The Carrollian Kaleidoscope

TL;DR

This review explores the emerging field of Carrollian symmetries, their construction, and diverse applications in holography, hydrodynamics, and condensed matter physics, highlighting recent theoretical advances and practical implications.

Contribution

It provides a comprehensive overview of Carrollian and conformal Carroll field theories, their role in flat spacetime holography, and connections to hydrodynamics and condensed matter phenomena.

Findings

Carrollian symmetries are fundamental in flat spacetime holography.

Hydrodynamics in the Carroll limit relates to ultrarelativistic flows.

Connections to fractons and phase separation in condensed matter are established.

Abstract

The Carroll group arises in the vanishing speed of light limit of the Poincar\'{e} group and was initially discarded as just a mathematical curiosity. However, recent developments have proved otherwise. Carroll and conformal Carroll symmetries are now ubiquitous, appearing in diverse physical phenomena starting from condensed matter physics to quantum gravity. This review aims to provide the reader a gateway into this fast-developing field. After an introduction and setting the stage with basics of the symmetry in question, we detail the construction of Carrollian and Carrollian Conformal field theories (CCFT). We then focus on applications. By far the most popular of these applications is in the context of the construction of holography in asymptotically flat spacetimes (AFS) in terms of a co-dimension one dual CCFT. We review the early work on AFS$_3$ /CCFT$_2$ before delving into an in-depth analysis for the construction of the dual to 4D AFS. Two other important sets of applications are in hydrodynamics and in condensed matter physics, which we discuss in detail. Carroll hydrodynamics is introduced as the $c\to 0$ limit of relativistic hydrodynamics first and then reconstructed from a symmetry based approach. Relations to ultrarelativistic flows and connections to the quark-gluon plasma are discussed with concrete examples of the Bjorken and Gubser flow models. In condensed matter applications, we cover connections to fractons, flat bands, and phase separation in Luttinger liquid models. To conclude, we give very brief outlines of other topics of interest including string theory and black hole horizons.