Inflation is a period during which the Universe expansion accelerated in the very 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 (CMB). Actually the most accepted models of inflation are the socalled standard singlefield models of slowroll inflation. The quantum field theory (QFT) description of such models consists in the presence during inflation of one scalar field, the inflaton, which slowly rolls down an almost flat potential and interacts with Einstein gravity. At the beginning of inflation both the inflaton and the metric tensor have linear oscillations around their background. During inflation these primordial perturbations are stretched by the accelerated expansion on very large (superhorizon) scales, where they get "frozen". They form the seeds for the formation of primordial perturbations in the scalar curvature of comoving hypersurfaces, which can expain the temperature anisotropies of the CMB, and perturbations of the metric tensor corresponding to primordial gravitational waves. The statistics of the primordial perturbations predicted by the standard slowroll models of inflation is almost Gaussian. If we try to develop a nonlinear extension of the slowroll theories we find that there is no possibility to observe the nonGaussianities predicted given the sensitivity of the actual measurements. In the last years the WMAP and Planck satellite has constrained with increasing precision the level of primordial nonGaussianity. The best constraints at present are those from the Planck measurements of the temperature (and polarization) CMB anisotropies. Such constraints are compatible with a zero level of primordial nonGaussianity as predicted by the slowroll models, but there is still a window of almost two orders of magnitude unexplored. For this reason it is interesting to think about modifications of slowroll models of inflation in order to achieve signals of nonGaussianity. Modified gravity theories are an example of such a modification. Beingopen minded about a modification of Einstein gravity during inflation is a well motivated question. In fact the high energies of the early universe are not accessible today in the colliders and we do not know if in these conditions gravity follows exactly the Einstein description. In this Thesis we have focused on the analysis of ChernSimons gravity during inflation, which is parity breaking and polarizes the primordial gravitational waves into circular polarizations. In this case a difference between the largescale power spectrum of the two different circular polarizations of the primordial gravitational waves arises. For the approximations made to develop the theory we argue that this difference is small but maybe observable with future experiments. As an original contribution we have focused on primordial nonGaussian signatures. We have computed a non trivial parity breaking pattern into the nonGaussianity of the primordial perturbations (specifically for the 3point function correlating primordial graviational waves and the scalar curvature perturbation). In the slowroll limit this signature can be large even if there is a small parity breaking in the power spectra. For this reason making a more detailed investigation about nonGaussianities provided by the ChernSimons gravity term during inflation can be an interesting iusse for the future.
Investigating modified gravity signatures during inflation
Orlando, Giorgio
2016/2017
Abstract
Inflation is a period during which the Universe expansion accelerated in the very 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 (CMB). Actually the most accepted models of inflation are the socalled standard singlefield models of slowroll inflation. The quantum field theory (QFT) description of such models consists in the presence during inflation of one scalar field, the inflaton, which slowly rolls down an almost flat potential and interacts with Einstein gravity. At the beginning of inflation both the inflaton and the metric tensor have linear oscillations around their background. During inflation these primordial perturbations are stretched by the accelerated expansion on very large (superhorizon) scales, where they get "frozen". They form the seeds for the formation of primordial perturbations in the scalar curvature of comoving hypersurfaces, which can expain the temperature anisotropies of the CMB, and perturbations of the metric tensor corresponding to primordial gravitational waves. The statistics of the primordial perturbations predicted by the standard slowroll models of inflation is almost Gaussian. If we try to develop a nonlinear extension of the slowroll theories we find that there is no possibility to observe the nonGaussianities predicted given the sensitivity of the actual measurements. In the last years the WMAP and Planck satellite has constrained with increasing precision the level of primordial nonGaussianity. The best constraints at present are those from the Planck measurements of the temperature (and polarization) CMB anisotropies. Such constraints are compatible with a zero level of primordial nonGaussianity as predicted by the slowroll models, but there is still a window of almost two orders of magnitude unexplored. For this reason it is interesting to think about modifications of slowroll models of inflation in order to achieve signals of nonGaussianity. Modified gravity theories are an example of such a modification. Beingopen minded about a modification of Einstein gravity during inflation is a well motivated question. In fact the high energies of the early universe are not accessible today in the colliders and we do not know if in these conditions gravity follows exactly the Einstein description. In this Thesis we have focused on the analysis of ChernSimons gravity during inflation, which is parity breaking and polarizes the primordial gravitational waves into circular polarizations. In this case a difference between the largescale power spectrum of the two different circular polarizations of the primordial gravitational waves arises. For the approximations made to develop the theory we argue that this difference is small but maybe observable with future experiments. As an original contribution we have focused on primordial nonGaussian signatures. We have computed a non trivial parity breaking pattern into the nonGaussianity of the primordial perturbations (specifically for the 3point function correlating primordial graviational waves and the scalar curvature perturbation). In the slowroll limit this signature can be large even if there is a small parity breaking in the power spectra. For this reason making a more detailed investigation about nonGaussianities provided by the ChernSimons gravity term during inflation can be an interesting iusse for the future.File  Dimensione  Formato  

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https://hdl.handle.net/20.500.12608/25604