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SUMMARY:Scalar-tensor theories of gravity beyond Horndesky - Prof. Juan Ga
 rcia-Bellido (IFT-UAM/CSIC)
DTSTART:20141031T130000Z
DTEND:20141031T140000Z
UID:TALK55974@talks.cam.ac.uk
CONTACT:Benson Way
DESCRIPTION:In this talk I will briefly review structure of scalar-tensor 
 theories of gravity based on derivative couplings between the scalar and t
 he matter degrees of freedom. Such interactions are classified by their te
 nsor structure into conformal (scalar)\, disformal (vector) and extended d
 isformal (traceless tensor)\, as well as by the derivative order of the sc
 alar field. Relations limited to first derivatives of the field ensure sec
 ond order equations of motion in the Einstein frame and hence the absence 
 of Ostrogradski ghost degrees of freedom. The existence of a mapping to th
 e Jordan frame is not trivial in the general case\, and can be addressed u
 sing the Jacobian of the frame transformation through its eigenvalues and 
 eigentensors. Although sane in the Einstein frame\, generic disformally co
 upled theories are described by higher order equations of motion in the Jo
 rdan frame. This apparent contradiction is solved by the use of a hidden c
 onstraint: the contraction of the metric equations with a Jacobian eigente
 nsor provides a constraint relation for the higher field derivatives\, whi
 ch allows one to express the dynamical equations in a second order form. T
 his signals a loophole in Horndeski's theorem and allows one to enlarge th
 e set of scalar-tensor theories which are Ostrogradski-stable.  
LOCATION:Pavilion B Potter Room (B1.19)
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