P(X, phi) scalar field theory ($P(X,\varphi )$ theory)

A $P(X,\varphi )$ scalar theory has Lagrangian density $P$ depending on the field and $X=-g^{\mu \nu }{\partial }_{\mu }\varphi {\partial }_{\nu }\varphi /2$. It includes canonical scalar theory and models with noncanonical kinetic interactions.

A k-essence model uses noncanonical kinetic terms for cosmological acceleration; a general P(X, phi) scalar field theory need not have that application.

On a homogeneous background with $X=\bar{X}$, a $P(X,\varphi )$ perturbation has sound speedThe denominator is its time-kinetic coefficient and $P_X$ is its spatial-gradient coefficient.

For $X={\partial }_{\mu }\varphi {\partial }^{\mu }\varphi /2$ in signature $(+---)$, the derivative part of the quadratic action on a timelike background is $\frac{1}{2}(P_{,X}+2X{P}_{,XX})\delta {\dot{\varphi }}^2-\frac{1}{2}{P}_{,X}{a}^{-2}(\nabla \delta \varphi {)}^2$. Absence of a ghost requires $P_{,X}+2X{P}_{,XX}>0$; positive gradient energy requires $P_{,X}>0$. The squared sound speed is their ratio $P_{,X}/(P_{,X}+2X{P}_{,XX})$.

In the limit where scalar field fluctuations dominate metric fluctuations, put $\zeta =-H\delta \varphi /\dot{\bar{\varphi }}$ and neglect derivatives of slowly varying background coefficients. Splitting $\delta X=\delta X_1+\delta X_2$ givesThe cubic terms $P_{XX}\delta X_1\delta X_2+P_{XXX}\delta X_1^3/6$ therefore yieldThe expression before factoring remains well-defined when $P_{XX}=0$. These interactions can generate substantial primordial non-Gaussianity when the Sound speed of a P(X, phi) scalar perturbation is small.

Neglect metric fluctuations and derivatives of $\dot{\bar{\varphi }}/H$, so $\delta X_1=-2\bar{X}\dot{\zeta }/H$ and $\delta X_2=\bar{X}[{\dot{\zeta }}^2-a^{-2}(\nabla \zeta {)}^2]/H^2$. The coefficient of ${\dot{\zeta }}^3$ in the cosmic-time action is $C=-a^3[2{\bar{X}}^2{P}_{,XX}+(4/3){\bar{X}}^3{P}_{,XXX}]/H^3$. This unfactored form is valid even when $P_{,XX}=0$ and a factored ratio would be undefined.

Varying the shift in the 3+1 decomposition of spacetime gives a nondynamical momentum constraint. For a $P(X,\varphi )$ field it equates the spatial divergence of $K_i{}^j-{\delta }_i^{j}K$ to the scalar momentum density.