Anthropic cosmology

Abstract

The cosmological constant problem is one of the most profound issues on the cosmic fine-tuning problem. The insights from inflation theory and string landscape produces the picture of multiverse on which anthropic reasonings should be involved. In the context of the cosmic multiverse, the cosmological constant problem turns out to be a statistical inference, which is equivalent to survey the probabilistic viability of the observed value. A concrete framework of the statistical inference includes two main parts: the prior probability of a certain type of universe and the anthropic probability of occurrence of a certain type of observer. The prior probability is to measure the occupation of a universe with observable data, primarily the cosmological constant, among the multiverse. The estimation of the prior probability is ambiguous and it is called ‘measure problem’ of inflationary multiverse. Among proposals trying to resolve the measure problem, the holographic measure which is based on the black hole complementarity looks attractive. It makes a proper description to be constrained only in the causal horizon of an observer from the complementarity so that it makes the measure problem and Boltzmann brain problem not too heavy. However, since the black hole complementarity gets in trouble with regular black holes or dynamical charged black holes, it is claimed that the holographic measure may not work in the multiverse. The anthropic probability is to measure the distribution of anthropic observers like us. In the context of astrophysics, the anthropic observer is specified as the condensation of gravitational fluctuations into proper scales and so its probability is quantified with the Press-Schecter formalism by adding formation time conditions and local structures. It is well known that the anthropic probability is dominant within a few hundred in the observed value of the cosmological constant but it is shown that by our method the production of the ant...