Cosmology can be surprising when you add more detailed physics
Cosmology has gone through a revolution during the last decade owing to the large amount of new precise observational data. These data strongly indicate the existence of two periods of accelerated expansion in the history of the Universe. One is in the primordial universe, the so-called inflationary phase, and one at the present era. Data and models are linked mostly using linear perturbation theory. However, beyond simple linear order many interesting effects such as non-Gaussianity appear. Take for instance, the CMB (Cosmic Microwave Background) and backreaction (BR) effects. Quantum BR of primordial fluctuations can significantly modify the dynamics of the early universe and its observational signatures, while classical BR of large scale structure (LSS) might contribute to the observed late time acceleration. The goal of the NEBRIC (Non-linear effects and backreaction in classical and quantum cosmology) project was to quantify these non-linear effects more precisely. There has been a lot of recent interest in the generation of cosmological seeds for the magnetic fields observed in galaxies and clusters during inflation. The project has demonstrated how the adiabatic renormalisation of the two-point function of the magnetic field is not viable for the generation of an inflationary magnetic field. In a chaotic inflationary model, the backreaction effects due to long wavelength scalar and tensor fluctuations on the effective Hubble factor and equation of state (with respect to different classes of observers) were investigated. It was shown how the observed dynamic for a test field observer can be different from the background equivalent. NEBRIC research has led to the formulation of new tools. This has applications in the field of relativistic cosmology, particularly in the study of the non-linear and relativistic physics to extract information from LSS surveys and CMB experiments. The project has shown how the impact of stochastic inhomogeneities on LSS cosmological observables cannot be responsible for the present acceleration of the Universe. The bispectrum of the galaxy number counts has been calculated for comparison with observations from LSS surveys. The study has shown how to take into consideration the impact of inhomogeneities on local measurements. This can explain the different values of the Hubble parameter obtained via different measurement techniques.
Keywords
Cosmology, linear perturbation theory, cosmic microwave background, backreaction, Hubble parameter
