Constraining models of inflation with a non-trivial field-space by using numerical calculations of their non-Gaussianities

Butchers, Sean (2020) Constraining models of inflation with a non-trivial field-space by using numerical calculations of their non-Gaussianities. Doctoral thesis (PhD), University of Sussex.

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An era of cosmological inflation is the preferred mechanism for producing primordial fluctuations that are later amplified to become the observed large-scale structure. Unfortunately, constraining inflation is problematic because there are numerous models that reproduce the Gaussian power-spectrum found in the CMB. This problem is further elevated when multifield models are considered because the kinetic part of their Lagrangian often contains a non-trivial field metric describing a curved geometry in the field-space. Additionally, these models can produce measurable primordial non-Gaussianities which are now constrained in the Planck data. The non-Gaussianities of an inflation model are measured using the bispectrum B and its amplitude fNL for a specific triangular template and the Gaussian statistics are described using the tensor-to-scalar ratio r and the scalar spectral index ns.

In the first part of this thesis, we extend the transport method, first introduced by Mulryne, Seery and Wesley, to be able to calculate the power spectrum and the bispectrum and their associated observables from an inflation model with a non-canonical field metric. We implement these in a publicly available code called CppTransport to automate the calculation of the statistical properties of the primordial fluctuations. The results of this code are tested using a model that can be described in both canonical and non-canonical field coordinates with excellent agreement. We also demonstrate the code’s accuracy by comparing our bispectrum results with a separate numerical implementation of the transport method called PyTransport 2.0, again finding good agreement. Lastly, we consider a class of gelaton models that were predicted to produce boosted equilateral configurations of the bispectrum and showed this is difficult to accomplish for models with simple potentials and a hyperbolic field-space.

In the second part of the thesis, we introduce a new code CpptSample, which is a CosmoSIS module that adds sampling functionality and Bayesian model selection to CppTransport. In this, we build an interface which allows the Monte-Carlo-Markov-Chain (MCMC) samplers in CosmoSIS to provide cosmological and Lagrangian initial conditions to CppTransport. The results for the primordial spectra are then passed on to the Boltzmann code CLASS to calculate the theoretical CMB power spectrum based on the underlying model and Bayesian evidence is found from the Planck2015 likelihood code. Our implementation of this retains all the extensions needed for models with a non-trivial field-space and does not rely on the slow-roll approximation. We demonstrate this by calculating marginalised statistics for the quadratic, quartic, Gelaton/QSFI and α-attractor models of inflation.

Item Type: Thesis (Doctoral)
Schools and Departments: School of Mathematical and Physical Sciences > Physics and Astronomy
Subjects: Q Science > QB Astronomy > QB0460 Astrophysics
Q Science > QB Astronomy > QB0980 Cosmogony. Cosmology > QB0991.L37 Large scale structure
Depositing User: Library Cataloguing
Date Deposited: 04 Nov 2020 14:14
Last Modified: 04 Nov 2020 14:14

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