MC 6460

## Candidate

HyungJin Kim | Applied Math, University of Waterloo

## Title

Cuscuton Gravity, Pulsar Timing, and Super Excited Modified Dispersion Relation

## Abstract

In this presentation, three different topics are covered. First is on Cuscuton Gravity. This model was introduced as a new approach to modify general relativity on large scales. Cuscuton field is incompressible by design, which leads to no second order time derivatives in its equation of motion. Therefore, cuscuton does not have its own dynamics. Scrutinizing this field under the cosmological perturbation theory, we can see that this field does not suffer from any ghost instabilities or instabilities that originate from violation of null energy condition. Second topic is the pulsar timing noise and the upper bound to mass of particles in standard model physics. Basic idea is that the power spectrum of the pulsar periods is related to the curvature fluctuation sourced by vacuum noise. Since the curvature affects geodesic and thus pulsar timing noise, we can use the pulsar timing data to set upper bounds on the mass of different quantum fields. Third topic is the modified dispersion relations that lead to highly populated excited states. By writing down the most general action that is consistent for a scalar field, we can obtain dispersion relations with higher order corrections. This presentation will show the detailed analysis of techniques and methods used.