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By Root 606 0

bang singularity—where the temperature of the universe was infinite. It is no surprise that the universe was very hot in the big bang phase; the expanding universe was, after all, much smaller and more densely compressed at those times than it is now, implying an enormous increase in temperature. But infinity as the result of a physical theory simply means that the theory has been stretched beyond its limits; its equations lose all meaning at such a place. In the case of the big bang model, one should not misunderstand the breakdown of equations as the prediction of a beginning of the world, even though it is often presented in this way. A point in time at which a mathematical equation results in an infinite value is not the beginning (nor the end) of time; it is rather a place where the theory shows its limitations. In spite of all its successes in other areas, the theory given by general relativity in combination with the quantum theory of matter remains to be extended.

The problem lies in the incompleteness of the revolution brought about by physical research during the last century. Quantum theory is used to describe matter in the universe, but not gravity or even space and time themselves. The latter are firmly in the domain of general relativity, largely independent of quantum physics. A successful combination of quantum theory and general relativity, even in the realms of space and time, would significantly extend known theories. Such a combination—quantum gravity—is particularly important for a description of the hot big bang phase of the universe, and it can explain, or so we hope, what happened at the infinity of the big bang singularity. Was this really the origin of the world and of time, or was there something before? And if there was something before the big bang, what was it?

Unfortunately, quantum gravity is extremely complicated. Even separately, general relativity and quantum theory are distinguished by a mathematical machinery unknown to preceding branches of physics. To make matters worse, these two areas require markedly different mathematical methods. A combination of the physical theories also requires a unification of the underlying mathematical principles, further magnifying their degree of difficulty. For this reason, no completely formulated quantum theory of gravity is available yet, in spite of many decades of research and vigorous efforts by a large number of scientists. What we have seen nonetheless, in particular in the last few years, are numerous promising indications for some properties of quantum gravity that can already be analyzed. As so often in research, the situation resembles the first stages of assembling a jigsaw puzzle, when one may already have an inkling of the emerging picture but could well be on the wrong track. Our current view indicates what a completion of physical theory can bring about: It allows us to see what possibly happened at and even before the big bang. We are granted a glimpse of the earliest times of our universe and can, for the first time, analyze how it may have arisen.

In this book, recent results of the theory as well as plans for satellite observations in the near future are explained, and it is shown how radically they could alter our worldview. Loop quantum gravity in particular, one of the variants currently put forward as a possible combination of general relativity and quantum theory, has provided first results concerning a nonsingular description of the big bang. In this framework, the universe existed before the big bang, and one can roughly estimate how it could have differed from what we see now. By its influence on later stages of the cosmic expansion, detectable by sensitive observations, this ancient prehistory of the universe can be explored. This book provides a firsthand report of this line of research, followed by a discussion of black holes, which also show fascinating effects. The final chapters touch upon further issues regarding a general understanding of the world, among them cosmogony, the riddle of time and its direction, and