In General > s.a. causality violations; modified lorentz symmetry; photon; Superluminal Propagation.
* Idea: Particles with negative squared mass; If they transform under the ordinary action of the Lorentz group, as often assumed, they travel along spacelike directions, i.e. faster than light or superluminally.
* Consequences: Existence of tachyons and Lorentz invariance imply causality violations, so a tachyon is considered to be sick in the context of particle mechanics; In quantum field theory, the appearance of tachyonic degrees of freedom indicates vacuum instability rather than the actual existence of faster-than-light particles.
* Description: Scalar ones can be described by a Klein-Gordon theory, vector ones by a Proca theory, with m2 < 0.
* Pseudo-tachyons: Particles which behave like tachyons in momentum space and like subluminal particles (v < c) in ordinary space.
> Related topics: see casimir effect; mass [negative-mass particles].
> Online resources: see MathWorld page; Wikipedia pages on Tachyonic particles and Tachyonic fields.

In Cosmology > s.a. observational cosmology.
@ General references: Davidson PRD(87), & Owen PLB(86) [particles as Kaluza-Klein tachyons]; Tomaschitz IJMPD(98) [in FLRW spacetime]; Feinstein PRD(02); Gibbons PLB(02)ht [rolling down potential]; Fairbairn & Tytgat PLB(02)ht; Frolov et al PLB(02)ht; Kofman & Linde JHEP(02)ht [problems]; Steer & Vernizzi PRD(04)ht/03; Gorini et al PRD(04); Vernizzi & Steer MGX(06)ap/04 [vs standard inflation]; Keresztes et al PRD(09)-a0901 [and Big Brake singularity]; Nozari & Rashidi PRD(13)-a1306 [inflation, perturbations, etc]; Novosyadlyj a1311; Agarwal & Afshordi PRD(14)-a1406 [Thermal Tachyacoustic Cosmology]; > s.a. Rigidity.
@ As dark matter: Shiu & Wasserman PLB(02)ht; Davies IJTP(04)ap.
@ As dark energy: Bagla et al PRD(03)ap/02; Padmanabhan PRD(02)ht; Copeland et al PRD(05)ht/04; de la Macorra et al PLB(06) [late-time behavior]; Calcagni & Liddle PRD(06); Novosyadlyj a1311 [homogeneous tachyonic scalar fields]; Martins & Moucherek PRD(16)-a1606 [constraints]; Schwartz MPLA(17)-a1703 [revised energy-momentum tensor].

References > s.a. special relativity.
@ Books, reviews: Kowalczyński 66; Kreisler AS(73); in Gribbin 77; Herbert 89; Stonier 90.
@ General: Feinberg PR(67); Feldman AJP(74)mar [bibliography]; Lévy-Leblond AJP(80)may; Arntzenius BJPS(90); González-Mestres ap/95 [and spacetime geometry]; Kowalczyński APPB(00)he/00 [proposed experimental searches]; Ehrlich AJP(03)nov [and teaching]; Romero et al MPLA(12)-a1112 [general action]; Rembieliński & Włodarczyk a1206 [Lorentz-covariant, consistent description]; Perepelitsa a1407 [theory, and response to objections]; Kapuścik IJTP(15)-a1412 [re error in Feinberg's paper].
@ Quantum: Fanchi FP(90) [parametrized relativistic quantum mechanics]; Schwartz IJMPA(16)-a1603 [towards a quantum field theory].
@ And field theory / matter: Tanaka PTP(61); Bilaniuk et al AJP(62)oct; Bers et al in(71); Moretti & Agresti NCB(95) [Klein-Gordon]; González-Mestres ap/96 [HECRs]; Bollini et al IJTP(99) [and electromagnetism]; Tomaschitz IJMPA(99); Schwartz JMP(11)-a1011 [in general relativity]; Fazlpour & Banijamali JCAP(15)-a1410 [tachyon non-minimally coupled to torsion in teleparallel gravity].
@ Formal properties, kinematics: Basano IJTP(77) [and causal paradoxes]; Asaro AJP(96)apr [and complex velocity]; Madarász & Székely JAL(14)-a1303 [consistency with special relativity]; Madarász et al Sigma(14)-a1309 [axiomatic framework in Minkowski spacetime].
@ Related topics: Davidson PE(01)qp/02 [motion of charged ones]; Salesi IJMPA(97)qp/02 [pseudotachyonic Fermions]; Schwartz MPLA(11)-a1104 [and gravitational Cerenkov radiation]; Chashchina & Silagadze APPB(12)-a1211 [proposed type of particles, "elvisebrions", that can be superluminal but are not tachyons]; Lee et al PRA(15)-a1503 [proposed experimental simulation of Dirac tachyons with trapped ions]; > s.a. Bradyons.

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