By Lawrence M. Krauss
This article first appeared on June 6, 2000 in The New York Times. Reprinted by permission of the author.
In the bitter cold of Antarctica and the blistering heat of Texas, scientists recently coaxed the universe to display its hidden structure. In the past weeks, two different high altitude balloon experiments studying the microwave radiation left over from the Big Bang independently reported that the universe is very close to being "flat."
This is a geometric concept. Einstein's general theory of relativity tells us that light rays bend in a gravitational field because space is "curved" in the presence of such a field, meaning that the standard Euclidean rules of geometry taught in high school need not always apply. The recent observations, however, imply that when crossing the entire visible universe, light seems to travel in straight lines.
The Antarctic announcement, first in print of the two, was impressive enough to be reported in local and national newspapers, but it did not, to my knowledge, produce even a slight blip in the stock market. Nor should it have. Nothing in the entire course of human history would have been noticeably different if the universe were not flat.
Yet precisely for this reason there is poetry in the recent discoveries that I believe is worth celebrating, even if one is not a cosmologist. They represent on the one hand a triumph of the human imagination, and on the other a valuable reminder that science can never be purely cerebral. The universe always seems to surprise us.
In a century full of scientific revolutions, Einstein's discovery of general relativity stands as a monument to the power of human reasoning. In 1915, not a single direct piece of evidence existed that clearly suggested that space might be curved in the presence of massive bodies. However, indirect reasoning, based on careful observations of the nature of light and the motion of massive bodies, led to the realization that the Newtonian gravity theory would have to be revised.
Empirically, the observable effects of general relativity were minuscule. Light, for example, was predicted to bend around the Sun by about five 10-thousandths of a degree. But small as these effects were, they changed the way we think about space and time. Einstein's theory paved the way for interpreting Edwin Hubble's remarkable discovery that the universe is expanding. For it is only within the context of general relativity that such a global expansion can be consistently described. But, there remained a problem: if the universe is currently expanding, will the expansion stop, or continue unabated forever?
The answer to this question appeared to depend upon the specific geometry of our universe, and could not be determined within the context of the theory alone, without further observations. Moreover, it seemed to depend on a very careful tuning of the initial conditions in the expanding universe.
Twenty years ago, when I was a graduate student beginning to get interested in cosmology, a poll of astronomers and physicists would have yielded almost no support for the possibility that we live in a flat universe.
In the first place, none of the data then seemed to point in such a direction. In the second, a flat universe seemed very special, merely the boundary between two more generic geometric possibilities, the so-called open or closed universes. In the former case, light rays would diverge as they traveled across the universe and in the latter case they would converge together.
But there was a problem, first voiced by the remarkable experimental physicist Dr. Robert H. Dicke and his colleague, the distinguished cosmological theorist Dr. P. James E. Peebles. They pointed out that living in a flat universe was like sitting atop a very steep mountain. If one moved slightly away from the top in either direction, one would very quickly come tumbling down.
But our universe is over 10 billion years old. Surely if the universe were not essentially flat, it would long ago either have collapsed back upon itself, or have expanded so fast that matter would have long ago been diluted to irrelevancy on a cosmic scale.
This theoretical argument led Dr. Alan Guth, a physicist now at Massachusetts Institute of Technology, to develop a new cosmological model, called inflation, which would naturally ensure that the early universe would be driven to being so close to being flat that it should remain indistinguishable from being flat for virtually an eternity thereafter. Dr. Guth's theory was itself so compelling that within a decade, again without any direct evidence for a flat universe, many cosmologists had been converted.
I remember progressing from graduate student to professor during this time, as we in the "flat-universe trenches" began to win the day, and others in the field and out began to come around. The argument seemed so compelling that it appeared that nature would have had no choice but to adopt it, even if there was always a slight equivocation that what seemed natural to us might not be so natural for the universe.
So had the discoveries in Antarctica taken place 20 years ago, most physicists would have been shocked, and perhaps dubious. Now, however, many in the cosmological community can be seen patting themselves on the back for their foresight.
When I ponder these developments, I remain amazed that we have come to understand the universe so intimately that we may have suspected in advance that it should be flat. We are, after all, confined to the immediate proximity of our spinning globe on the outskirts of the Milky Way galaxy.
No one could guess by simply peering with a naked eye at the night sky that our galaxy is one of 100 billion or so in the visible universe, or that the universe is expanding, much less that space on large scales might be curved. Fortune indeed favors the prepared mind, and it is hard to over-emphasize the intellectual journey required before the very question of the naturalness of a flat universe could even be discussed.
But this essay is written in praise of cosmology, not cosmologists. I have made it sound as if the universe willingly followed the demands of human reason. Nothing could be further from the truth. The models we invented in the 1980's to try to bring a flat universe into accord with the observations at the time are clearly wrong. Much to our surprise the energy that dominates in the expanding universe is not associated matter of any sort, pedestrian or exotic.
Rather, it appears that our universe is only flat because empty space is endowed with some sort of funny energy, whose origin we can only begin to imagine at the present time. Although Einstein had been the first to speculate about such a possibility by introducing what became known as a cosmological constant (in order to explain why an apparently static universe might not collapse of its own gravity) he later found it so abhorrent that he dismissed it as his greatest blunder.
It is fair to say that if observations had not driven us to this precipice, no one would have traveled there in advance or later revisited it. Indeed, theoretical a priori arguments suggest that a cosmological constant tuned to produce a flat universe today is as unnatural on fundamental grounds as a flat universe now seems natural.
It gets worse. We were driven to determine whether the universe is flat or not with one main goal in mind: to constrain eternity. The classical arguments in pre-1995 cosmology books, mine included, stressed that if we could determine the geometry of the universe, we would know its ultimate destiny. How very marvelous to imagine that in our lifetimes we might determine, for certain, whether the universe would end with a bang or a whimper.
But the universe has outsmarted us once again. The realization that empty space might in fact provide the dominant source of energy in the universe has again changed everything. We now recognize that any universe, open, closed or flat, can collapse or expand forever, depending upon the magnitude of this new form of energy. Geometry and destiny have been disentangled.
Indeed, my colleague Michael Turner and I have argued that there will never be any finite set of astronomical measurements made over a finite time that will allow us to determine the ultimate fate of our universe. Some things, it seems, may be forever shrouded in mystery.
So we find ourselves at the dawn of the 21st century strangely self-satisfied and at the same time confused. A flat universe doesn't matter a tinker's damn, an expression my wife likes to use, in the everyday course of human events. Yet the lessons that arise from these discoveries can leave us with a completely new perspective of our place in the cosmos. That is what the progress of culture is all about. Ultimately this, not technology, may be the greatest legacy of science.
Finally, we learn that the universe is a stranger and more interesting place than human imagination alone can ever foretell. If we stop looking outward, we are likely to end up going nowhere.
Dr. Lawrence M. Krauss is a professor of physics at Case Western Reserve University. About Lawrence M. Krauss
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