Inflation: A graceful entrance from loop quantum cosmology

The evolution of a scalar field is explored taking into account the presence of a background fluid in a positively curved universe in the framework of loop quantum cosmology. Though the mechanism that provides the initial conditions for inflation extensively studied in the literature is still available in this setup, it demands that the initial kinetic energy of the field be comparable to the energy density of the background fluid if the field is initially situated at the minimum of the potential. It is found, however, that for potentials with a minimum such as the chaotic inflation model, there is an additional mechanism that can provide the correct initial conditions for successful inflation even if initially the kinetic energy of the field is subdominant by many orders of magnitude. In this latter mechanism the field switches direction when the Universe is still in the expanding phase. The kinetic energy gained while the field rolls down the potential is subsequently enhanced when the Universe enters the collapsing phase pushing the field one step up the potential. This behavior is repeated on every cycle of contraction and expansion of the Universe until the field becomes dominant and inflation follows.