There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened!
--Douglas Adams, The Restaurant at the End of Universe.
Theories of the universe have abounded throughout human history, but the last fifty years have been exceptional. A single theory, the hot Big Bang picture, has dominated scientific and public discourse and has even become part of the standard curriculum for school children. Its central tenet, the idea that the Universe emerged from a very hot, dense state 14 billion years ago and has been expanding and cooling ever since, has been firmly established through many independent measurements. But nearly every other feature of the theory has had to be modified. One ingredient after another -"dark matter", "inflation", "dark energy"- has been added and separately adjusted to fit the observations, and each of these adjustments has critically altered our conception of the history of Universe. Even so, the picture remains far from complete.
The Big Bang is conjectured to be the beginning of Space and Time, but there is no clue as to how or why the Big Bang occurred. Nor is there a firm prediction about the future of the Universe. Most cosmologists do not consider these flaws to be worrisome. They think that the theory will ultimately be simplified and made more complete. And perhaps they are right, Douglas Adam's joke not withstanding.
There is a new theory of the Universe according to which our cosmic history consists of repeating cycles of evolution. Each cycle Begins with a Bang, but the bang is not the beginning of space and time. Rather, it is an event with a "Before" and an "After" that can be described by the laws of physics. Each cycle influences the next. The events that occurred before the last bang shaped the large-scale structure of the universe observed today, and the events that are occurring today will determine the structure of the universe in the cycle to come. Perhaps space and time sprang into being many cycles ago, but it is also possible that they are literally "endless".
In this new, more integrated picture, the components that had to be added one by one to the conventional picture are either jettisoned, as in the case of "inflation", or become essential, interwoven elements of the machinery that keeps the universe cycling. Most remarkably, this new theory of a cyclic universe is able to match all current astronomical observations with the same accuracy as the modified Big Bang picture, and it may also explain some aspects of the universe that the big bang picture cannot.
Chapter One : 2001
" He was moving through a new order of creation of which few men ever dreamed. Beyond the realms of sea and land and air and space lay the realms of fire, which he alone had been privileged to glimpse. It was too much to expect that he would also understand".
--Arthur C. Clarke, 2001: A Space Odyssey
Cosmologists celebrate 2001 as the year the U.S. National Aeronautics and Space Administration launched a satellite mission from Cape Canaveral to investigate not the black monolith of Kubrick's film but a thin, dark layer of space at the outermost edge of the visible universe. The mission was called WMAP ( pronounced "W-map"), which stands for Wilkinson Microwave Anisotropy Probe. On board was a bank of highly sensitive detectors designed to gather some of the ancient light emitted from the dark layer nearly 14 billion years ago, at a time when the first atoms were just beginning to form. Every 2.2 minutes, the satellite spins once around its axis, and every hour the axis itself traces out a circle.
From the combination of motions, light from a narrow ring ring on the sky is collected. Over the course of six months, the entire satellite keeps shifting, until the detectors have covered the entire sky. The sequence will be repeated until enough light has been gathered to make a detailed portrait of the infant universe. (WMAP is a follow-up to the pioneering NASA satellite launched in 1989 called COBE - The Cosmic Background Explorer, which had made an initial low-resolution image of the early universe; in 2006, the leaders of the COBE team, John Mather at the NASA Goodard Space Center and George Smoot at the University of California at Berkeley, were awarded the Nobel Prize in Physics).
Nineteen months after the WMAP launch, in February 2003, mission head Charles Bennet and his team had collected and analyzed sufficient light to announce their findings at NASA's Washington Headquarters, in a press conference broadcast throughout the world. Bennet and his team unveiled the image that had emerged after a yearlong exposure. Just like the fictional astronaut peering into the monolith, the WMAP satellite had gazed into the primordial layer and obtained the first clear view of the infant universe. What the great thinkers in history - from Plato to Newton - to Einstein - could only speculate about was suddenly there for all to see, bringing humanity closer to answering the ultimate question: Where did it all come from?
At the end of the broadcast, world-renowned astrophysicist John Bahcall summarized the sentiments of the scientists watching: " Every astronomer will remember where he or she was when they first heard the WMAP results. For cosmology, the announcement today represents a rite of passage from speculation to precision science." Bahcall's point was that not only are the measurements marvelously accurate, but they are also in astonishing agreement with what cosmologists had been expecting.
By the time of the WMAP announcement, most scientists had come to accept a cosmological theory known as the inflationary model of the universe. In Scientific discussions, "model" is often used to mean "theory", especially cases where the idea includes aspects that are qualitative or incomplete. The inflationary model refers to a combination of three concepts: the hot big bang model, developed in the early twentieth century; the inflation mechanism, introduced in the 1980s; and the dark energy hypothesis, added in the 1990s.
In this picture, the Big Bang itself is not explained. It is simply imagined that space and time emerged somehow. Next, it is assumed that just after the bang, a small region of the universe underwent a dramatic process called inflation, during which it expanded a googol (10^100) times or more within a billionth of a trillionth of a second. Once this period of inflation ended, the energy causing the inflation was transformed into a dense gas of hot radiation. [Energy converting into mass? what about the matter and antimatter?] The gas cooled and the expansion slowed, allowing atoms and molecules to clump into Galaxies and Stars.
This picture of the inflationary universe was originally conceived in the 1980s and is now presented in many textbooks. However, recent astronomical discoveries have led to a major amendment to the story - that 9 billion years after the big bang, a mysterious force called dark energy took over and started to accelerate the expansion again. In the standard picture, the expansion of universe will accelerate forever, turning all of space into a vast and nearly perfect vacuum.
In the WMAP press conference, the inflationary theory had scored a major victory, the model now fits an enormous range of measurements - the clustering of Galaxies, the distribution of infra-red radiation and X-rays, the expansion rate of the universe and its age, the abundances of the elements - to within ten percent or better. To have a theory that can so accurately describe events occurring billions of years ago is a stunning success; fortunately, compared to the Earth's atmosphere, the conditions in the early universe are uncomplicated, and the physical laws that govern them are remarkably simple to analyze.
Cosmology, the study of the origin and evolution of universe, has some unique limitations that call for a high degree of caution. Scientists cannot perform direct experiments on the universe, and they cannot travel back in time. The best they can do is gather indirect information about the history of the universe through painstaking observations of distant objects that emitted their light a long time ago and try to piece together a logical account. But the evidence is uneven, with highly detailed information about some epochs and little or no information about others. Even if one story fits all the available evidence well, there is always the possibility that another story might fit just as well, or better. Sometimes, as with Clarke and Kubrick's astronaut, a closer look will reveal that the original idea is wrong. Just as the giant monolith proved different when viewed from up close, more precise snapshots of the embryonic universe could, in the foreseeable future, lead to an entirely different explanation of the origin and evolution of the cosmos.
A number of flaws and untested predictions remained in the inflationary model and then emerged a new alternative known as the cyclic model. According to this picture, the big bang is not the beginning of space and time but, rather, an event that is, in principle, fully describable using physical laws. Nor does the Big Bang happen only once. Instead, the universe undergoes cycles of evolution. In each cycle, a big bang creates hot matter and radiation, which expand and cool to form the galaxies and stars observed today. Then the expansion of the universe speeds up, causing the matter to become so spread out that the space approaches a nearly perfect vacuum. Finally, after a trillion years or so, a new big bang occurs and the cycle begins anew. The cyclic model accounts for the WMAP results and all other current astronomical observations with the same accuracy as the inflationary model, but the interpretation it offers is drastically different. From the cyclic view, the WMAP image is nearly as strange as Clarke's monolith. The image takes all of us on a trans-dimensional journey to view events ranging from before the big bang to the distant future.
Flaws Too Important to Ignore
In recent years, many experts have downplayed the flaws of the inflationary model, especially two disturbing factors. First, they, say the inflationary model appears contrived, a patchwork of disconnected ingredients that have been added one by one to fit observations. The first ingredient is ordinary visible matter, composed of atoms, their subatomic components. A second ingredient is dark matter, an invisible entity that surrounds galaxies and accounts for most of their mass. Third ingredient is dark energy, a completely different invisible substance that is spread uniformly across space and that produces an anti-gravity force causing the expansion of the universe to accelerate.
To match the universe as it is seen today, all three components must exist in a particular combination. In addition, to explain the past evolution of the universe, the inflationary model requires still another ingredient, known as inflationary energy. This ingredient is required to drive the explosive expansion of the universe for a few instants after the big bang, and then it must decay away in order not to interfere with the subsequent evolution of the universe.
As the inflationary model has matured and more components have had to be added, no one has been able to explain how the components might be related. Each new ingredient has required its own awkward adjustment to fit with the rest of the model. Scientists are suspicious of such procedures. They seek simple, all-encompassing explanations for the fundamental mechanisms underlying nature, like Newton's three laws of motion, which summarize everything known about motion and forces before Special relativity; or the four-letter language in which the genetic code of life is written; or Schrodinger's equation in quantum theory, which describes with one stroke the structure of every type of atom and molecule. None of these great advances was made by adding awkwardly tweaked components to a pre-existing idea. Every new ingredient and every special adjustment in the inflationary model may be a indication that it is not the final answer.
The best hope of finding a more compelling explanation lies in the realm of fundamental physics: the attempt to describe all the forces and particles of nature in a single, unified theory. The second disturbing feature of the inflationary model is the idea that "time" has a "beginning". How did the universe start, if there was nothing before it? The notion sounds contradictory, and may be even nonsensical. Philosophers have grappled with the issue for thousands of years without making much progress. Some people appeal to religious faith for arguments supporting the creation of something from nothing. Even here, however, a careful reading of the original texts reveals a certain ambiguity.
Dreams of a Better Theory
History should encourage one to think boldly. Several times in the last century, cosmologists converged on what they believed to be the true model of the universe, only to discover that, with new observations and advances in theoretical physics, they had to abandon it in favor of a new idea.
Before Einstein, many astronomers had concluded that only a single, isolated galaxy of stars, the Milky Way, existed, surrounded by an infinite expanse of empty space. When Einstein developed his own model of the cosmos based on his new theory of gravity, known as general relativity, he assumed a different picture in which the matter in the universe is spread uniformly throughout space and the universe has existed in its present state for all eternity, neither expanding nor contracting. Over the next decade, the astronomer Edwin Hubble showed that the astronomers' picture was wrong. In 1923, Hubble showed that stars are not spread out uniformly, but are instead clumped into galaxies that are spread throughout the universe far beyond the boundaries of the Milky way.
In 1929, Hubble further proved that, contrary to Einstein's concept, the state of the universe is changing because the galaxies are all moving apart from one another and space is expanding. In the 1950s, Fred Hoyle, Herman Bondi, and Thomas Gold, working together at the University of Cambridge, revived the idea of an unchanging universe, convincing many astronomers that matter is created at just the right rate to balance the expansion of universe and keep it in a steady state for all time. But the 1963 discovery of the cosmic background radiation by Bell Laboratory astronomers Arno Penzias and Robert Wilson shattered the steady-state model. The radiation, which comes from the early universe, is direct evidence that there was once a period when the temperature and density were much greater than today.
The discovery of the cosmic background radiation immediately convinced almost all cosmologists of the hot big bang model, first proposed by the Russian mathematical physicist Alexander Friedmann and Belgian cosmologist Georges Lemaitre in the 1920s, based on Einstein's theory of gravity, and developed by George Gamow and collaborators at George Washington University in the 1940s. Their model was simple compared to today's inflationary model: it assumed a universe containing only ordinary matter composed of atoms and their components. By the 1980s, cosmologists had been forced to introduce dark matter and inflation to explain the observed motions within galaxies and galaxy clusters and the formation of large-scale structure in the universe. Then in the early 1990s, just as cosmologists were becoming convinced that the inflationary big bang model was the answer, they were shocked to discover the existence of dark energy and cosmic acceleration in today's universe.
Although this history suggests that radical changes in cosmology are possible, the prospect of constructing an alternative theory seemed daunting. Such a model would require completely novel features. But the features need to be well justified, scientifically sensible, and mathematically consistent. A new model that tries to embrace the entire history of the universe would probably not emerge fully developed. More likely, it would first appear as a tenuous framework mixing a few solid ideas with a lot of bold speculations. Building from foundations would probably require several years. And at any time a fatal flaw might appear- due to either a mathematical surprise or a new observation- and topple the whole idea.
In a particular form of string theory, the ordinary three-dimensional "world" - everything in the universe that can be touched, felt, seen - is separated from another inaccessible three-dimensional "world" by a tiny gap along a forth dimension that cannot be experienced. Atoms and light can move in the length, breadth and height of this world but are forbidden by the laws of string theory from moving into the extra dimension. The other world has its own kinds of matter and light, which cannot travel through the extra dimension either. Thus the two worlds are disconnected from each other except for one factor: they interact through the force of gravity. Burt Ovrut, working with the assumption that the worlds lie at a fixed, minuscule distance from one another along the extra dimension, described how this bold new idea explained many of the detailed features of elementary particles.
pondering over the possibility for these two worlds to move along the extra dimension, generated the conclusion that if such a thing is possible, then big bang is nothing more than a collision between these two worlds. Ovrut's compelling conceptual picture suggested that the big bang might not be the "beginning: of the universe after all, but instead a physically explicable event with a "before" and an "after". Furthermore, if there was no inflation to spread apart and dilute the matter and structures produced at the collision and one could observe them today, there might even be direct observational evidence of events that occurred before the big bang.
At the conceptual level, the cyclic model differs from the consensus picture in three key respects. First, the big bang is transformed from a singular beginning of space and time into a collision between worlds. Second, the big bang is not an one-time occurrence. The worlds are drawn together and collide at regular intervals of about a trillion years. Each bang creates new hot matter and radiation and initiates a new period of cosmic expansion, leading to the formation of new galaxies, stars, planets and life. Finally, what happens before a big bang cannot be ignored. The spatial arrangements of galaxies and the patterns observed in the cosmic background radiation today are set in place by events that took place a cycle ago. Likewise, the events taking place today are setting the scene for the pattern of galaxies and radiation in the cycle to come.
A Cosmic Competition
The story begins with the period of cosmic history on which the two models agree, between the one-second mark after the last big bang and the present time. A wide range of astronomical observations, especially those made during the last decade, leave little room for doubt about what happened during this epoch. But when one asks what happened before the one-second mark and what will occur in the future, the cyclic model and the inflationary model provide dramatically different answers.
A critical aspect in judging the two models is how they relate to the fundamental laws of physics. The inflationary model arose from the view that matter is composed of indivisible particles, interacting through forces. The cyclic model was motivated by the revolutionary ideas of string theory, the new approach to fundamental physics that grabbed the attention of leading theorists in recent decades. The basic tenets of string theory that matter is composed of vibrating string like objects and that space contains extra hidden dimensions imply a new geometrical view of the universe. That vision led directly to the formulation of the cyclic picture.
Are the laws of physics the same everywhere? Do Space and Time last forever? Is there only one universe? What will happen to the matter and light observed today?The scientific community is beginning to accumulate the observations that could settle these issues but the ultimate arbiter will be nature, as it is for all scientific debates.
--Douglas Adams, The Restaurant at the End of Universe.
Theories of the universe have abounded throughout human history, but the last fifty years have been exceptional. A single theory, the hot Big Bang picture, has dominated scientific and public discourse and has even become part of the standard curriculum for school children. Its central tenet, the idea that the Universe emerged from a very hot, dense state 14 billion years ago and has been expanding and cooling ever since, has been firmly established through many independent measurements. But nearly every other feature of the theory has had to be modified. One ingredient after another -"dark matter", "inflation", "dark energy"- has been added and separately adjusted to fit the observations, and each of these adjustments has critically altered our conception of the history of Universe. Even so, the picture remains far from complete.
The Big Bang is conjectured to be the beginning of Space and Time, but there is no clue as to how or why the Big Bang occurred. Nor is there a firm prediction about the future of the Universe. Most cosmologists do not consider these flaws to be worrisome. They think that the theory will ultimately be simplified and made more complete. And perhaps they are right, Douglas Adam's joke not withstanding.
There is a new theory of the Universe according to which our cosmic history consists of repeating cycles of evolution. Each cycle Begins with a Bang, but the bang is not the beginning of space and time. Rather, it is an event with a "Before" and an "After" that can be described by the laws of physics. Each cycle influences the next. The events that occurred before the last bang shaped the large-scale structure of the universe observed today, and the events that are occurring today will determine the structure of the universe in the cycle to come. Perhaps space and time sprang into being many cycles ago, but it is also possible that they are literally "endless".
In this new, more integrated picture, the components that had to be added one by one to the conventional picture are either jettisoned, as in the case of "inflation", or become essential, interwoven elements of the machinery that keeps the universe cycling. Most remarkably, this new theory of a cyclic universe is able to match all current astronomical observations with the same accuracy as the modified Big Bang picture, and it may also explain some aspects of the universe that the big bang picture cannot.
Chapter One : 2001
" He was moving through a new order of creation of which few men ever dreamed. Beyond the realms of sea and land and air and space lay the realms of fire, which he alone had been privileged to glimpse. It was too much to expect that he would also understand".
--Arthur C. Clarke, 2001: A Space Odyssey
Cosmologists celebrate 2001 as the year the U.S. National Aeronautics and Space Administration launched a satellite mission from Cape Canaveral to investigate not the black monolith of Kubrick's film but a thin, dark layer of space at the outermost edge of the visible universe. The mission was called WMAP ( pronounced "W-map"), which stands for Wilkinson Microwave Anisotropy Probe. On board was a bank of highly sensitive detectors designed to gather some of the ancient light emitted from the dark layer nearly 14 billion years ago, at a time when the first atoms were just beginning to form. Every 2.2 minutes, the satellite spins once around its axis, and every hour the axis itself traces out a circle.
From the combination of motions, light from a narrow ring ring on the sky is collected. Over the course of six months, the entire satellite keeps shifting, until the detectors have covered the entire sky. The sequence will be repeated until enough light has been gathered to make a detailed portrait of the infant universe. (WMAP is a follow-up to the pioneering NASA satellite launched in 1989 called COBE - The Cosmic Background Explorer, which had made an initial low-resolution image of the early universe; in 2006, the leaders of the COBE team, John Mather at the NASA Goodard Space Center and George Smoot at the University of California at Berkeley, were awarded the Nobel Prize in Physics).
Nineteen months after the WMAP launch, in February 2003, mission head Charles Bennet and his team had collected and analyzed sufficient light to announce their findings at NASA's Washington Headquarters, in a press conference broadcast throughout the world. Bennet and his team unveiled the image that had emerged after a yearlong exposure. Just like the fictional astronaut peering into the monolith, the WMAP satellite had gazed into the primordial layer and obtained the first clear view of the infant universe. What the great thinkers in history - from Plato to Newton - to Einstein - could only speculate about was suddenly there for all to see, bringing humanity closer to answering the ultimate question: Where did it all come from?
At the end of the broadcast, world-renowned astrophysicist John Bahcall summarized the sentiments of the scientists watching: " Every astronomer will remember where he or she was when they first heard the WMAP results. For cosmology, the announcement today represents a rite of passage from speculation to precision science." Bahcall's point was that not only are the measurements marvelously accurate, but they are also in astonishing agreement with what cosmologists had been expecting.
By the time of the WMAP announcement, most scientists had come to accept a cosmological theory known as the inflationary model of the universe. In Scientific discussions, "model" is often used to mean "theory", especially cases where the idea includes aspects that are qualitative or incomplete. The inflationary model refers to a combination of three concepts: the hot big bang model, developed in the early twentieth century; the inflation mechanism, introduced in the 1980s; and the dark energy hypothesis, added in the 1990s.
In this picture, the Big Bang itself is not explained. It is simply imagined that space and time emerged somehow. Next, it is assumed that just after the bang, a small region of the universe underwent a dramatic process called inflation, during which it expanded a googol (10^100) times or more within a billionth of a trillionth of a second. Once this period of inflation ended, the energy causing the inflation was transformed into a dense gas of hot radiation. [Energy converting into mass? what about the matter and antimatter?] The gas cooled and the expansion slowed, allowing atoms and molecules to clump into Galaxies and Stars.
This picture of the inflationary universe was originally conceived in the 1980s and is now presented in many textbooks. However, recent astronomical discoveries have led to a major amendment to the story - that 9 billion years after the big bang, a mysterious force called dark energy took over and started to accelerate the expansion again. In the standard picture, the expansion of universe will accelerate forever, turning all of space into a vast and nearly perfect vacuum.
In the WMAP press conference, the inflationary theory had scored a major victory, the model now fits an enormous range of measurements - the clustering of Galaxies, the distribution of infra-red radiation and X-rays, the expansion rate of the universe and its age, the abundances of the elements - to within ten percent or better. To have a theory that can so accurately describe events occurring billions of years ago is a stunning success; fortunately, compared to the Earth's atmosphere, the conditions in the early universe are uncomplicated, and the physical laws that govern them are remarkably simple to analyze.
Cosmology, the study of the origin and evolution of universe, has some unique limitations that call for a high degree of caution. Scientists cannot perform direct experiments on the universe, and they cannot travel back in time. The best they can do is gather indirect information about the history of the universe through painstaking observations of distant objects that emitted their light a long time ago and try to piece together a logical account. But the evidence is uneven, with highly detailed information about some epochs and little or no information about others. Even if one story fits all the available evidence well, there is always the possibility that another story might fit just as well, or better. Sometimes, as with Clarke and Kubrick's astronaut, a closer look will reveal that the original idea is wrong. Just as the giant monolith proved different when viewed from up close, more precise snapshots of the embryonic universe could, in the foreseeable future, lead to an entirely different explanation of the origin and evolution of the cosmos.
A number of flaws and untested predictions remained in the inflationary model and then emerged a new alternative known as the cyclic model. According to this picture, the big bang is not the beginning of space and time but, rather, an event that is, in principle, fully describable using physical laws. Nor does the Big Bang happen only once. Instead, the universe undergoes cycles of evolution. In each cycle, a big bang creates hot matter and radiation, which expand and cool to form the galaxies and stars observed today. Then the expansion of the universe speeds up, causing the matter to become so spread out that the space approaches a nearly perfect vacuum. Finally, after a trillion years or so, a new big bang occurs and the cycle begins anew. The cyclic model accounts for the WMAP results and all other current astronomical observations with the same accuracy as the inflationary model, but the interpretation it offers is drastically different. From the cyclic view, the WMAP image is nearly as strange as Clarke's monolith. The image takes all of us on a trans-dimensional journey to view events ranging from before the big bang to the distant future.
Flaws Too Important to Ignore
In recent years, many experts have downplayed the flaws of the inflationary model, especially two disturbing factors. First, they, say the inflationary model appears contrived, a patchwork of disconnected ingredients that have been added one by one to fit observations. The first ingredient is ordinary visible matter, composed of atoms, their subatomic components. A second ingredient is dark matter, an invisible entity that surrounds galaxies and accounts for most of their mass. Third ingredient is dark energy, a completely different invisible substance that is spread uniformly across space and that produces an anti-gravity force causing the expansion of the universe to accelerate.
To match the universe as it is seen today, all three components must exist in a particular combination. In addition, to explain the past evolution of the universe, the inflationary model requires still another ingredient, known as inflationary energy. This ingredient is required to drive the explosive expansion of the universe for a few instants after the big bang, and then it must decay away in order not to interfere with the subsequent evolution of the universe.
As the inflationary model has matured and more components have had to be added, no one has been able to explain how the components might be related. Each new ingredient has required its own awkward adjustment to fit with the rest of the model. Scientists are suspicious of such procedures. They seek simple, all-encompassing explanations for the fundamental mechanisms underlying nature, like Newton's three laws of motion, which summarize everything known about motion and forces before Special relativity; or the four-letter language in which the genetic code of life is written; or Schrodinger's equation in quantum theory, which describes with one stroke the structure of every type of atom and molecule. None of these great advances was made by adding awkwardly tweaked components to a pre-existing idea. Every new ingredient and every special adjustment in the inflationary model may be a indication that it is not the final answer.
The best hope of finding a more compelling explanation lies in the realm of fundamental physics: the attempt to describe all the forces and particles of nature in a single, unified theory. The second disturbing feature of the inflationary model is the idea that "time" has a "beginning". How did the universe start, if there was nothing before it? The notion sounds contradictory, and may be even nonsensical. Philosophers have grappled with the issue for thousands of years without making much progress. Some people appeal to religious faith for arguments supporting the creation of something from nothing. Even here, however, a careful reading of the original texts reveals a certain ambiguity.
Dreams of a Better Theory
History should encourage one to think boldly. Several times in the last century, cosmologists converged on what they believed to be the true model of the universe, only to discover that, with new observations and advances in theoretical physics, they had to abandon it in favor of a new idea.
Before Einstein, many astronomers had concluded that only a single, isolated galaxy of stars, the Milky Way, existed, surrounded by an infinite expanse of empty space. When Einstein developed his own model of the cosmos based on his new theory of gravity, known as general relativity, he assumed a different picture in which the matter in the universe is spread uniformly throughout space and the universe has existed in its present state for all eternity, neither expanding nor contracting. Over the next decade, the astronomer Edwin Hubble showed that the astronomers' picture was wrong. In 1923, Hubble showed that stars are not spread out uniformly, but are instead clumped into galaxies that are spread throughout the universe far beyond the boundaries of the Milky way.
In 1929, Hubble further proved that, contrary to Einstein's concept, the state of the universe is changing because the galaxies are all moving apart from one another and space is expanding. In the 1950s, Fred Hoyle, Herman Bondi, and Thomas Gold, working together at the University of Cambridge, revived the idea of an unchanging universe, convincing many astronomers that matter is created at just the right rate to balance the expansion of universe and keep it in a steady state for all time. But the 1963 discovery of the cosmic background radiation by Bell Laboratory astronomers Arno Penzias and Robert Wilson shattered the steady-state model. The radiation, which comes from the early universe, is direct evidence that there was once a period when the temperature and density were much greater than today.
The discovery of the cosmic background radiation immediately convinced almost all cosmologists of the hot big bang model, first proposed by the Russian mathematical physicist Alexander Friedmann and Belgian cosmologist Georges Lemaitre in the 1920s, based on Einstein's theory of gravity, and developed by George Gamow and collaborators at George Washington University in the 1940s. Their model was simple compared to today's inflationary model: it assumed a universe containing only ordinary matter composed of atoms and their components. By the 1980s, cosmologists had been forced to introduce dark matter and inflation to explain the observed motions within galaxies and galaxy clusters and the formation of large-scale structure in the universe. Then in the early 1990s, just as cosmologists were becoming convinced that the inflationary big bang model was the answer, they were shocked to discover the existence of dark energy and cosmic acceleration in today's universe.
Although this history suggests that radical changes in cosmology are possible, the prospect of constructing an alternative theory seemed daunting. Such a model would require completely novel features. But the features need to be well justified, scientifically sensible, and mathematically consistent. A new model that tries to embrace the entire history of the universe would probably not emerge fully developed. More likely, it would first appear as a tenuous framework mixing a few solid ideas with a lot of bold speculations. Building from foundations would probably require several years. And at any time a fatal flaw might appear- due to either a mathematical surprise or a new observation- and topple the whole idea.
In a particular form of string theory, the ordinary three-dimensional "world" - everything in the universe that can be touched, felt, seen - is separated from another inaccessible three-dimensional "world" by a tiny gap along a forth dimension that cannot be experienced. Atoms and light can move in the length, breadth and height of this world but are forbidden by the laws of string theory from moving into the extra dimension. The other world has its own kinds of matter and light, which cannot travel through the extra dimension either. Thus the two worlds are disconnected from each other except for one factor: they interact through the force of gravity. Burt Ovrut, working with the assumption that the worlds lie at a fixed, minuscule distance from one another along the extra dimension, described how this bold new idea explained many of the detailed features of elementary particles.
pondering over the possibility for these two worlds to move along the extra dimension, generated the conclusion that if such a thing is possible, then big bang is nothing more than a collision between these two worlds. Ovrut's compelling conceptual picture suggested that the big bang might not be the "beginning: of the universe after all, but instead a physically explicable event with a "before" and an "after". Furthermore, if there was no inflation to spread apart and dilute the matter and structures produced at the collision and one could observe them today, there might even be direct observational evidence of events that occurred before the big bang.
At the conceptual level, the cyclic model differs from the consensus picture in three key respects. First, the big bang is transformed from a singular beginning of space and time into a collision between worlds. Second, the big bang is not an one-time occurrence. The worlds are drawn together and collide at regular intervals of about a trillion years. Each bang creates new hot matter and radiation and initiates a new period of cosmic expansion, leading to the formation of new galaxies, stars, planets and life. Finally, what happens before a big bang cannot be ignored. The spatial arrangements of galaxies and the patterns observed in the cosmic background radiation today are set in place by events that took place a cycle ago. Likewise, the events taking place today are setting the scene for the pattern of galaxies and radiation in the cycle to come.
A Cosmic Competition
The story begins with the period of cosmic history on which the two models agree, between the one-second mark after the last big bang and the present time. A wide range of astronomical observations, especially those made during the last decade, leave little room for doubt about what happened during this epoch. But when one asks what happened before the one-second mark and what will occur in the future, the cyclic model and the inflationary model provide dramatically different answers.
A critical aspect in judging the two models is how they relate to the fundamental laws of physics. The inflationary model arose from the view that matter is composed of indivisible particles, interacting through forces. The cyclic model was motivated by the revolutionary ideas of string theory, the new approach to fundamental physics that grabbed the attention of leading theorists in recent decades. The basic tenets of string theory that matter is composed of vibrating string like objects and that space contains extra hidden dimensions imply a new geometrical view of the universe. That vision led directly to the formulation of the cyclic picture.
Are the laws of physics the same everywhere? Do Space and Time last forever? Is there only one universe? What will happen to the matter and light observed today?The scientific community is beginning to accumulate the observations that could settle these issues but the ultimate arbiter will be nature, as it is for all scientific debates.
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