Informal Pizza Seminar

Cuscuton Cosmology: Dark Energy meets Modified Gravity

Ghazal Geshnizjani

University of Wisconsin-Madison

The origin of the current accelerating expansion of the Universe is of central importance to understanding of modern cosmology. In this talk I introduce a model of scalar field dark energy, Cuscuton, which can be realized as the incompressible limit of a scalar field theory with a non-canonical kinetic term. Even though perturbations of Cuscuton seem to propagate superluminally, I show why the theory is causal and even coupling to ordinary scalar fields cannot lead to acausal signal propagation. Next I show that the Cuscuton action can model the continuum limit of the evolution of a field with discrete degrees of freedom. In second part of my talk I focus on Cuscuton cosmology and show its perturbations do not introduce any additional dynamical degree of freedom, amounting to a minimal theory of evolving dark energy. I will then briefly discuss two simple examples of Cuscuton models with quadratic and exponential potentials which lead to expansion histories as LCDM and DGP self-accelerating brane-world model and explore their signature on cosmic microwave background (CMB).