the kind of foundation that so much of the rest of science enjoys, but it also constrains theories that people thought up when there wasn't enough data to say whether they were wrong or not. No science achieves maturity without it.

Let there be cosmology.