steady state theory has 1 distinct synonym spread across 1 of its 1 sense. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
(cosmology) the theory that the universe maintains a constant average density with matter created to fill the void left by galaxies that are receding from each other
“the steady state theory has been abandoned in favor of the big bang theory”
Synonyms, antonyms and the broader and narrower relations for steady state theory come from Princeton University’s WordNet. Full sources and licences.