Marinucci, Marco (2018) Inflationary tensor fossils and their implications. [Magistrali biennali]
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Inflation is a period during which the Universe expansion accelerated in the early universe. Originally introduced to solve the fine tuning problems of the cosmological Hot Big Bang model, it has been a great success in explaining the origin of the small temperature anisotropies of the Cosmic Microwave Background. The most accepted models are the standard single-field models of slow-roll inflation. The quantum field theory description of such models consists in the presence during inflation of one scalar field which slowly rolls down an almost flat potential. At the beginning of inflation both the scalar field and the metric tensor have small oscillations around their backgrounds. During inflation these primordial perturbations are stretched by the accelerated expansion on very large scales, where they get frozen. They form the seeds for the formation of primordial scalar (curvature) perturbations, associated to primordial density perturbation, which explain the temperature anisotropies observed in the CMB. Another fundamental prediction of Inflation is the production of a background of tensor perturbations, the primordial gravitational waves. The statistics of the primordial perturbations predicted by the standard slow-roll models of Inflation is almost Gaussian. The development of a non-linear extension of the slow-roll theories makes clear that there is no possibility to observe the non-Gaussianities predicted given the sensitivity of the actual measurements. The best constraints at present are those from the Planck measurements of the temperature CMB anisotropies. We have studied an indirect way to detect the primordial Gravitational Waves given by a new physical quantity: tensor fossils. These are primordial degrees of freedom that no longer interacts during late-time cosmic evolution. The only observational effect might be its imprint in the primordial curvature perturbation. Indeed the effect of these tensor fossils would entail a quadrupole perturbation in the mass distribution in the Universe. In order to measure their contribution it is possible to define a parametrization strictly connected both with the bispectrum, i.e., the scalar-scalar-tensor (fossil) three-point correlation function, and with the power spectrum of the new tensor (fossil) background. A new model, Gaugid Inflation, has been studied. In this model the field responsible for Inflation is a gauge vector field with vev that manifestly breaks invariance under spatial rotations and translations. Imposing additional symmetries on the field, it allows to restore, the wanted background isometries, and to study the inflationary phase driven by this field. Perturbing the field we found that, besides the metric tensor perturbations, there are additional tensor degrees of freedom, which could play the role of tensor fossils. The original contribution of this work was finding that these new tensor degrees coupled to the metric ones in a non-trivial way. In our generalization we tried to add a parity-breaking term in the Lagrangian. We expect this term to modify the power spectrum of gravitational waves, polarizing the primordial GW into left and right polarization states in the sense that the statistics of such polarization states becomes different.
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