In various approaches to quantum gravity, spacetime geometry is understood to emerge from more fundamental discrete structures at the Planck scale. As sometimes posited, their presence could lead to dissipative effects in the smooth effective sector. In this paper, we develop the idea of nonconservation in gravity by introducing an effective cosmological model within unimodular gravity in which a varying cosmological constant arises as a consequence of dissipation. We show that this requires the incorporation of hidden degrees of freedom—termed quantum gravity defects—that act as an effective bath for the matter fields. To illustrate the viability of the framework, we study the case of an Ohmic bath inspired by the Caldeira-Leggett model for Brownian motion, leading to a diffusion equation for the matter energy density. The results show that, starting from a primordial Universe with no dark energy, dissipation can account for the generation of a small positive cosmological constant.

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• Dark energy
• Fluctuation-dissipation theorem
• Gravitation
• Quantum cosmology