The impact of the cosmological constant on past and future star formation

It is well known that the peculiarly small observed value of the cosmological constant gives rise to coincidences that cry out for an explanation. Anthropic reasoning conjectures that cosmic coincidences arise as a selection effect: we are more likely to observe conditions that promote the generation of observers, such as ourselves. Building up on our understanding of galaxy formation in a cosmological context, and additionally using star formation as a proxy for observers’ generation, it is possible to rigorously test anthropic arguments by determining the values of cosmological parameters that maximise cosmic star formation efficiency. I will review recent analytical and numerical works in the subject. I will present the results of my analytic model of cosmic star formation (Sorini & Peacock 2021) on the past and future star formation history in universes with a cosmological constant between 0 and 10^5 times the observed value. I find that although the cosmic efficiency of star formation peaks near the observed cosmological constant, the probability that an observer exists peaks at ~800 times larger values. Instead, the probability of observing a cosmological constant as small as the one we measure would be just ~0.5%. This surprising result calls for a critical reflection on the viability of anthropic reasoning as an explanation for the peculiarly small observed value of the cosmological constant.