It could perhaps be a reference to boundaries of the observed universe, which has a moving but eventually finite boundary (due to dark energy) that is called the cosmological (infinite future) horizon. (IIRC, you may want to google it.)

The cosmological horizon features in speculative physics of so called holography (which has an established physical basis, FWIW; you may want to google that too), and of the speculative causal diamond solution to the statistical measurement problem of multiverse cosmologies (and either inspired by, or attempting to resolve, similar problems of black holes, I think). [ https://en.wikipedia.org/wiki/Measure_problem_(cosmology)#Causal_diamond ]