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Cosmological Imprints of the Ultra-Large Scale Universe

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Inflation provides a dynamical mechanism to seed density fluctuations that eventually collapse to form all of the structure in the observable universe. However, microphysical theories of inflation often predict that on scales much larger than our present horizon, the universe may be extremely inhomogeneous. In general, this inhomogeneity may manifest in both adiabatic and isocurvature modes.

One possible source of such ultra-large scale inhomogeneity is from the initial conditions preceding the inflationary phase. Using numerical relativity, I will discuss the ultra-large scale structure arising from initial fluctuations in the inflaton, in particular the contribution to the CMB quadrupole (known as the Grischuk-Zel’dovich effect). For large fluctuations, the resulting distribution is strongly distorted from the Gaussian form usually assumed in the literature. Surprisingly, we find that this leads to weaker constraints on large amplitude initial fluctuations than small amplitude ones.

I will also briefly discuss an additional source of inhomogeneity: the nonlinear conversion of isocurvature perturbations into adiabatic modes during the post-inflation phase of preheating. This produces a generalised form of local nonGaussian density perturbations, which exhibits a high degree of spatial intermittency. Remarkably, I will show how the main features can be understood from the dynamics of decoupled field trajectories undergoing chaotic ballistic motion in a potential.

This talk is part of the Cosmology Lunch series.

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