Exotic Branes in String Theory (1209.6056v3)

Abstract: Besides the familiar D-branes, string theory contains a vast number of other non-perturbative objects. While a complete classification is lacking, many of these objects are related to each other through various dualities. Codimension two objects play a special role, because their charges are no longer additive but are instead expressed in terms of holonomies of scalar fields, which is given by an element of the relevant duality group. In this paper we present a detailed exposition of these "exotic" objects, the charges they carry, and their connection to non-geometric compactifications. Despite the name "exotic branes," these objects are in fact ubiquitous in string theory, as they can automatically appear when describing bound states of conventional branes, and as such may be of particular importance in describing the microscopic degrees of freedom of black holes.

Exotic Branes in String Theory

The paper entitled "Exotic Branes in String Theory" by Jan de Boer and Masaki Shigemori offers an exhaustive investigation into the lesser-known concept of "exotic branes" within string theory. These objects extend beyond the canonical D-branes and introduce novel structural characteristics, notably through their interactions with the duality groups intrinsic to string theory and M-theory.

The paper initially delineates the broad presence of extended non-perturbative objects in string theory, highlighting that a full classification remains unresolved. Exotic branes are specifically intriguing due to their distinct codimensions, where charges manifest not additively but through holonomies in scalar fields linked to elements of duality groups like $U$-duality. These structures relate to non-geometric compactifications within the landscape of string theory.

The authors methodically explore the attributes and interrelations of these exotic branes, underscoring their pertinence in contexts such as the non-geometric backgrounds and possible configurations known as $U$-folds. This richer architecture is thereby critical for representing bound states of conventional branes, clarifying the microscopic degrees of freedom of black holes.

Dualities and Symmetries

A significant focus of the paper is the role of dualities, such as $T$-duality and $U$-duality, that connect various brane configurations and elucidate higher-dimensional origins of these exotic states. When string/M-theory is compactified to lower dimensions on tori (denoted $T^k$), the paper describes an enhancement in the symmetry groups (e.g., reaching $E_{k(k)}$ symmetries), hinting at the intricate relationships among charged particles and the branes' geometric or non-geometric nature. Crucial case studies include exotic $5^2_2$-branes or generalizations.

Implications in String Theory and Black Holes

This research bears theoretical and practical implications. The presence of exotic branes is ubiquitous and influences the dynamics and stability of other well-studied systems, including configurations relevant to black hole microstates. Exotic branes can, for instance, emerge through a phenomenon akin to the supertube effect, where ordinary branes polarize into exotic ones. This may lead to more complex stringy source arrangements like the "superstrata," potentially elucidating the microstate structure in the fuzzball paradigm for black holes.

The analysis extends to how these branes contribute to the myriad aspects of string theory, from duality cascades to the polarizations' effect in non-trivial exotic configurations. Additionally, challenges in maintaining coherence within these frameworks point to broader implications for non-geometric compactifications and the intriguing mathematical landscapes these entail.

Future Directions

The paper hints at rich avenues for continued investigation: the mapping of brane monodromies, detailed examination of their microstate configurations and implications on cosmology, and exploration within non-perturbative frameworks such as double field theory. The dialog between geometric and non-geometric formulations invites questions about how these branes interact with large-scale structures, global symmetries, and even holographic or duality-based formulations in string theory.

In essence, this paper provides a firm foundation for understanding the exotic branes' place within theoretical physics and highlights their importance in addressing long-standing challenges in the fundamental underpinnings of the universe. Further explorations along the suggested pathways will likely lead to more profound insights into the inner workings of the string landscape, including potential resolutions to enigma such as the black hole information paradox within the field of quantum gravity.