Embedding ultraslow-roll inflaton dynamics in warm inflation

Slow roll of the inflaton field defines the standard dynamics of the inflationary epoch. However, the inflationary system deviates from slow roll when it encounters an extremely flat region of the inflaton potential, and enters a phase dubbed ultraslow roll. In this article, we explore the possibility of realizing an ultraslow-roll phase in a particularly interesting inflationary scenario, called warm inflation. In the warm inflationary scenario a thermalized, subdominant radiation bath coexists with the inflaton energy density as an effect of dissipative dynamics. We show in this article that though the background dynamics indicate ultraslow roll when the potential becomes extremely flat, in warm inflation models, where the dissipation coefficient is a sole function of the temperature of the radiation bath, the system fails to maintain the thermal equilibrium as soon as it enters the ultraslow-roll phase. As thermal equilibrium is a key feature of warm inflation, and as it is not yet known how to deal with warm inflation without thermal equilibrium, we could not analyze such systems any further in this article. However, we demonstrate that brief periods of the ultraslow-roll phase, which smoothly ends in a standard slow roll, can be accommodated in warm inflation (WI) models where the dissipation coefficient is not only a function of the temperature of the radiation bath but also depends on the amplitude of the inflaton field. We theoretically determine the criteria of successfully embedding ultraslow roll in WI while the system remains in thermal equilibrium, and also demonstrate numerically that such short ultraslow-roll phases can indeed be embedded in specific warm inflation models which comply with the theoretically determined criteria.