Higher derivative scalar-vector-tensor theories from Kaluza-Klein reductions of Horndeski theory

It was recently pointed out that some precise photon-Galileon couplings in four dimensions (4D)—inspired by a higher dimensional reduction—are enough to obtain a Horndeski theory that is less constrained by the stringent experimental bounds on the speed of gravitational waves. They imply the constancy of the ratio of speed of gravity to light throughout cosmic evolution. This holds even if we include the general scalar potentials <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>G</a:mi><a:mn>4</a:mn></a:msub><a:mo stretchy="false">(</a:mo><a:mi>π</a:mi><a:mo>,</a:mo><a:mi>X</a:mi><a:mo stretchy="false">)</a:mo></a:math> and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mi>G</e:mi><e:mn>5</e:mn></e:msub><e:mo stretchy="false">(</e:mo><e:mi>π</e:mi><e:mo stretchy="false">)</e:mo></e:math>. In this paper we go into the details of this 4D luminal extension of Horndeski theory including its scalar sector. We also present the complete action including the general <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msub><i:mi>G</i:mi><i:mn>5</i:mn></i:msub><i:mo stretchy="false">(</i:mo><i:mi>π</i:mi><i:mo>,</i:mo><i:mi>X</i:mi><i:mo stretchy="false">)</i:mo><i:mo>,</i:mo><i:msub><i:mi>G</i:mi><i:mn>6</i:mn></i:msub><i:mo stretchy="false">(</i:mo><i:mi>π</i:mi><i:mo>,</i:mo><i:mi>X</i:mi><i:mo stretchy="false">)</i:mo></i:math> scalar potentials. Thus we show all the <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mi>U</o:mi><o:mo stretchy="false">(</o:mo><o:mn>1</o:mn><o:mo stretchy="false">)</o:mo></o:math> gauge invariant vector Galileons in 4D that result from a Kaluza-Klein dimensional reduction from 5D Horndeski. They provide a coupling of a higher derivative vector to scalar modifications of gravity—namely, without inducing Ostrogradsky ghosts and keeping gauge invariance—in the aim to explore more universal couplings of dark energy to other matter, such as vectors and in particular the photon.