2022 Theses Doctoral

A symmetry’s tale: from the material to the celestial

Sun, Guanhao

Symmetry has played a crucial role in our understanding of physical systems. In this thesis, we review several works based on investigating the symmetry properties of theories. We examine and improve the Noether's theorem and the coset construction, both powerful tools when studying the symmetry aspects of a physical system. We manipulate the intrinsic ambiguities in the derivation of the stress-energy tensor using Noether's theorem to systematically compute, without any guesswork, the necessary ``improvement terms'' which make the tensor satisfy certain algebraic properties such as symmetry and tracelessness, even off-shell. We then construct a new type of coset construction, which can accommodate relativistic particles with arbitrary spins.

This is the first work we know of to incorporate arbitrary spin degrees of freedom into coset construction. We then present two interesting examples of condensed matter systems described by effective field theories that come from spontaneous symmetry breaking. For the so-called framid, we present the peculiar behavior of its stress-energy tensor that it is Lorentz-invariant even though the system breaks Lorentz boosts spontaneously. An analogy is drawn to the cosmological constant problem since the vacuum energy there and the Lorentz-breaking terms here are all surprisingly zero. Lastly, we describe how the inflation of the universe can be driven by a solid. We focus on the icosahedral inflation model, where the isotropies of background evolution and scalar power spectrum are guaranteed although the system is anisotropic. We discuss some observational signatures of this model.