The Spectre of Underdetermination in Modern Cosmology

The scientific status of physical cosmology has been the subject of philosophical debate ever since detailed mathematical models of the Universe emerged from Einstein's general theory of relativity. Such debates have revolved around whether and to what extent cosmology meets established demarcation criteria for a discipline to be scientific, as well as determining how to best characterize cosmology as a science, given the unique challenges and limitations faced by a discipline which aims to study the origin, composition, and fate of the Universe itself. The present article revisits, in light of the dramatic progress in cosmology in recent decades, an earlier debate held in the 1950s between Herman Bondi and Gerald Whitrow regarding the scientific status of cosmology. We analyse cosmology's transition from an emerging science to a cornerstone of modern physics, highlighting its empirical successes in establishing the $\Lambda$-Cold Dark Matter ($\Lambda$CDM) model and in its delivery of various successful novel predictions. Despite this remarkable scientific success and progress, we argue that modern cosmology faces a further profound challenge: the permanent underdetermination of the microphysical nature of its exotic energy components: inflation, dark matter, and dark energy. Drawing historical parallels with the role of spectroscopy in revealing the microphysical nature of atomic physics, we argue that the epistemic barriers obstructing us from ascertaining the microphysical nature of these exotic energy components are significant, in turn casting doubt upon whether cosmology can ever transcend these particular epistemic challenges. We conclude by reflecting on the prospects for future breakthroughs and/or non-empirical arguments which could decide this issue conclusively.