Frame-invariant formulation for multifield and higher-derivative scalar–tensor theories

Extend frame-invariant formulations from single-field scalar–tensor theories to multifield and higher-derivative scalar–tensor theories and determine their implementation in realistic dark-energy and early-Universe scenarios.

Background

The existing discussion primarily concerns scalar–tensor theories with one scalar field and comparatively standard derivative structure. The dissertation identifies the extension of frame-invariant methods to multifield systems and higher-derivative scalar–tensor theories as unresolved, particularly when these generalized theories are applied to realistic models of dark energy and the early Universe.

References

There are several issues that remain open and require further investigation:

— The alternative scalar field dark sector of the Universe: Cosmological inflation, quintessence and ekpyrotic model in the framework of scalar-tensor cosmology  (2609.37293 - Postolak, 29 Sep 2026) in Section 2, subsection “Open directions”