Time After Time — Big Bang Cosmology and the Arrows of Time

Abstract

Time, as familiar as it seems to us in everyday life, is one of the greatest puzzles of science and philosophy. In physics and cosmology it is especially mysterious why time appears to be directed, that is, why there seems to be an essential difference between the past and the future. The most basic known laws of nature do not contain this asymmetry. And yet, several arrows of time can be distinguished at least ten, in fact. However, it is unclear whether any of them are fundamental or whether others can be reduced to these, and it is not known how the direction of time could be explained convincingly. From the growing but still astonishingly low entropy of the observable universe, it seems plausible that the solution of the mystery is connected with cosmology and an explanation of the big bang. This could require a new fundamental law of nature (which might be related to a particular geometry) or specific boundary conditions (which might be comprehensible within the framework of a multiverse theory). Or it may be that times direction is fundamental and irreducible, or an illusion and not explicable, but can only be explained away. It is even more confusing that not all of these alternatives are mutually exclusive. Furthermore, there is a plethora of approaches to explain the big bang. Some models postulate an absolute beginning of time, others an everlasting universe or multiverse in which the big bang is a phase transition, and maybe there are myriads of big bangs. So the low entropy of the observable universe might be a random fluctuation whereas elsewhere even opposite thermodynamic directions of time may arise. Perhaps the (or our) big bang just created the arrows of time, if it originated as some sort of pseudo-beginning in a quantum vacuum that has no direction of time. Thus it seems useful to conceptually distinguish an undirected microtime and a directed macrotime. It is even possible that time ends although paradoxically, it may do so only temporarily.