Once Before Time - Martin Bojowald [69]
Positive values can be computed in simple models, sometimes easily, sometimes with considerably more effort. There are more models than one can imagine in which some computable parameters can be found, much appreciated by researchers as rich sources of scientific publications. Especially in the beginning stages of a newly emerging field, many possibilities for concrete calculations remain open. But the flood of data does not necessarily foster a deeper understanding. Often, known results are simply reproduced in a slightly changed model, but then one still remains ignorant about whether such a property is generally valid or the result of a selection effect. A transition to the pessimistic view, as recently initiated for loop quantum cosmology, is thus a sign of the maturity of a scientific field. One starts to take limits seriously, and turns to more general results within the studied theory. Besides, addressing and admitting limits is an important contribution to the honesty of science.
5. OBSERVATIONAL COSMOLOGY
PROBING THE WHOLE UNIVERSE, PAST AND FUTURE
Any theory about space and time must be consistent with what we can see of the whole universe. Modern cosmology started in the 1920s with pioneering work by Edwin Hubble, but it remained an imprecise science for many decades. For most quantities characterizing the form of the universe, such as its expansion rate (now called the Hubble parameter), estimates were made. But according to the available data, they could easily have been changed by a factor of two or so. There were times when certain stars were estimated to be older than the age of the whole universe. With refined methods and advanced technologies, the situation eventually changed. Especially in recent times, observational cosmology has managed to achieve impressive progress, providing valuable contributions to an understanding of the universe. Today, it is one of the most exciting parts of physics.
Cosmology plays a special role within physics because the universe represents a unique system. It has not been prepared by an experimentalist, and so the usual procedure—generating many similar systems, such as groups of elementary particles in numerous accelerator reactions, to measure their properties repeatedly—is precluded. The common methodology of experimental physics allows one to considerably reduce the influence of mistaken measurements, which are bound to happen in a single unrepeated and unchecked experiment. With the universe, the situation is certainly different: Cosmologists must accept what the universe once and for all presents them.
There is only one branch in cosmological experiments: that of observation. Preparation cannot happen. Some observations are thus fraught with errors that cannot be eliminated completely. It is not always clear whether a measured value is the way it has been seen just by chance, or for a good reason. With repeated experiments one can easily detect and eliminate the role of randomness because it cannot give rise to systematic effects. Repetitions, however, are impossible in cosmology.
Cosmological parameters often have such surprising values that they almost cry out for a deeper explanation. For instance, as we will soon see, the universe has accelerated its expansion relatively recently, as inferred from the escape velocities of distant star explosions. It is sometimes speculated, perhaps in jest, that there may be a connection between the acceleration and the emergence of intelligent life able to perceive it. But how can one rule out that this is not just a result of chance? Making a decision about the role of chance becomes more complicated by the fact that the reason for the acceleration (mysteriously called “dark energy”) is barely understood.
One should keep in mind, however, that many such coincidences exist, to which at times enormous importance has been attributed. For instance, the lunar and solar radii correspond so well with the radii of the earth and moon orbits that nearly perfect