On the Fate of the Inflaton after Inflation

The transition from cosmic inflation to the hot Big Bang phase, known as reheating, remains a key open question in inflationary cosmology. During its early 'preheating' stage, the inflaton field is expected to decay explosively via parametric resonance into lighter bosonic offspring fields. However, when such external couplings are very weak, strong enough self-interaction causes the oscillating inflaton condensate to fragment, forming extremely long-lived scalar-field lumps, known as Oscillons. In this talk, we discuss the conditions for oscillon formation during preheating, particularly in the presence of an external coupling, within the class of inflationary potentials favored by the latest CMB observations. Using high-resolution (3+1)-dimensional lattice simulations performed on the CosmoLattice platform, we systematically map the parameter space that supports oscillon formation. Our results suggest that preheating may have proceeded through both oscillon decay and the conventional decay of the inflaton condensate, offering new insights into this early preheating dynamics. The talk will conclude by highlighting some of the open problems in the field, and discussing possible future directions.