Speaker
Description
Amidst a myriad of sophisticated alternatives to general relativity, unimodular gravity stands unique as a relatively simple extension. In the Henneaux-Teitelboim (HT) formulation of unimodular gravity, the cosmological constant $\Lambda$ is promoted to a scalar field $\Lambda(x)$ at the level of the action. However, a non-dynamical vector density $\mathcal{T^{\mu}}$ ensures the constancy of $\Lambda$ on shell and consequently, the retention of the original Einstein field equations.
In 4 space-time dimensions, the vector density $\mathcal{T^{\mu}}$ can be interpreted as a topological 3-form gauge field which exists in a non-standard $U(1)$ representation. In the regular electrodynamics for a $U(1)$ gauge field $A_{\mu}$, the addition of a mass term or Proca term $m^{2}A_{\mu}A^{\mu}$ increases the internal d.o.f of $A_{\mu}$. Analogously, when $m^{2}\mathcal{T_{\mu}}\mathcal{T^{\mu}}$ is added to the HT action unimodular symmetry is broken. Curiously, $\mathcal{T^{\mu}}$ is still non-dynamical, rather, the scalar field $\Lambda(x)$ now obeys a wave equation. The Lambdon is born.
Based on:
https://arxiv.org/pdf/2305.09380
https://arxiv.org/pdf/2311.11160
