Fine-tuning free paradigm of two-measures theory:k-essence, absence of initial singularity of the curvature, and inflation with graceful exit to the zero cosmological constant state

Abstract

The dilaton-gravity sector of the two-measures field theory (TMT) is explored in detail in the context of spatially flat Friedman-Robertson-Walker (FRW) cosmology. The model possesses scale invariance which is spontaneously broken due to the intrinsic features of the TMT dynamics. The dilaton $\ensuremath{\phi}$ dependence of the effective Lagrangian appears only as a result of the spontaneous breakdown of the scale invariance. If no fine-tuning is made, the effective $\ensuremath{\phi}$-Lagrangian $p(\ensuremath{\phi},X)$ depends quadratically upon the kinetic term $X$. Hence TMT represents an explicit example of the effective $k$-essence resulting from first principles without any exotic term in the underlying action intended for obtaining this result. Depending of the choice of regions in the parameter space (but without fine-tuning), TMT exhibits different possible outputs for cosmological dynamics: (a) Absence of initial singularity of the curvature while its time derivative is singular. This is a sort of sudden singularities studied by Barrow on purely kinematic grounds. (b) Power law inflation in the subsequent stage of evolution. Depending on the region in the parameter space the inflation ends with a graceful exit either into the state with zero cosmological constant (CC) or into the state driven by both a small CC and the field $\ensuremath{\phi}$ with a quintessencelike potential. (c) Possibility of resolution of the old CC problem. From the point of view of TMT, it becomes clear why the old CC problem cannot be solved (without fine-tuning) in conventional field theories. (d) TMT enables two ways for achieving small CC without fine-tuning of dimensionful parameters: either by a seesaw type mechanism or due to a correspondence principle between TMT and conventional field theories (i.e. theories with only the measure of integration $\sqrt{\ensuremath{-}g}$ in the action). (e) There is a wide range of the parameters such that in the late time universe: the equation of state $w=p/\ensuremath{\rho}<\ensuremath{-}1$; $w$ asymptotically (as $t\ensuremath{\rightarrow}\ensuremath{\infty}$) approaches $\ensuremath{-}1$ from below; $\ensuremath{\rho}$ approaches a constant, the smallness of which does not require fine-tuning of dimensionful parameters.