Hamilton-Jacobi analysis of noncanonical inflation in $f(R, T)$ gravity: Constraints from Planck/ACT data, and theoretical bounds
The latest CMB data from ACT DR6, combined with Planck, DESI, and BICEP/Keck, indicate a slight upward shift in the scalar spectral index, placing several previously favored inflationary models under tension. We study an inflationary scenario within the framework of $f(R, T)$ gravity, featuring a nonminimal matter-curvature coupling, where the inflaton is a noncanonical scalar field with a generalized kinetic energy. Using the Hamilton-Jacobi formalism, we express the Hubble parameter as a function of the scalar field and consider two forms of $H(ϕ)$, a power-law and an exponential one, deriving the scalar spectral index $n_s$ and tensor-to-scalar ratio $r$. Comparison with ACT DR6 allows us to explore the parameter space, showing that the power-law case is compatible with the data across a wide range, while the exponential form requires a large number of e-folds. We then study reheating, noting its close link with the inflationary dynamics. By imposing the bound on overproduction of primordial gravitational waves encoded in the constraint on $ΔN_{\text{eff}}$, we obtain a lower limit on the reheating temperature, which becomes particularly restrictive for the stiff reheating equation of state $ω_{\text{re}}$. This bound implies that the total number of e-folds should not exceed $N\lesssim 64(65)$. The predicted gravitational-wave spectrum shows an enhanced high-frequency amplitude, potentially observable by future detectors. We also examine consistency with the Swampland conjectures and the Trans-Planckian Censorship Conjecture, finding that combining $f(R, T)$ gravity with noncanonical dynamics provides a rich and testable framework for the early universe.