In a trillion years, our home will be well on its way toward a vacuous oblivion. Virtually all the galaxies we see today will still exist, but the stars will be gradually burning out. There will in all likelihood still be stars, planets, and life. But the accelerated expansion due to dark energy will have spread out the galaxies so much that nothing beyond the Andromeda Galaxy be visible to us.
The surviving civilizations in the Milky Way will know from the historical record that the universe was once filled with billions of galaxies, which emerged from tiny fluctuations in a hot plasma uniformly spread over space. But all the observational clues available today will be long gone by then. It is hard to imagine that a newly emerging civilization could piece together cosmic history on its own. In the inflationary model, therefore, the present is a unique epoch in the evolution of the universe where we can see both substantial amounts of the matter and radiation that dominated our past and the dark energy that will dominate our future. At other epochs, only one or the other would be detectable.
The same trillion-year prospectus applies if the Cyclic model is correct, but it holds nearly everywhere in space, not just in isolated pockets. After a trillion years, however, the story changes dramatically. The branes begin to approach each other, the dark energy decreases, and expansion slowly grinds to a halt. There will be no galaxies or other distant sources that future observers can use to detect the expansion rate, unless the future civilizations send regular test probes beyond the Milky Way. Yet there will be some novel physical effects to indicate that the end of the cycle is near. First, many fundamental physical constants of nature, like the strength of gravity and the strong, weak, and electromagnetic forces, will begin to change noticeably because their values depend on the separation between the branes. They don’t change during earlier stages, like today, because the brane separation is frozen. That is why they are interpreted as constants of nature. However, once the branes start to rush toward each other, all the physical constants will start to change in concert. A number of sensitive experiments exist today that monitor these constants and search for time variation. So far, no conclusive evidence for change has been found. According to the cyclic model, physicists performing those same experiments during the last 10 billion years before the next brane collision would detect a large variation of the constants, whose rate would increase as the branes speed up. In the final moments before the crunch, the rapid changes would become dramatic: particles would lose their mass and the laws of nature would be restored to a much simpler and more symmetrical form.
In reality, what is happening is that something enormous is approaching fast along a dimension we cannot see. The realization will come in a flash when, suddenly, everywhere in space lights up with new matter and radiation from the collision. The temperature soars to 10^15 times the surface temperature of the sun, evaporating any remnant structures from the previous cycle. The quarks and gluons of which we all are made join the flood of new quarks and gluons created at the bang, and the cycle of the cosmos is renewed.
One Hundred Years
The cosmological debate between the inflationary and the cyclic models is only just beginning to simmer in the scientific community. Many cosmologists have not yet given the issue much consideration because they see no reason for thinking about an alternative until some observation or experiment contradicts the inflationary picture. Others are reluctant to consider a model so deeply rooted in concepts like extra dimensions or branes because they regard these ideas as too far-fetched, even though string theorists are finding these concepts to be essential for unifying our understanding of the fundamental forces. In fact, contemporary versions of the inflationary model are now using the same stringy building blocks.
The reluctance of some to introduce so many new elements into cosmology is understandable. Science usually advances through small variations on an established idea. Radically new directions are not considered unless the scientific case is compelling. For that to happen, the problems with the conventional picture (which might be swept under the rug if there were no competing idea) have to become recognized, and the novel components underlying the new approach have to become familiar. Historically, this conservative approach has served science well, enabling it to make steady progress without getting diverted. In cosmology, for example, the main elements of the current inflationary model – the big bang picture, inflationary expansion, dark matter, and dark energy – were all subject to the same resistance when they were first introduced, and it took many years for them to be accepted.
The cyclic model, if it is worthy, will require similar patience. As discussions and investigations of the cyclic picture continue over the next few years and some of the weaknesses of the inflationary model become more exposed, interest will grow. The fact that two such dissimilar models can predict such similar results is too intriguing to ignore. Creative experiments will feel compelled to mount the decisive test between the two views of cosmic history because the issues at stake are too captivating to be ignored.
In settling the debate, cosmologists will have come to grips with the most fundamental questions about space and our place in the cosmos; about time and our moment in cosmic history; and about nature and our ultimate ability to figure out its laws. The answers will be our legacy to future generations. One hundred years from now, they will be taught to every schoolchild. They will permeate human discourse and inform our philosophical and religious views. And they will motivate many of the scientific advances of the twenty-second century.
Every elementary science textbook will include the WMAP snapshot or some improved image of the cosmic background radiation across the sky. The authors of the textbook will point to it as one of the great achievements of the twenty-first century. What will they claim about its significance?
If the inflationary model is proven correct, they will write that the image shows primordial wrinkles created at the end of inflation, about 10^-35 seconds after the big bang, when the temperature of the universe was about 10^27 degrees. The universe had a beginning of some sort, perhaps the big bang, but the period of rapid expansion diluted all information about what happened before inflation. Because human-made particle accelerators cannot possibly reach the energies needed to probe conditions before inflation, there is a limit to how much we can learn through observations or experiments about the fundamental laws of the universe. If the inflationary landscape picture survives, it may be impossible to discover the secrets of the universe because everything we see, no matter how far we look, has little in common with the rest of the cosmos, which consists of a combination of inflating regions and pocket universes with different physical properties. As for our own island, the likely outcome is that we are approaching a vacuous, uninhabitable state that will last forever. Perhaps we live in a misanthropic universe.
If the cyclic model proves to be correct, the textbook authors will write that the image shows the splatter of matter and radiation created at the big bang itself. The big bang was not the beginning but the moment separating our current period of expansion and cooling from a previous one. They will explain that the universe has an extra dimension, that the extra dimension is bounded by branes, and that the branes collided with each other to create the bang. They will show how the image can be used to determine the collision speed of the branes and to check that all the matter and radiation we see was created by the collision.
They will write that the WMAP image is also a window on the previous cycle. The small wrinkles in distribution of matter and energy were created billions of years before by random quantum waves that spontaneously appeared on the surfaces of the branes. A similar effect is beginning now that will eventually give birth to new galaxies and new stars in the next cycle. Because conditions everywhere in the universe are similar to what we observe here and because we can collect observable and measureable traces from an entire cycle, the whole cosmos can be comprehended from our single vantage point.
In 2010, it is too early to say which, if any, of these models will appear in the textbooks of the next century. But all of us can watch as a new theory blossoms into maturity and a mature theory is reinvigorated by the challenge. We can have the fun of debating the two visions of the universe and weighing in with our personal convictions while the matter remains in doubt. And we can do all this secure in the knowledge that the debate will not be endless.
No comments:
Post a Comment