J García-Bellido
The nature of the cosmological constant is a mystery. We don’t understand its quantum origin but we associate it with the actual acceleration of the universe because it is the simplest description we had until recently of the present cosmological observations. However, this may change with the next generation of experiments. If we can convince ourselves that the cosmic acceleration is not due to a constant, this would open up new fascinating avenues. By exploring the simplest cosmological model in the bulk, that of an empty and flat space with a cosmological constant Λ, we find that its holographic correspondence makes sense as a theory of fundamental quantum degrees of freedom at the boundary. Moreover, we find that an observer in the bulk, making long-range gravitational observations, cannot distinguish the acceleration induced by the cosmological constant Λ from that induced by the thermodynamic properties of the boundary, the de Sitter horizon, strictly at the level of the background cosmology. By including matter in the bulk we extend this holographic correspondence to GREA, where the quantum d.o.f. associated with the evolving boundary of the causal horizon induces an entropic acceleration that varies in time. Upcoming surveys such as DESI, Euclid, and the Vera Rubin Observatory (LSST) will test this framework through the growth of large-scale structures in the late universe, where GREA and ΛCDM differ quantitatively.