Role of quantum coherence and dissipation in cosmology

Abstract

This thesis looks at different manifestations that the non-unitary dynamics proper of dissipation can have during the inflationary era and the late-time universe. For starters, we formalise the calculation of the primordial spectrum in warm inflation, which significantly differs in terms of phenomenology from the standard inflationary picture because of the extra degrees of freedom that originate due to the dissipation of energy from the inflaton field into a thermal radiation bath.

Then, turning our attention to inflation in the context of string theory, we point out how the fluctuation-dissipation dynamic in warm inflation makes it robust against most of the so-called swampland conjectures. Nevertheless, that is not the case for the trans-Planckian censorship conjecture (TCC), which severely limits the duration of inflation to avoid trans-Planckian (TP) modes becoming observable, threatening the EFT description of inflation. In general, only models of inflation with a small energy scale can satisfy the TCC, effectively destroying any hope of experimental confirmation of inflation. To deal with this, we proposed a multi-stage warm inflation scenario with radiation-dominated eras in between. Such a model proved successful in opening a wider range of available energies that could make a model satisfying the TCC produce sizeable tensor perturbations. However, we also argue in favour of refinements of TCC that could make most high-energy models consistent with the conjecture. To do this, we looked at several mechanisms of subhorizon decoherence, like preheating and warm inflation itself, ultimately proving that keeping TP modes hidden inside the horizon is not enough to prevent their classicalisation, negating in this way the original premise of TCC.

Next, in a different direction, we study inflationary perturbations as an open quantum system, with an environment composed of subhorizon fluctuations and a system of superhorizon modes. We argue that this is the most appropriate way to study the physics of inflationary perturbations, as opposed to standard Wilsonian EFTs. We use the technology of open quantum systems in two big setups: scalar and tensor perturbations. In both cases, we compute the corrections to the two-point correlation function due to gravitational nonlinearities present in the Einstein-Hilbert action. This allowed us to explore topics such as long-time IR behaviour, (non-)Markovian behaviour, and the relation between this method and a standard loop expansion.

Finally, we changed our gears and looked at the viability of establishing quantum communication channels mediated by photons across astronomical and cosmological distances. For this, we survey multiple factors that could potentially disrupt the quantum state of the photons, like charged particles in space or the gravitational field of astrophysical bodies. We concluded that the x-ray portion of the electromagnetic spectrum would be ideal for establishing a quantum communication channel.