Nothing is certain in string compactifications

A bubble of nothing is a spacetime instability where a compact dimension collapses. After nucleation, it expands at the speed of light, leaving “nothing” behind. We argue that the topological and dynamical mechanisms which could protect a compactification against decay to nothing seem to be absent in string compactifications once supersymmetry is broken. The topological obstruction lies in a bordism group and, surprisingly, it can disappear even for a SUSY-compatible spin structure. As a proof of principle, we construct an explicit bubble of nothing for a T3 with completely periodic (SUSY-compatible) spin structure in an Einstein dilaton Gauss-Bonnet theory, which arises in the low-energy limit of certain heterotic and type II flux compactifications. Without the topological protection, supersymmetric compactifications are purely stabilized by a Coleman-deLuccia mechanism, which relies on a certain local energy condition. This is violated in our example by the nonsupersymmetric GB term. In the presence of fluxes this energy condition gets modified and its violation might be related to the Weak Gravity Conjecture.We expect that our techniques can be used to construct a plethora of new bubbles of nothing in any setup where the low-energy bordism group vanishes, including type II compactifications on CY3, AdS flux compactifications on 5-manifolds, and M-theory on 7-manifolds. This lends further evidence to the conjecture that any non-supersymmetric vacuum of quantum gravity is ultimately unstable.

Topological superconductors on superstring worldsheets

We point out that different choices of Gliozzi-Scherk-Olive (GSO) projections in superstring theory can be conveniently understood by the inclusion of fermionic invertible phases, or equivalently topological superconductors, on the worldsheet. This allows us to find that the unoriented Type 00 string theory with \Omega^2=(-1)^{f}Ω2=(−1)f admits different GSO projections parameterized by nn mod 8, depending on the number of Kitaev chains on the worldsheet. The presence of nn boundary Majorana fermions then leads to the classification of D-branes by KO^n(X)\oplus KO^{-n}(X)KOn(X)⊕KO−n(X) in these theories, which we also confirm by the study of the D-brane boundary states. Finally, we show that there is no essentially new GSO projection for the Type II worldsheet theory by studying the relevant bordism group, which classifies corresponding invertible phases. In two appendixes the relevant bordism group is computed in two ways.

О свойствах группы кобордизма стабильно-оснащённых погружений в коразмерности k

Изучается понятие „intermediate bordism group" , которое было введено П. Дж. Экклзом для исследовании фильтраций в стабильных гомотопических группах сфер. Введено новое понятие группы кобордизма стабильно-оснащенных погружений. Строится представляю щее пространство для новых групп и вычисляются ранги этих групп кобордизма. Инварианты Хопфа и гомоморфизм Кана-Придди обобщаются на группы кобордизма стабильно-оснащенных погружений.

The bordism group of unbounded KK-cycles

We consider Hilsum’s notion of bordism as an equivalence relation on unbounded [Formula: see text]-cycles and study the equivalence classes. Upon fixing two [Formula: see text]-algebras, and a ∗-subalgebra dense in the first [Formula: see text]-algebra, a [Formula: see text]-graded abelian group is obtained; it maps to the Kasparov [Formula: see text]-group of the two [Formula: see text]-algebras via the bounded transform. We study properties of this map both in general and in specific examples. In particular, it is an isomorphism if the first [Formula: see text]-algebra is the complex numbers (i.e. for [Formula: see text]-theory) and is a split surjection if the first [Formula: see text]-algebra is the continuous functions on a compact manifold with boundary when one uses the Lipschitz functions as the dense ∗-subalgebra.

Oriented bordism groups of immersions

Let IΩn, k denote the bordism group of immersions of closed oriented n-manifolds into (n + k)-space. The object of this paper is to study certain group extension problems arising from Pastor's calculations of IΩn, k.The bordism group of immersions was first studied by Wells [12] who calculated the unoriented bordism group I Rn, k for k = n and k = n − 1 ≡ 3(4). Later these unoriented bordism groups were completely determined by Koschorke and Olk for k ≥ n − 2 with the help of an exact sequence measuring the difference between IRn, k and Rn (see [4]). A similar program has been carried out by Pastor [7] to determine the oriented bordism group I Ωn, k for k ≥ n − 2 except for certain group extension problems and some low dimensional cases.

TYPE OF 3-MANIFOLDS AND ADDITION OF RELATIVISTIC KINKS

The type of a closed, connected, orientable 3-manifold M was first considered in the classification of relativistic kinks over the space-time 4-manifold M × R. In this paper theorems are developed relating the type of M to H1 (M; Z), which lead to the determination of the type of large families of 3-manifolds. The relation of type to connected sum is established, and the connected sum is also used to define addition of kinks. The kink addition is related to the sum of the kink numbers, as well as addition in the bordism group Ω3(P3).

Relative semicharacteristic classes

In 1973 Ronnie Lee introduced the notion of semicharacteristic classes, which are invariants of the bordism group ℜ*(Bπ) of closed manifolds equipped with a free action of a finite group π. In this paper we relativize his theory. Associated to a homomorphism G → π of finite groups, there is the relative bordism group ℜ*(BG → Bπ), which is the bordism group of compact manifolds M with a free π-action, so that the action on ∂M is induced from a free G-action, i.e. ∂M = π xGN for some manifold N with a free G-action. The invariants defined here are invariants of this relative group.

Multiple points of codimension one immersions of oriented manifolds

Work by L.S.Pontrjagin(18) and M.W.Hirsch(7) allows us to identify the stablen-stemwith the bordism group of oriented compact closed smoothn-manifolds immersed in ℝn+1. In a recent paper (11), U. Koschorke discusses invariants thereby defined onby analysing the self-intersections of immersed manifolds. In particular he discusses the homomorphismdefined by assigning to a generic immersionMn→ ℝn+1number (modulo 2) of its (n+ 1)-fold points.