The existence of cosmological horizons resulting from the expansion of the universe is a foundational concept in standard Big Bang cosmology, making it common knowledge that does not require individual citation.
The claim states that cosmic expansion creates a cosmological horizon beyond which galaxy light cannot reach us. This is a fundamental, textbook definition in physical cosmology (specifically concerning particle and event horizons in expanding spacetimes like FLRW). While papers discuss cosmological models, horizons, and boundaries (such as [5], [6], [7]), the core phenomenon is a basic tenet of standard cosmological theory. Therefore, it is categorized as common knowledge.