On under-determination in cosmology

•Cosmology illustrates under-determination of theoretical hypotheses by data, in ways that are different from most philosophical discussions.•I argue that cosmology is distinctively concerned with what data could in principle be collected by a single observer.•I discuss sceptically the appeal to the cosmological principle as a way of breaking the under-determination.

I discuss how modern cosmology illustrates under-determination of theoretical hypotheses by data, in ways that are different from most philosophical discussions. I emphasise cosmology's concern with what data could in principle be collected by a single observer (Section 2); and I give a broadly sceptical discussion of cosmology's appeal to the cosmological principle as a way of breaking the under-determination (Section 3).I confine most of the discussion to the history of the observable universe from about one second after the Big Bang, as described by the mainstream cosmological model: in effect, what cosmologists in the early 1970s dubbed the ‘standard model’, as elaborated since then. But in the closing Section 4, I broach some questions about times earlier than one second.