Quantum and thermodynamic windows into the universe

C

osmology is the branch of physics that deals with the study of the universe as a whole. At first glance our understanding of the universe seems to be solely anchored in classical gravity. Indeed, general relativity is a powerful tool that provides a successful geometric description of the cosmos. However, if one scratches beneath the surface, the universe becomes a fascinating playground for thermal and quantum phenomena.

On the one hand, the quantum origin of primordial fluctuations and, ultimately, the structure of the universe, is a spectacular and unavoidable prediction of the inflationary paradigm. The universe may look classical, but it is certainly quantum at a fundamental level. On the other hand, in an expanding universe many out-of-equilibrium thermodynamic processes take place, which allows us to introduce an arrow of time: the very concepts of past and future. The aim of the research collected in this thesis is to provide results and insight regarding phenomena that transcend the bare geometric cosmic description and are true quantum and thermodynamic windows into the universe.

On the quantum window we explore topics that merge quantum information techniques in real space and physics of the early universe. One can view the amplification of quantum fluctuations during inflation as a process of particle creation. We argue that due to this process distant regions share long-range correlations, as opposed to the standard short- range entanglement present in the Minkowski vacuum. We elaborate on this by showing the enhancement of the perturbative mutual information between two arbitrary regions of an inflating or radiation-dominated universe. Unlike the fast power decay found in the Minkowski vacuum, in a cosmological setup the decay is logarithmic and long-range share of information is possible. Furthermore, we study Bell inequalities in real space, showing that they are not violated by the Bunch-Davies vacuum of de Sitter spacetime, despite several hints on the existence of genuine quantum correlations in the quantum state of the Mukhanov-Sasaki field.

On the thermodynamic window, we develop a framework that allows to formulate non- equilibrium thermodynamics within general relativity in a consistent way. The Einstein field equations or, equivalently, the Hamilton equations in the (3+1)-formalism are mod- ified in compliance with the laws of thermodynamics. One immediate and fundamental consequence of this is the breaking of symmetry under time inversion and the emergence of the arrow of time. Furthermore, the Raychaudhuri equation is also modified and, thus, the way in which gravitational collapse takes place. When applied to cosmology, the Friedmann equations include an entropic force, which is always of accelerating nature when the universe is expanding. Even though most of the expansion history of the universe is isentropic, such a force may become relevant in out-of-equilibrium cosmic phenomena: for instance (p)reheating, phase transitions and gravitational collapse.

Moreover, we propose an explanation to the current accelerated expansion of the uni- verse as a sustained entropic force coming from the growth of the causal horizon in an open inflation scenario. We name this the general relativistic entropic acceleration (GREA) theory. Cosmological data in absence of priors on H0 strongly favours GREA in comparison with ΛCDM. Future cosmological surveys will further constrain cosmological parameters and may clearly support one model over the other.

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Llorenç Espinosa PhD

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Quantum and thermodynamic windows into the universe