Endless Universe Chapter Two

"You can't make a man unsee what he has seen"
                --Bertolt Bercht, The Life of Galileo

        The history of the universe can be compared to a play in which the actors- matter and radiation, stars and galaxies - dance across the cosmic stage according to a script set by the laws of physics. The challenge for the cosmologist is to figure out the story line after arriving at the show 14 billion years too late, long past the crucial opening scenes.

        Observations of nearby galaxies and stars provide an accurate picture of the present scene. By gathering light emitted long ago from more distant objects and applying the physical principles learned and tested on Earth, astronomers have been able to reconstruct more and more of what happened in the past. The epoch [ Act Two] which began just one-second after the big bang and continues to the present day, is the period of cosmic history that is best understood. During this nearly 14-billion-year span, the universe expanded more than a billion times in size, and the hot primordial gas that filled the infant universe cooled to less than a billionth of its initial temperature, condensing into structures of increasing mass and complexity, the first planets, stars, galaxies, galaxy clusters and super clusters.

        One might imagine that, armed with this detailed knowledge of Act two, scientists could straightforwardly determine what happened before or what will happen next. The big surprise is that this is not the case. The inflationary and cyclic models both incorporate Act Two, but they sandwich it between completely different first and third acts. In the case of cyclic model, Act Three is not even the final act: the plot eventually leads to a new Act One, in which the story begins all over again. To appreciate how two radically different views of the history of the universe can both be consistent with the plethora of observations available today, one must first understand what is known and what is not known about the Act two.

Endless Universe

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".