PhD Comprehensive Seminar | Hyungjin Kim, Mysteries in Cosmology: Dark Energy and the Beginning of the UniverseExport this event to calendar

Friday, August 31, 2018 — 10:00 AM EDT

MC 6460

Speaker

Hyungjin Kim | Applied Math, University of Waterloo

Title

Mysteries in Cosmology: Dark Energy and the Beginning of the Universe

 Abstract

In this presentation, I will talk about two of many mysteries in cosmology: the beginning of the unvierse, and the dark energy. Currently, the very popular paradigm of the early universe, Inflation, which postulates a phase of an early exponentially expansion of the universe, answers some theoretical puzzles we had such as the horizon problem and successfully fits the observational data. Nevertheless, it does not answer the question of initial conditions or the singularity problem. There has been many attempts to resolve these issues, and one of the popular approaches is the bouncing cosmology. Here, we introduce cuscuton field and show that an action with this field can violate the Null Energy Condition without provoking instabilities such as the ghost or gradient instabilities. Furthermore, we show that we are able to produce the bouncing universe with this field. Theoretically, we can avoid the singularity problem or the transplanckian problem. However, we still have to show that Cuscuton can reproduce the current observational data, such as the scale-invariant power spectrum or small non-gaussianity. Now, for the dark energy, we introduce a model-independent approach to setting constraints to the possible values of equation of state, w. Looking at the supernova data, and the growth history of the universe, we can set constraints to w without assuming any dark energy model. We show that the constraint may not be too strong with the current data. However, knowing that more precise data will come with better telescopes such as DESI, we can forecast how strong the constraints may be in future.

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