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SUMMARY:Stability of monopole perturbations in Horndeski theory on a dynam
 ical spherically symmetric background
DTSTART;VALUE=DATE-TIME:20261029T144000Z
DTEND;VALUE=DATE-TIME:20261029T145500Z
DTSTAMP;VALUE=DATE-TIME:20261029T032136Z
UID:indico-contribution-6256@cern.ch
DESCRIPTION:Speakers: Kirill Kiselev (M.V. Lomonosov Moscow State Universi
 ty)\nConstructing physically viable models within Horndeski and Beyond Hor
 ndeski theories requires strict checks to avoid ghost and gradient instabi
 lities\, yet the corresponding stability constraints for dynamical spheric
 ally symmetric backgrounds remain incomplete. Previous studies of perturba
 tions over such backgrounds focused exclusively on the odd-parity sector\,
  which yields stability conditions solely for the tensor (gravitational) m
 ode. Consequently\, the behavior of the scalar degree of freedom remained 
 unconstrained. \n\nIn this work\, we address this gap by investigating the
  even-parity sector of perturbations in both Horndeski and Beyond Horndesk
 i gravity over a general time-dependent\, spherically symmetric background
 . By utilizing the Regge-Wheeler formalism and focusing on monopole ($\\el
 l=0$) perturbations\, we isolate the dynamics of the scalar degree of free
 dom. We successfully integrate out non-dynamical fields and derive the exp
 licit quadratic action for the remaining physical scalar mode. \n\nAs a ma
 in result\, we obtain exact no-ghost and gradient stability conditions\, a
 s well as the expression for the radial propagation speed of the scalar mo
 de. These novel constraints strictly limit the scalar sector and complemen
 t the previously known tensor-mode conditions from the odd sector. Togethe
 r\, they establish a stringent theoretical framework for the parameter spa
 ce of Horndeski and Beyond Horndeski theories\, which is essential for bui
 lding physically consistent astrophysical and cosmological models.\n\nhttp
 s://indico.particle.mephi.ru/event/626/contributions/6256/
LOCATION: Conference Hall 5
URL:https://indico.particle.mephi.ru/event/626/contributions/6256/
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