Observing Brane Inflation In The Sky

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Brane inflation is a natural cosmic inflation scenario from string theory. Explicitly constructed brane inflation models enable us to probe string theory using cosmological data. This is an exciting area where observations make contact with fundamental physics at or around the string scale. In this work, I review the basic setup of warped brane inflation models, discuss how novel kinetic term, the Dirac-Born-Infeld (DBI) action, naturally arises and what inflaton potential string theory gives us. Depending on the magnitude of the inflaton mass relative to the Hubble scales, there are three different scenarios: the slow-roll scenario, the Ultraviolet DBI scenario, and the Infrared DBI scenario. I will review each scenario in detail, and discuss their predictions on the cosmic microwave background (CMB) sky. Some predictions, such as primordial non-Gaussianities, are so distinctive, that if detected in the foreseeable future, will reveal the nature of cosmic inflation in great detail. In fact, as we will show, current observational bounds on non-Gaussianity is already powerful enough to rule out Ultraviolet DBI inflation in the relativistic regime. Some predictions have very stringy natures. Such as the stringy phase transition during the early stage of Infrared DBI inflation that leads to significant running of the spectral index, and the sharp features in the warp factor as a gravity dual of Sieberg duality cascade. All these observable features opens the door for cosmological observations to test the nature of the theory. In this work, we will present cosmological data analysis which constrain specific brane inflation models in great detail. Aside from the observational prospects, we will also discuss the conceptual issue of eternal inflation in brane inflation scenarios. It is generically believed that eternal inflation is unavoidable in field theory inflation models; however, string theory offers new insights. As we will show, a bound on the inflaton field range from the size of the compactified manifold forbids stochastic eternal inflation in the slow-roll scenario. Eternal inflation in the DBI scenarios are also not generic, either due to the relativistic motion of the brane or due to the stringy phase transition at the tip of the throat. The last chapter of the work discuss our recent efforts in building inflation models on the string landscape. It is now widely accepted that string theory allows an enormous number of vacua. In the early universe, if the inflaton field is mobile in the landscape, the resulting cosmic inflation picture is quite different from usual points of view. Here we expect a multi-dimensional inflaton potential with random features. Such randomness in the potential causes the inflaton to undergo Brownian-like motion; the detours always increase the number of e-folds, ameliorating the fine tuning of a flat potential in slowroll model building. Meanwhile, detours lead to fluctuations in the primordial power spectrum, which could show up in the CMB power spectrum. If we are fortunate, the PLANCK satellite will be able to reveal some of these features for us. We will conclude this work with summaries and remarks on future research.

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