1
Expanding into Universe (13.7 Billion-4.6 Billion Years Ago)
We are all whirling about in space on a small planet, bathed for part of each day in the light and warmth of a nearby star we call the sun. We are traveling 2 million miles a day around the center of the Milky Way galaxy, which is whirling in a universe of more than 100 billion galaxies, each home to 100 billion stars (Fig. 1.1).
This universe in which we whirl began as a single point 13.7 billion years ago; it has been expanding ever since, with its temperature steadily decreasing. Our universe has at least four dimensions, three of space and one of time, meaning that time and space are interconnected. Just now the size of our observable universe is roughly 13.7 billion light-years on each of three dimensions by 13.7 billion years on the dimension of time, increasing as I write and you read.
Ever since human beings developed, they have been looking at points of light in the nighttime sky with awe and respect, learning what they could from direct observations and using this knowledge to make predictions, to travel on land, and to navigate by sea. Without specialized instruments, however, people could not detect much about the origin of our immense universe and the nature of matter, because the scale of the universe and of matter is so different from that of everyday life. By the late twentieth century, scientists had invented instruments that could begin to view the macroscopic heavens and the microscopic domain. Knowledge about these worlds has recently expanded exponentially. Now everyone can understand the amazing universe that is our home—if we use our imaginations and absorb the photographic images and diagrams that are currently available.
11.1 The Milky Way Galaxy
Fog and Transparency
It all began with an inconceivable event: the big bang. (This name was given by the British astrophysicist Fred Hoyle on a BBC radio broadcast in 1952.)
2 The universe erupted from a single point, perhaps the size of an atom, in which all known matter and energy and space and time were squeezed together in unimaginable density. Compressed space unfurled like a tidal wave, expanding in all directions and cooling, carrying along matter and energy to this very day. The power in this initial expansion was sufficient to fling a hundred billion galaxies for 13.7 billion years and counting. The billowing universe was under way.
Where did this eruption take place? Everywhere, including where each of us is right now. In the beginning all the locations that we see as separate were the same location.
Initially the universe was composed of “cosmic plasma,” a homogeneous substance so hot that it had no known structure at all. Matter and energy are interchangeable at temperatures of many trillion degrees; no one knows what energy is, but matter is energy at rest. As the universe cooled, the smallest constituents of matter that we know about, called quarks, began to clump together in groups of three, forming both protons and neutrons (Fig. 1.2). This took place at about one hundred thousandths of a second after the big bang, when the temperature had cooled to about a million times hotter than the sun’s interior. A hundredth of a second later, these protons and neutrons began hanging together to form what would later become the nuclei of the two lightest elements, hydrogen and helium.
1.2 The Constituents of Matter
Matter is composed of atoms, each of which is composed of electrons circling a nucleus containing protons and neutrons, both of which are made of quarks. Whether quarks are composed of something smaller is currently unknown.
Before one second had elapsed the four fundamental forces that govern matter had come into being: gravitational force, electromagnetic force, the strong nuclear force, and the weak nuclear force. Gravitational force, or gravity, is the weakest of the four forces. It was described by Newton’s theory of gravity and by Einstein’s general theory of relativity, but it still cannot be defined. Electromagnetic force is a union of the electric and the magnetic forces. The strong nuclear force, the strongest of the four, is responsible for keeping quarks locked inside of protons and neutrons and for keeping protons and neutrons crammed inside of atomic nuclei. The weak nuclear force mediates the decay (or disintegration of the atomic nuclei) of radioactive elements. Scientists believe that all four forces must be aspects of one force, but they have not yet been able to create a unifying theory.
These four forces work in perfect balance to allow the universe to exist and expand at a sustainable rate. If the gravitational force were a tiny bit stronger, all matter would likely implode in on itself. If gravity were slightly weaker, atoms could not form. If the temperature of the universe had dropped more slowly, the protons and neutrons might not have stopped at helium and lithium but continued to bond until they formed iron, too heavy to form galaxies and stars. The exquisite balance provided by the four forces seems to be the only way in which the universe can maintain itself. Scientists wonder if perhaps many other universes came into existence but vanished before this one survived. The newborn universe evolved with phenomenal speed, setting in place in a tiny fraction of a second the fundamental properties that have remained stable since.
During about 300,000 years of expanding and cooling, the wildly streaming electrons, negatively charged, slowed down. The atomic nuclei, protons and neutrons, were positively charged. When the electrons had slowed down sufficiently, the nuclei could attract them by their electric charge and form the first electrically neutral atoms: hydrogen (H) and helium (He), the lightest elements, the first matter. Hydrogen consists of one proton and one electron; helium consists of two protons and two electrons.
This became a pivotal moment in the story of the universe. Before the formation of stable atoms, the universe was filled with so many zigzagging particles, some negative, some positive, that light (consisting of subatomic particles called photons) could not move through the bath of charged particles. This was so because photons interact with electrically charged particles and are either deflected or absorbed. If anyone had been there to see it, the universe would have appeared as a dense fog or a blinding snowstorm.
As soon as atoms formed, binding the negative electrons and positive neutrons together, the photons of light could travel freely. The dense fog of radiation lifted. Matter had formed, and the universe became transparent. Its full expanse came into view—if anyone had been there to see it—consisting mostly of vast empty space filled with huge clouds of hydrogen and helium with immense amounts of energy pouring through them.
Today we can see some of the photons left from the big bang—as “snow” on our television screens. To do so we must disconnect the cable feed and tune to a channel the set does not receive. About 1 percent of the “snow” we see is residual light/heat left from the big bang that forms a cosmic sea of background microwave radiation.
3 If our eyes were sensitive to microwaves, which they are not, we would see a diffuse glow in the world around us.
By using radio equipment, scientists have documented the background microwave radiation. By the 1950s and 1960s physicists realized, from what they already knew about the universe, that the present universe should be filled with primordial photons, cooled over 13.5 billion years to a few degrees above absolute zero. In the spring of 1965 two radio astronomers, Arno A. Penzias and Robert W. Wilson, working for Bell Laboratories in New Jersey, accidentally detected this afterglow as a background hissing noise while they were testing a new microwave antenna to be used with communication satellites. In 1989 NASA sent up the Cosmic Background Explorer (COBE) satellite, which collected information that confirmed with high precision that there are about 400 million photons in every cubic meter of the universe—an invisible cosmic sea of microwave radiation, at 3 degrees Celsius, just as predicted by the theory of the big bang.
In 2002 NASA sent a sixteen-foot probe called the Wilkinson Microwave Anistropy Probe, or WMAP, a million miles out from Earth. For a year WMAP took time exposures of the entire sky, showing in high resolution the map of the cosmic background radiation (CBR) from 380,000 years after the big bang and confirming again the big bang account of the universe.
Fortunately for astronomers, on the scale of the universe, distance is a time machine. The farther away something is, the younger we see it; this is because the more distant something is, the longer its radiation takes to reach us. We can never see the universe as it is today, only as it once was, because it takes millions and billions of years for the light of distant galaxies and stars, traveling at nearly 6 trillion miles a year, to reach us. Hence, we can see far back into the past. By picking up microwave radiation, we can “see” back nearly to the beginning of the universe (Fig. 1.3).
Think of it this way. The light from our nearest star, the sun, takes eight minutes and twenty seconds to reach us. Light from Jupiter takes about thirty-five minutes when it is closest to us, about an hour when it is farthest away in its orbit. The light of the brightest star in the night sky, Sirius, takes 8.6 years to reach us. (The distance the light travels is 8.6 light-years, or 50.5 trillion miles). The light from stars we can see without optical aid takes from four years to 4,000 years to reach us. If we should see a star exploding 3,000 light-years away, then that explosion occurred 3,000 years ago—the time it takes for the light to reach us.
Twinkling Galaxies
As described earlier, the universe became transparent some 300,000 years after the big bang. Immense clouds of hydrogen and helium drifted until these clouds broke into about a trillion separate clouds, each with its own dynamics, each escaping from the universe’s expansion in that the diameter of each cloud remained the same while the space between the clouds increased.
1.3 Our View of the Universe
From our position in the Milky Way galaxy—one of the galaxies in the Local Group—we see the universe in the distant past, because the light from remote galaxies takes billions of years to reach us. In this distant past the universe was smaller, and galaxies collided more often. Quasars are very distant objects thought to be the nuclei of younger galaxies, possibly in collision.
As the universe cooled and calmed down, each separate cloud of hydrogen and helium became a separate galaxy of stars joined by gravity. This happened as the atoms of hydrogen and helium collided with each other. As they collided, the friction created temperatures so high that the atoms were stripped of their electrons. The hydrogen nuclei started to fuse, forming helium ions. These fusion reactions released a huge amount of heat/energy, according to Einstein’s equation E = mc
2, in which the loss of a tiny bit of mass results in energy multiplied by the speed of light squared. As the hydrogen begins to burn, millions of tons of matter are transformed into energy each second, and a star is born. The earliest stars formed only about 200,000 years after the big bang.
The universe is filled with an enormous range of objects as measured by their mass. The largest objects are stars, which produce their own energy. The largest stars are up to twenty times more massive than the star that is our sun. The smallest objects in the universe are dust particles visible only under a microscope and which rain down into the Earth’s atmosphere at the rate of a hundred tons a day. The silt in the eaves of any house probably contains a minute amount of interstellar material. Planets are middle-range objects; their mass is not sufficient to produce their own energy through hydrogen-fusion reactions.
Stars come in a vast range of sizes and densities, and they evolve over time from one type to another. Most of the stars nearest us are red stars, but the one we know best, the sun, is a stable yellow star burning hydrogen, called hydrogen fusion as described earlier. When its hydrogen is used up, in about 5 billion years, our sun will switch to burning helium, called helium fusion. Since helium fusion is a hotter process with a greater energy output, the pressure from the extra energy will expand the sun until it becomes what is called a red giant. When the helium fuel is used up, the red giant will collapse to a white dwarf. Then it will slowly cool until it becomes a cinder called a black dwarf, about the size of Earth and 200,000 times its mass. No black dwarf has yet been found because the universe is not old enough for any to have completed the slow process of cooling down.
Some yellow stars, the ones that are larger than our sun at their inception, become larger red giants than our sun will. When their red-giant stage is over, they do not shrink into white dwarfs. In them heavier elements are created and burned: carbon, nitrogen, oxygen, magnesium, and finally iron. But iron cannot be used as a stellar fuel. Energy production stops and gravity takes over. The star’s core implodes and triggers an immense explosion of the outer layers that blasts most of the star to smithereens. Only the core survives as a white dwarf, a neutron star (tiny and incredibly dense), or a black hole, which is an object so dense that light cannot escape its gravitational field. This explosive self-annihilation of a star is called a supernova; only stars at least six times more massive than our sun can become supernovas.
These supernovas play an immense role in the creativity of the universe. They are the cosmic furnaces out of which new elements are formed and, and we have seen, they initiate the formation of black holes. When a star of more than ten times the mass of our sun explodes, the imploding core that is left may be larger than four times the mass of the sun. If it is, then gravity is so immense that all the matter disappears and a black hole remains, leaving only a gravitational field so strong that it prevents light from escaping. No one knows where the matter goes. The center of a black hole is called a singularity; a black hole created by a star of ten solar masses has a diameter of only forty miles. Around the singularity is a field of gravitational force so powerful that anything that enters the field disappears into the hole. This field of gravity is called the event horizon.
Astronomers suspect that massive black holes exist at the core of most galaxies, as one seems to at the center of our Milky Way galaxy. Our black hole, more than six solar masses, is called SgA because it appears to lie in the southern hemisphere constellation Sagitarius. Scientists, working for over ten years at the Very Large Telescope in Chile’s Atacama Desert, confirmed in 2002 the presence of SgA.
Enormous supernovas become black holes. Smaller ones, those between three and six solar masses, explode outward rather than implode inward. In their burning cores hydrogen is burned into helium, then helium to carbon; nuclei are fused into ever larger nuclei, like oxygen, calcium, and on through the periodic table of elements. At some point an explosion occurs, spewing most of the star back into space as gas, but now containing complex, life-supporting atoms, not merely hydrogen and helium.
Only supernovas can create elements higher than iron. Gradually, over roughly 9 billion years, all the elements of the periodic table were built up in this way. Every scrap of gold on our planet originated in giant stars that exploded before the sun was born. The gold in the ring on your finger has to be more than 4.5 billion years old. Thus explosions of stars created the elements that make life on Earth possible. We quite literally are made of stardust.
Coming back to our story, several hundred thousand years after the big bang, galaxies consolidated as density waves moved though space, shocking the clouds of hydrogen and helium into star formation. Space began to twinkle, with billions of stars flowing in spidery filaments of whirling spirals. Most galaxies took the shape of spirals, but in the early universe matter was crowded, and galaxies often bumped into one another. When they did, the large one absorbed the smaller, but the large one could never recover its spiral shape. Instead, it became a sphere or an ellipse (oval), called an elliptical galaxy. Elliptical galaxies do not produce new stars, since density waves do not move through them to shock the clouds of gases into forming new stars. Our Milky Way galaxy is a perfect spiral, the lucky accident of being in a noncongested area of the early universe about 12 billion years ago.
For some 9 billion years, the first two-thirds of its lifetime thus far, the universe consisted of unimaginable celestial fireworks. Galaxies wheeled and collided. Density waves surged through galaxies, causing new stars to form. Supernovas exploded, scattering new gaseous elements ready to be shocked into new stars by other supernovas or imploding into black holes, losing their matter to who knows where. All the while, space was expanding and the temperature cooling. The universe was a sparkling dance of death and resurrection, ruin and elegance, overwhelming violence and destruction cycled with dazzling beauty and creativity.
The Sun/El Sol/Helios/Die Sonne
About 4.6 billion years ago, in the Milky Way galaxy, a supernova exploded, and a new star—our sun—emerged from the debris. We know this because moon rocks and meteorites, all originating in that supernova, consistently date about 4.56 billion years ago.
This sun was bigger and brighter than average, among the top 5 percent of stars in our galaxy. It was also distinguished by not having a companion star (about two-thirds of the stars in our section of the Milky Way are multiple-star systems). The sun is located two-fifths of the way out on one of the spiral arms, about 30,000 light years from the center of the Milky Way. It takes about 225 to 250 million years to circle around the center of the galaxy in an elliptical, or oval, orbit, traveling about 200,000 miles a day. Accompanied by its system of planets and other bodies, the sun has orbited the center of the Milky Way about twenty times since its origin. Its size indicates it will burn about 10 billion years; it has now burned for about 4.6 billion of those years.
Around our early sun spun a disk of leftover materials—nebulous dust and gases of many elements created by our exploding supernova. As all these gaseous elements collided, they formed small grains whose instabilities shaped the disk into bands. As centers of concentration developed in these bands, the planets emerged, with the sun’s gravity making the inner four (Mercury, Venus, Earth, and Mars) heavier and rockier, while the outer ones (Jupiter, Saturn, Uranus, and Neptune) are lighter and more gaseous. Pluto, smaller than our moon, has been declared not large enough to be considered a planet. Jupiter, about 300 times the mass of Earth, is almost, but not quite, large enough to become a star.
(There is no practical way to draw the solar system to scale without using distances the size of city blocks. If Earth were reduced to the size of a pea, Jupiter would be over 1,000 feet away and Neptune would be over a mile away.)
4The planets in their earliest state were molten or gaseous. Each planet arranged itself by gravitational interaction; the heaviest elements, such as iron and nickel, sank into the core, while the lighter elements, such as hydrogen and helium, formed the outer layers. The static, gravitational order was broken by the unstable, radioactive elements. When these elements broke apart, their energy kept the planets in a boil, bringing materials up from the deep inside to the surface.
On the three smallest planets—Mercury, Venus, and Mars—all activity came to a halt within a billion years with the formation of rocks. On the four largest planets—Jupiter, Saturn, Uranus, and Neptune—the boiling gaseous activity continues today, similar to what it was at the beginning of the solar system. Only Earth has a size that produces a gravitational and electromagnetic balance, which allows a solid rock crust to form around a burning core. Only Earth has a position in respect to the sun, a mean distance of 93 million miles, that establishes a temperature range in which complex molecules can form. Within our solar system, only here on Earth does chemical activity continue in constant change.
We measure time by the amount of it that Earth takes to circle the sun, called one year. Earth spins on an axis while it circles the sun. This axis is tilted somewhat, about 23.5 degrees, so that Earth’s electromagnetic poles are not perpendicular to the sun. Our tilted axis means that while Earth is on one side of the sun, one hemisphere leans toward the sun and receives more sunlight, and while Earth is on the other side of the sun, the other hemisphere does. This tilt of our axis as we spin creates the seasons here on Earth, for if we spun on a vertical axis both hemispheres would receive the same amount of sunlight all year round. (All other planets revolve on a vertical axis except Saturn, which revolves on a horizontal axis.)
During its first half billion years the early Earth suffered the shock of collisions with meteors, asteroids, and planetoids. We need only look at the surface of our moon to see a rockscape with the imprints of these early collisions; the moon is so small that it quickly lost its internal heat and preserved its original surface. Earth was sufficiently large—with a core hot enough that the heat of those early impacts kept it boiling day and night—that no imprints of the collisions could form.
When Earth had cooled down enough for rocks to form on its surface, plumes of molten lava rose up from within, bringing chemicals forged in the interior to the surface, changing continually the Earth’s atmosphere, composed mostly of methane, hydrogen, ammonia, and carbon. Gigantic electrical storms, with immense bolts of lightning and thunder, stirred the chemical pot. After some half billion years of gestation, Mother Earth lay poised to bring forth living molecules.
Unanswered Questions
My story thus far has been based on what scientists know about our universe, called the Standard Model, developed in the 1960s and 1970s. I have not knowingly strayed into speculation. Yet everything that we think we know needs to be viewed in the context of what we do not know. Many significant questions remain unanswered.
Even the origin of our moon is uncertain. Some say it is a piece broken off from Earth, but most believe that the moon arose when a planetoid crashed against Earth, could not escape its gravity, and went into orbit, knocking Earth off its vertical axis to the slightly tilted one that creates our seasons.
More difficult questions come to mind, such as: “Why do mathematical equations work to account for things like the trajectory of the moon and of the Andromeda galaxy?” and “What came before the big bang?” To the first question, mathematicians just shrug and joke, “God is a mathematician.” It is simply amazing that we are able to understand anything about the universe, that our minds can create equations that correlate with reality. As for the second and other questions:
1. What came before the big bang?
No one knows what the initial conditions of the universe were. Some physicists believe the answers to this question lie forever beyond the grasp of the human mind and any of its theories. But theories abound. One, posed by Lee Smolin of Pennsylvania State University,
5 proposes that the initial condition of our universe may have been a black hole in some other universe. The description of a black hole seems similar to the story of the beginning of the universe, except in reverse—matter, energy, space, and time becoming more compacted until they disappear. Physicists who are considering Smolin’s idea are theorizing that matter, energy, space, and time may disappear out of the fabric of our universe to reappear somewhere else as a new universe. Perhaps we live in a “multiverse” of many universes popping out of each other. This is just one of several current theoretical scenarios based on many universes.
2. How did the universe start expanding in the first place?
One likely hypothesis says that in its first instant of existence the universe inflated—that is, it expanded exponentially, at a rate far exceeding the velocity of light, repeatedly doubling its radius over equal intervals of time. This spasm was over in less than a second, and thereafter the universe settled into a steady linear expansion rate, until about 5 billion years ago when its expansion rate began to accelerate. This inflationary hypothesis helps explain several problems in big bang theory, but it has not been conclusively established.
3. How can theories dealing with vast, astronomical scales, called general relativity, and theories dealing with the microscopic properties of the universe, called quantum mechanics, be reconciled?
These two groups of theories contain contradictions, which cannot yet be resolved into one grand unifying theory of everything. Yet, when considering black holes or the universe at the moment of the big bang, physicists need to use both general relativity and quantum mechanics together. When they do, the answers to their equations often equal infinity. This indicates a problem, which can be stated simply in the following way: Quantum mechanics tells us that the universe on the microscopic scale is a chaotic, frenzied arena with everything appearing and disappearing unpredictably. In contrast, general relativity is based on the principle of a smooth spatial geometry. In practice, avoiding extremes of scale, the theories of quantum mechanics and general relativity work perfectly to predict perceivable outcomes; the random, violent undulations of the microscopic world cancel each other out to behave like smooth fabric.
Physicists feel that their knowledge must be considered incomplete until there are no contradictions and inconsistencies in their theories. In 1984 two physicists, Michael Green and John Schwarz, provided the first piece of evidence for a new unifying theory, called superstring or string theory, for short. This idea posits that the most elementary ingredients of the universe are not point particles but wriggling strands, or strings, of energy whose properties depend on their mode of vibration. These miniscule strings are so small—about 10-35 centimeters long—that they appear as points, even to the most powerful available equipment. This theory also posits that the universe has more than three dimensions plus time—maybe ten (or more) dimensions plus time. Theoretically, string theory provides a truly unifying theory, positing that all matter and all forces arise from one ingredient: oscillating strings of energy. Since 1984 additional pieces of evidence have fallen in place to reinforce the idea of strings, but experimental evidence to validate the theory has not yet been found.
4. Ever since scientists began, in the 1960s and 1970s, to feel certain that the universe had a specific beginning, they have been wondering: “How will our universe end?”
There seem to be three possibilities. The universe could expand forever until all the light is gone from all the galaxies and every star is a cinder; the expansion of the universe could come to a halt and reverse itself, with all the matter of the universe imploding in on itself in a horrific implosion; or somehow the expansion of the universe could reach a delicate balance at which it slows down but never quite reverses.
In the last few decades physicists have learned that the expansion of the universe is not slowing down but rather is accelerating. Something unknown is pushing the universe farther apart. Scientists are calling this unknown antigravitational force “dark energy,” or the energy of nothingness. They also believe there is something called “dark matter,” unlike anything on Earth. No one knows yet what dark matter and energy are; scientists currently think that they may constitute more than 90 percent of the universe. The search has only just begun.
