{"work":{"id":"9b8ae859-e5cd-49c1-9992-0ab561976e83","openalex_id":null,"doi":null,"arxiv_id":"1404.6495","raw_key":null,"title":"Healthy theories beyond Horndeski","authors":null,"authors_text":"Gleyzes, J","year":2014,"venue":"hep-th","abstract":"We introduce a new class of scalar-tensor theories that extend Horndeski, or \"generalized galileon\", models. Despite possessing equations of motion of higher order in derivatives, we show that the true propagating degrees of freedom obey well-behaved second-order equations and are thus free from Ostrogradski instabilities, in contrast to the standard lore. Remarkably, the covariant versions of the original galileon Lagrangians-obtained by direct replacement of derivatives with covariant derivatives-belong to this class of theories. These extensions of Horndeski theories exhibit an uncommon, interesting phenomenology: the scalar degree of freedom affects the speed of sound of matter, even when the latter is minimally coupled to gravity.","external_url":"https://arxiv.org/abs/1404.6495","cited_by_count":null,"metadata_source":"pith","metadata_fetched_at":"2026-07-04T08:29:42.137994+00:00","pith_arxiv_id":"1404.6495","created_at":"2026-05-09T06:10:40.499384+00:00","updated_at":"2026-07-04T08:29:42.137994+00:00","title_quality_ok":true,"display_title":"Healthy theories beyond Horndeski","render_title":"Healthy theories beyond Horndeski"},"hub":{"state":{"work_id":"9b8ae859-e5cd-49c1-9992-0ab561976e83","tier":"hub","tier_reason":"10+ Pith inbound or 1,000+ external citations","pith_inbound_count":14,"external_cited_by_count":null,"distinct_field_count":3,"first_pith_cited_at":"2018-07-17T04:05:07+00:00","last_pith_cited_at":"2026-07-02T09:32:10+00:00","author_build_status":"not_needed","summary_status":"needed","contexts_status":"needed","graph_status":"needed","ask_index_status":"not_needed","reader_status":"not_needed","recognition_status":"not_needed","updated_at":"2026-07-04T15:46:55.417803+00:00","tier_text":"hub"},"tier":"hub","role_counts":[{"context_role":"background","n":3}],"polarity_counts":[{"context_polarity":"background","n":3}],"runs":{},"summary":{},"graph":{},"authors":[]}}