Theoretical Schema for Detecting Quantum Nature of Primordial Gravitational Waves by Probing the Quantum Features of Electromagnetic Waves

Soutenance de thèse de Fatima Shojaei-Arani (Université de Isfahan, Iran)

Résumé de la thèse :

One of the fundamental outcomes of inflationary cosmology is that tensorial pertur- bations of spacetime, similar to the large-scale structure of our Universe, originate from quantum fluctuations during the inflationary epoch. Creation and amplifica- tion of inflationary-generated primordial gravitational waves lie in the middle of the crossroad where quantum mechanics meet cosmology. Thus, detecting their quan- tumness, especially their squeezed nature, not only paves the way to search for the quantum essence of gravitational waves but also may provide a hitherto inaccessible source of information about the early Universe. In this dissertation, we present a comprehensive investigation of the interaction between quantum gravitational waves and electromagnetic fields that are supposed to probe quantum spacetime. Using the optical medium analogy framework, we not only recover the usual results of a typical interferometer detector in terms of the phase shift, but we may also pro- mote the investigation to a fully quantum mechanical one, where the application of the well-known quantum optical language in analog systems, helps understand the electromagnetic-gravitational wave interaction in a close-up. In particular, we assess the quadrature variance of the electromagnetic field and demonstrate the impossi- bility of witnessing the quantum nature of gravitational waves based on the revivals of optical squeezing. On the contrary, we show that primordial gravitational waves induce decoherence of the electromagnetic field and ruin temporal correlations after a characteristic time scale, which depends on the cosmological parameters, namely the inflationary index and the tensor-to-scalar ratio. Moreover, the apparition of sidebands in the optical spectrum may be viewed as a signature of the squeezed nature of relic gravitational waves. On top of that, we promote the idea of using spatial correlations of light as a new tool to constrain the gravitational wave back- ground. In particular, we show how the angular size-redshift data of a set of quasars, measured by the long baseline interferometers, may provide an exquisite constraint on the tensor-to-scalar ratio.

Composition du jury de thèse :

• Vincent VENNIN (Rapporteur)                                       LPENS (Paris)
• Behrouz MIRZA (Rapporteur)                                        Université technologique de Isfahan (Iran)
• Rasoul ROKNIZADEH (Examinateur)                            Université de Isfahan (Iran)
• Ruth Durrer (Examinatrice)                                            Université de Genève
• Jean-Philippe UZAN (Examinateur)                               IAP (Paris)
• Alain Blanchard (Directeur de thèse)                             IRAP (Toulouse)
• Malek BAGHERI HAROUNI (Co-directeur de thèse)     Université de Isfahan (Iran)
• Brahim Lamine (Co-directeur de thèse)                         IRAP (Toulouse)
• Ali MAHDIFAR (invité)                                                    Université de Isfahan (Iran)