Dynamics and observational signatures of warm Dirac–Born–Infeld inflation with nonminimal derivative coupling

Abstract This paper investigates a warm Dirac–Born–Infeld (DBI) inflationary model with nonminimal derivative coupling (NMDC) to gravity, where the inflaton kinetic term interacts with the Einstein tensor, thereby improving the effective gravitational friction. This model seamlessly integrates the noncanonical DBI kinetic structure, the NMDC-induced gravitational friction, and thermal dissipation. We formulate the background evolution equations along with the corresponding slow-roll stability conditions, leading to analytic results for the scalar spectral index $$n_s$$ n s and the tensor-to-scalar ratio R . By applying these results to power-law potentials with $$n=2$$ n = 2 and $$n=4$$ n = 4 , we constrain the model parameter space using the Planck 2018 data together with BK15 and BAO measurements. The findings indicate that the interaction between NMDC-induced gravitational friction and thermal dissipation effectively modulates $$n_s$$ n s and significantly expands the viable parameter space. In the $$(n_s,R)$$ ( n s , R ) plane, the predictions for $$N=50$$ N = 50 fall within the 95% confidence-level (CL) region, while those for $$N=60$$ N = 60 extend into the 68% CL region and approach the observationally preferred central values. For the representative parameter choices examined, the tensor-to-scalar ratio is notably suppressed, generally within the range $$10^{-8}\\lesssim R\\lesssim 10^{-5}$$ 10 - 8 ≲ R ≲ 10 - 5 . We also compare our results with the recent ACT DR6 observations, showing that the tighter constraints do not introduce significant observational tension for our model. Moreover, the combined damping mechanism relaxes the slow-roll condition related to $$\\eta $$ η and limits the inflaton field excursion, thus addressing the $$\\eta $$ η problem without incurring super-Planckian field variations. These results indicate that warm DBI inflation with NMDC offers a theoretically coherent and observationally viable inflationary model, showcasing the complementary effects of thermal dissipation and enhanced gravitational friction in the context of modified gravity.