2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/53141We show explicitly that free quantum field theory in de Sitter background restricted on the cosmological horizon produces another quantum field theory unitarily equivalent with the original one. Symmetry properties descending from the dual theory are also remarked. In the restricted theory the thermal properties, known for de Sitter quantum field theory, can be proved straightforwardly.14 pages, 1 figure, minor corrections, added referencesHigh Energy Physics - TheoryGeneral Relativity and Quantum CosmologyMathematical Physicstext