The Big Bang

Most scientists agree that the universe began some 12 to 20 billion years ago in what has come to be known as the Big Bang (a term coined by the English astrophysicist Fred Hoyle in 1950. Hoyle, who championed a rival cosmological theory, meant the "Big Bang" to be a term of derision, but the name was so catchy that it stuck.). Though the Big Bang suggests a colossal explosion, it wasn't really an "explosion" in the sense that we understand it. Space itself exploded. At the instant of the Big Bang, the universe was infinitely dense and unimaginably hot. Cosmologists believe that all forms of matter and energy, as well as space and time itself, were formed at this instant. Since "before" is a temporal concept, one cannot ask what came before the Big Bang and therefore "caused" it, at least not within the context of any known physics. (At least one cosmological theory, however, predicts that our universe's Big Bang is part of a chain reaction in which the demise of one universe spawns the birth of many parallel universes. According to this scenario, our universe may simply be part of a huge, infinitely growing fractal.) Science tells us nothing about the way space, time and matter behaved in our universe's earliest instant, from the time of the Big Bang to 10^-43 seconds later. Space was certainly expanding--violently--and from this expansion of space was formed a highly energetic soup of particles and antiparticles. The energy was so great during this so-called Grand Unification Epoch--a fine-sounding name for the period from 10^-43 to 10^-35 seconds after the Big Bang--that all matter and energy was essentially interchangeable and in equilibrium. What's more, electromagnetism and the two nuclear forces were as one (gravity, the fourth and weakest force, had separated from the other three at the beginning of the Grand Unification Epoch). As the universe expanded, it cooled down. At 10^-35 seconds, the temperature was a mere 10^27 degrees K (water boils at 373.16 K or 3.7316^2!). At this critical temperature, the universe underwent a phase transition, something like the process that happens when liquid water freezes into ice. The strong nuclear force--which acts at very short distances and holds protons and neutrons together--split off from the other forces. Physicists call this process "symmetry breaking," and it released an enormous amount of energy. Then, in an extraordinary instant that theorists have dubbed "inflation," the universe expanded exponentially. During this time, the universe grew by a factor of 10^50 in 10^-33 seconds. Talk about runaway inflation! This scenario, much as it strains credulity, neatly explains several different observations made during the last 20 years--the large-scale smoothness and apparent flatness of the universe among them--that had weakened the original Big Bang theory of cosmology based on a much more leisurely period of expansion. Things slowed up a bit after the inflationary epoch. A number of observations, well supported by theory, suggest that our universe continued to expand, albeit more slowly, and that it is expanding still. As space expanded, it continued to cool down. Matter--at first photons, quarks, neutrinos, and electrons, and then protons and neutrons--condensed out, all less than one second after the Big Bang. It was not until one billion years later, when the universe was one-fifth the size it is today, that the matter would form the first stars and galaxies The ancient Greek philosophers Democritus and Leucippus were ahead of their time when they proposed that matter are made up of tiny grains, or atoms; there was no solid empirical evidence for atoms until the late nineteenth century. Now we know that neither atoms nor the protons and neutrons that make up atomic nuclei are fundamental particles. Particle physicists have constructed a family of 12 fundamental particles, divided into two groups of six: quarks and leptons. "Up" and "down" quarks (the names are strictly arbitrary; there is no up or down orientation peculiar to either particle) are the building blocks of protons and neutrons. One of the leptons is the familiar electron. Also in the lepton class are wispy, nearly massless neutrinos that interact only very weakly with other particles. Elusive as they are, neutrinos are abundant in the universe and may be candidates for the missing dark matter. At 10^-12 second, the weak and electromagnetic forces separated, leaving us with the four separate forces we know today. The weak force, which governs radioactive decay, acts only over very short distances. The more familiar electromagnetic force governs electricity, magnetism, and the propagation of electromagnetic radiation such as visible light. During the Grand Unification Epoch, quarks and leptons and their corresponding anti-matter particles were constantly colliding and annihilating each other with a release of energy; two colliding photons could likewise create new matter and antimatter. What's more, quarks could decay into leptons, and vice versa. Matter, anti-matter, and radiation existed in nearly equal amounts. There's hardly any antimatter left today (and a good thing, too, or we wouldn't exist: everything would have been annihilated long ago!). What happened to it? Just before the end of the Grand Unification Epoch, there was a very slight excess--perhaps one in a billion--of matter over antimatter. Quarks could decay, or be created, without the corresponding decay or creation of antiquarks. As the universe continued expanding and cooling and the strong force separated from the electroweak force (which would later separate into the electromagnetic and weak forces), quarks and antiquarks condensed into hadrons. The most stable, and therefore familiar, hadrons are called baryons, also known as protons and neutrons. baryons (a unit of three quarks) and antibaryons, mesons (a unit of two quarks) and antimesons--with a slight excess of baryons and mesons over antibaryons and antimesons. Mesons and some kinds of baryons are highly unstable; they and their antimatter cousins, the dinosaur particles of the universe disappeared long ago (although they can be re-created, Jurassic Park-like, in the laboratory). When the universe was 10^-4 (1/10,000th) second old, there was no longer enough energy to create new, baryon-antibaryon pairs. In a kind of cosmic shoot-out, the pairs continued to collide with and annihilate each other, producing a huge number of photons and leaving a few surviving baryons. When the universe was about one second old, the universe cooled to the point that electron-antielectron pairs could no longer be created. The result was another mass annihilation, creating even more photons and leaving behind a few electrons. The weak force was losing its effectiveness to mediate interactions between neutrinos, antineutrinos, and the other particles. Neutrinos no longer interacted with other particles or even antineutrinos, thereby escaping annihilation. They began--and continue today--to drift through space in enormous numbers; some scientists believe they might account for a significant portion of the missing dark matter. Now the stage is set for the formation of atomic nuclei--from which all the matter we see in the universe today originates. Almost all of the helium, deuterium (heavy hydrogen), and some of the lithium nuclei in our universe today were created during the "Era of Nucleosynthesis," which began about 1 second after the Big Bang and ended just 100 seconds later (ordinary hydrogen nuclei, each consisting of just one proton, did not have to be "created;" they already existed!). One hundred seconds after the Big Bang, the temperature dropped to the point where protons and neutrons could stick together without being torn apart by highly energetic photons. The neutrinos and antineutrinos had lost their ability to interact with protons or neutrons, and all of the positrons (electrons' antimatter particle), had been annihilated. The conditions were suddenly ripe for nucleus formation, the most stable being that of two protons and two neutrons (helium). At the end of the nucleosynthesis period, all of the neutrons had paired with protons to form helium (24% of the primordial light elements) and trace amounts of deuterium (2 protons), tritium (3 protons), helium-3 (two protons and one neutron) and lithium (three protons and four neutrons). The protons that were left over were destined to become hydrogen nuclei, which made up 75% of these new atomic nuclei. These nuclei, being composed of baryons, are known as baryonic matter. The ultimate fate of the universe--whether it continues to expand forever, eventually reaches a steady state, or collapses in one big crunch--depends on the density of baryonic matter. Astrophysicists at Johns Hopkins University recently detected, in the intergalactic medium, the helium formed in the first two minutes after the Big Bang. This matter, along with the primordial hydrogen that is almost sure to accompany the helium, is sparsely scattered throughout intergalactic space. These atomic nuclei--joined with electrons many years later--would eventually become the seeds of stars. All of the other elements--from the carbon, nitrogen, and oxygen upon which life is based to metals like iron, copper and gold--were forged in repeating cycles of starbirth and death. And, although stars continue to produce helium, scientists believe that 98% of the helium in the universe today was produced in those first few seconds. For 300,000 years, protons and atomic nuclei continued to roam about in an almost totally opaque sea of photons, electrons and neutrinos--opaque because the photons couldn't travel far without bumping into another particle. Any electron that combined with a proton or atomic nucleus was immediately knocked off by a traveling photon. Matter and radiation were intimately linked. But after about 300,000 years, the opaque soup of nuclear matter and radiation began to clear. By this time, the temperature of the universe dropped to 3,000 K. Photons no longer had enough energy to knock electrons free from atomic nuclei and protons. Now the photons were free to travel through the universe, at last decoupled from matter. This era, called recombination, actually lasted about a million years. The vast sea of photons created during the earliest epochs prior to recombination persists to this day, in the form of cosmic microwave background that pervades the universe. No longer so energetic after being stretched by the expansion of the universe for roughly 20 billion years, this radiation has cooled to a chilly 2.73 K (minus 270.43 degrees Celsius!). It's nonetheless considered by cosmologists to be one of the clearest signatures of the Big Bang. The uniformity of this radiation--to within a few parts in 100,000--indicates that the universe was extremely smooth around the time of recombination. Tiny variations have recently been found in this background radiation, indicating minute fluctuations in density of matter and energy at recombination. These fluctuations were eventually amplified by gravity to form the objects that make up our hierarchical universe: stars, galaxies, galaxy clusters and superclusters. Accompanying those minute fluctuations in radiation, scientists believe, were tiny fluctuations of matter, or, more precisely, baryonic matter, mainly hydrogen and helium gas. Gravitational attraction between the atoms concentrated them into faint clouds of gas. As the universe expanded, the surrounding matter gradually thinned out, with the result that the internal gravity of the gas clouds grew relatively stronger. Slowly, then faster and faster, the clouds pulled in more and more material from the surrounding medium. Eventually, the clouds began to collapse under their own gravity, evolving into galaxies. About one billion years after the Big Bang, the first galaxies and the stars they contain were born. Peering back 12 billion years in time and space, scientists using the Hubble Space Telescope recently observed what appear to be "baby galaxies." Despite their "age," these 12 billion year-old galaxies have patterns of light emission remarkably similar to today's fully formed elliptical galaxies -- round collections of stars that formed fairly rapidly. The Hubble telescope has also detected nascent spiral galaxies 9 billion light years away. Far from being neat spirals, these early galaxies resemble a zoo of tadpole-like objects, disturbed systems called "train-wrecks" and a multitude of other strange formations. The bluish color of these early galaxies, corresponding to hot gas, indicates active starbirth. Taken together, these findings give scientists important clues about the age of the universe. The billion years it took for galaxies to form, added to the initial one billion years after the Big Bang and the 12 billion years it took for the galaxy's light to reach the Hubble Telescope, implies that the universe is about 14 billion years old. This estimate is in approximate agreement with other estimates based on the age of the oldest known stars. But it stands in stark contrast with other observations, also based on Hubble telescope observations, suggesting that the universe is only 8 billion years old. Quasars--quasi-stellar radio sources--may be another kind of infant galaxy. They look like bright stars, and yet they are billions of light years from earth. They are relatively small--about the size of a solar system--and yet they emit prodigious amounts of energy, particularly in the radio region of the electromagnetic spectrum. One quasar may emit as much energy as hundreds of galaxies put together! What could power the enormous amounts of radio energy pouring out of these distant objects? One distinct possibility is that gigantic black holes weighing up to a hundred million suns lurk at the centers of rapidly rotating disks of gas from which galaxies later evolve. Some scientists theorize that such supermassive black holes may form when a young galaxy is only a mass of gas and contains but a few stars. A star in the densely packed core explodes, its core collapsing into a black hole which then grows rapidly, devour ring neighboring stars and gas. Some of the gas sucked in by the black hole is transformed into vast amounts of energy, particularly radiation at radio wavelengths. It's this radiation we can detect with the aid of radio telescopes. Many astronomers believe that quasars eventually settle down and become radio galaxies--larger, elliptical structures that continue to emit strongly in the radio region of the spectrum, but are no longer as bright as their quasar precursors. Eventually these radio galaxies might evolve into galaxies. Because of their brightness and distance, quasars are used by cosmologists as beacons to estimate distances on grand scales and to study faraway gas clouds, which absorb some of the quasar's radiation. The characteristics of this absorption can reveal a lot about the mass, density and composition of the intervening gas. These gas clouds may be the stuff from which galaxies are condensing. There's evidence that, shortly after the Big Bang, the universe was essentially uniform in its density and appearance. When we peer out to the cosmos today, it's evident that the distribution of matter is far from uniform. In fact it's positively lumpy, even on a large scale, and clearly exhibits a hierarchical organization. As far as we can tell, planets formed sometime during starbirth, giving rise to solar systems such our own. Stars are organized into galaxies, which in turn appear to be bound gravitationally together in clusters. Superclusters of galaxies stretch as gigantic sheets across hundreds of billions of light years, bounded by enormous voids with like dimensions. How can this evident "lumpiness" be explained? That's but one of the questions challenging cosmologists, as they try to explain the universe we see today. Other difficult questions about cosmic origins and evolution preoccupy their minds, such as: How much matter does the universe contain?, What kind of matter fills it?, What is the shape of the cosmos?, How old is it?, Will it expand forever? To answer these questions, cosmologists are turning to an impressive array of tools. On the one hand, new, powerful new telescopes, earth-bound and spaceborn, enable them to peer out (and back in time) as never before. On the other, alternative models of cosmic creation and evolution can be tested as simulations in advanced computers. Better observations, new theories and now computation hold the keys to solving these ancient mysteries.