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The quantum nature of the big bang is reexamined in the framework of loop quantum cosmology. The strict application of a regularization procedure to the Hamiltonian, originally developed for the Hamiltonian in loop quantum gravity, leads to a qualitative modification of the bounce paradigm. Quantum gravity effects still lead to a quantum bounce connecting deterministically large classical universes. However, the evolution features a large epoch of a de Sitter universe, with an emergent cosmological constant of Planckian order, smoothly transiting into a spatially flat expanding universe.