Degenerate Higher-Order Scalar-Tensor theories

Degenerate Higher-Order Scalar-Tensor theories (or DHOST theories) are theories of modified gravity. They have a Lagrangian containing second-order derivatives of a scalar field but do not generate ghosts (kinetic excitations with negative kinetic energy), because they only contain one propagating scalar mode (as well as the two usual tensor modes).^[1]^[2]

^ Frusciante, Noemi; Kase, Kyotaro; Koyama, Kazuya; Tsujikawa, Shinji; Vernieri, Daniele (2019). "Tracker and scaling solutions in DHOST theories". Physics Letters B. 790: 167–175. arXiv:1812.05204. Bibcode:2019PhLB..790..167F. doi:10.1016/j.physletb.2019.01.009.
^ Langlois, David; Mancarella, Michele; Noui, Karim; Vernizzi, Filippo (2019). "Mimetic gravity as DHOST theories". Journal of Cosmology and Astroparticle Physics. 2019 (2): 036. arXiv:1802.03394. Bibcode:2019JCAP...02..036L. doi:10.1088/1475-7516/2019/02/036. S2CID 54994853.

[1] Frusciante, Noemi; Kase, Kyotaro; Koyama, Kazuya; Tsujikawa, Shinji; Vernieri, Daniele (2019). "Tracker and scaling solutions in DHOST theories". Physics Letters B. 790: 167–175. arXiv:1812.05204. Bibcode:2019PhLB..790..167F. doi:10.1016/j.physletb.2019.01.009.

[2] Langlois, David; Mancarella, Michele; Noui, Karim; Vernizzi, Filippo (2019). "Mimetic gravity as DHOST theories". Journal of Cosmology and Astroparticle Physics. 2019 (2): 036. arXiv:1802.03394. Bibcode:2019JCAP...02..036L. doi:10.1088/1475-7516/2019/02/036. S2CID 54994853.

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