Stephen Hawking's Universe
Contributed by Stephen Hawking, Richard Talcott, Michio Kaku, Alan Guth, Lee Smolin, Marcelo Gleiser, Seth Shostak, Carlos Frenk, Barry Levine, Mohammad Riza, David Filkin, William Grant, Ellen Mendlow, David McCarthy, Gina Niemiec, Janette Afsharian, et al.
“For thousands of years, people have wondered about the universe. Did it stretch out forever or was there a limit? And where did it all come from? Did the universe have a beginning, a moment of creation? Or had the universe existed forever? The debate between these two views raged for centuries without reaching any conclusions. Personally, I’m sure that the universe began with a hot Big Bang. But will it go on forever? If not, how will it end? I’m much less certain about that. The expansion of the universe spreads everything out, but gravity tries to pull it all back together again. Our destiny depends on which force will win.” Stephen Hawking
How did the universe really begin?
Most astronomers would say that the debate is now over: The universe started with a giant explosion, called the Big Bang. The big-bang theory got its start with the observations by Edwin Hubble that showed the universe to be expanding. If you imagine the history of the universe as a long-running movie, what happens when you show the movie in reverse? All the galaxies would move closer and closer together, until eventually they all get crushed together into one massive yet tiny sphere. It was just this sort of thinking that led to the concept of the Big Bang.
The Big Bang marks the instant at which the universe began, when space and time came into existence and all the matter in the cosmos started to expand. Amazingly, theorists have deduced the history of the universe dating back to just 10 -43 second (10 million trillion trillion trillionths of a second) after the Big Bang. Before this time all four fundamental forces—gravity, electromagnetism, and the strong and weak nuclear forces—were unified, but physicists have yet to develop a workable theory that can describe these conditions.
During the first second or so of the universe, protons, neutrons, and electrons—the building blocks of atoms—formed when photons collided and converted their energy into mass, and the four forces split into their separate identities. The temperature of the universe also cooled during this time, from about 10 32 (100 million trillion trillion) degrees to 10 billion degrees. Approximately three minutes after the Big Bang, when the temperature fell to a cool one billion degrees, protons and neutrons combined to form the nuclei of a few heavier elements, most notably helium.
The next major step didn’t take place until roughly 300,000 years after the Big Bang, when the universe had cooled to a not-quite comfortable 3000 degrees. At this temperature, electrons could combine with atomic nuclei to form neutral atoms. With no free electrons left to scatter photons of light, the universe became transparent to radiation. (It is this light that we see today as the cosmic background radiation.) Stars and galaxies began to form about one billion years following the Big Bang, and since then the universe has simply continued to grow larger and cooler, creating conditions conducive to life.
Three excellent reasons exist for believing in the big-bang theory. First, and most obvious, the universe is expanding. Second, the theory predicts that 25 percent of the total mass of the universe should be the helium that formed during the first few minutes, an amount that agrees with observations. Finally, and most convincing, is the presence of the cosmic background radiation. The big-bang theory predicted this remnant radiation, which now glows at a temperature just 3 degrees above absolute zero, well before radio astronomers chanced upon it.
The Big Bang
The explosive beginning of our universe, the Big Bang marks the earliest time we can probe with current physical theory. Theory has to guide our understanding of the first fraction of a second, since we can’t recreate the extremely high temperatures that existed during the earliest history of the universe in any earthly laboratory. What theory tells us is that from an initial state in which matter and radiation are both in an extremely hot and dense form, the universe expands and the matter cools. At that time, it is believed that all four of the fundamental forces of nature—gravity, electromagnetism, and the strong and weak nuclear forces—were unified.
The evolution of the earliest universe is not well understood because it is not clear exactly what laws were at work. However, it is known that by the end of the first second of time, the building blocks of matter had formed. By the end of the first three minutes, helium and other light nuclei (like deuterium) had formed but for a long time, temperatures remained too high for the formation of most atoms. At around one million years following the Big Bang, nuclei and electrons were at low enough temperatures to coalesce to form atoms. But the universe didn’t start to look like it does today until small perturbations in the matter distribution were able to condense to form the stars and galaxies we know today.
The destiny of all matter that falls into a black hole is to get crushed to a point of zero volume and infinite densitya singularity. General relativity also implies that our expanding universe began from a singularity.
A singularity is a region of space-time in which gravitational forces are so strong that even general relativity, the well-proven gravitational theory of Einstein, and the best theory we have for describing the structure of the universe, breaks down there. A singularity marks a point where the curvature of space-time is infinite, or, in other words, it possesses zero volume and infinite density. General relativity demands that singularities arise under two circumstances.
First, a singularity must form during the creation of a black hole. When a very massive star reaches the end of its life, its core, which was previously held up by the pressure of the nuclear fusion that was taking place, collapses and all the matter in the core gets crushed out of existence at the singularity. Second, general relativity shows that under certain reasonable assumptions, an expanding universe like ours must have begun as a singularity.
In the early 1920s, Russian physicist and mathematician Alexander Friedmann became the first person to embrace the idea that the equations of Einstein’s general theory of relativity called for a universe in motion. Einstein (and most other scientists, for that matter) believed that the universe was static, and he modified his equations by including a “cosmological constant” to keep it so.
Closed Universe: The Big Bang’s momentum is offset by gravity, producing a "Big Crunch."
Friedmann made two simple assumptions about the universe: that when viewed at large enough scales, it appears the same both in every direction and from every location. From these assumptions (called the cosmological principle) and Einstein’s equations, he developed the first model of a universe in motion. The Friedmann universe begins with a Big Bang and continues expanding for untold billions of years—that’s the stage we’re in now. But after a long enough period of time, the mutual gravitational attraction of all the matter slows the expansion to a stop. The universe then starts to fall in on itself, replaying the expansion in reverse. Eventually all the matter collapses back into a singularity, in what physicist John Wheeler likes to call the “Big Crunch.”
Open Universe: There is not enough matter to stop the universe from expanding forever.
Although Friedmann found only this one solution, called a closed universe because the size of the universe is finite, two similar solutions exist. In an open universe, there’s not enough matter to bring the expansion to a halt. Galaxies continue to separate from one another, although more slowly as time passes. Eventually all the stars go out, and the universe becomes cold and dark. Intermediate between the open and closed universes is the flat universe. In this case, the universe expands forever, but the speed at which the galaxies separate eventually approaches zero. What kind of universe do we live in? Observations of the universe’s density should eventually tell us, but they are not yet accurate enough to distinguish among the three possibilities.
Flat Universe: Expansion slows until the rate approaches zero.
Cosmic Background Radiation
Predicted by George Gamow and his collaborators in the 1940s and detected by Arno Penzias and Robert Wilson in the 1960s, the cosmic background radiation is the faint echo of the Big Bang. Following the explosive birth of our cosmos, the universe both expanded and cooled off rapidly. After roughly 300,000 years, its temperature had fallen to about 3000 kelvin (5000° Fahrenheit) and a big change was taking place. Before this time, conditions were too hot for atoms to form—protons and electrons each went their separate ways—and photons of light could travel only short distances before interacting with the free electrons. It was as if the universe existed in a thick fog that kept light from penetrating.
Tiny temperature fluctuations in the otherwise smooth cosmic background radiation represent the gravitational seeds in the early universe around which galaxies and galaxy clusters ultimately formed.
But when the temperature reached 3000 kelvin, atomic nuclei finally captured electrons and formed stable atoms. Photons were then able to travel unimpeded—the fog lifted—and the universe became transparent to light. It’s that light we see as the background radiation, coming at us from all directions. However, in the 10 billion or more years since the Big Bang, the universe has expanded by a factor of a thousand, causing the temperature of the radiation to fall by the same amount. It now glows at just 3 kelvin (3° Celsius above absolute zero) in the microwave part of the electromagnetic spectrum, a faint reminder of our universe’s hot start. The background appears very smooth, varying by only one part in 100,000 across the sky.
Arno Penzias and Robert Wilson
A pair of radio astronomers working at Bell Laboratories, Arno Penzias (1933-) and Robert Wilson (1936-) are credited with discovering the cosmic microwave background radiation. Using an antenna originally designed to detect signals from the Echo satellite, the two chanced upon an annoying radio hiss that seemed to be coming from everywhere. After accounting for all possible sources of error, including pigeon droppings inside the antenna, they concluded they were seeing signals coming from all directions of space. After discussing their findings with Princeton physicist Robert Dicke, they realized they were seeing the faint echo of the Big Bang predicted by George Gamow and his colleagues in the 1940s, now glowing softly at a temperature of just 3 degrees above absolute zero. For the discovery, Penzias and Wilson shared the 1978 Nobel Prize for physics.
"Where do we come from? How did the universe begin? Why is the universe the way it is? How will it end? All my life, I have been fascinated by the big questions that face us, and have tried to find scientific answers to them. If, like me, you have looked at the stars, and tried to make sense of what you see, you too have started to wonder what makes the universe exist. The questions are clear, and deceptively simple. But the answers have always seemed well beyond our reach. Until now.
"The ideas which had grown over two thousand years of observation have had to be radically revised. In less than a hundred years, we have found a new way to think of ourselves. From sitting at the center of the universe, we now find ourselves orbiting an average-sized sun, which is just one of millions of stars in our own Milky Way galaxy. And our galaxy itself is just one of billions of galaxies, in a universe that is infinite and expanding. But this is far from the end of a long history of inquiry. Huge questions remain to be answered, before we can hope to have a complete picture of the universe we live in.
"I want you to share my excitement at the discoveries, past and present, which have revolutionized the way we think. From the Big Bang to black holes, from dark matter to a possible Big Crunch, our image of the universe today is full of strange sounding ideas, and remarkable truths. The story of how we arrived at this picture is the story of learning to understand what we see."
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