(also nonabelian homological algebra)
A stability condition (Bridgeland 02) on a triangulated category (hence on a stable model category/stable (∞,1)-category) $\mathcal{D}$ (Def. below) singles out some of its objects, to be called the “stable objects” (Def. below), that behave, to some extent, like indecomposable objects (whence “stable”), in that a version of Schur's lemma applies to them (Prop. below).
Indeed, in the degenerate case that $\mathcal{D}$ is the derived category of a semisimple category, the stable objects are just the irreducible objects.
In the case where $\mathcal{D}$ is the bounded derived category of coherent sheaves on some complex manifold, the general notion of stability reduces to the classical notion of slope-stability of coherent sheaves (“$\mu$-stability”, see Example below).
In other examples Bridgeland stability reproduces the derived analogue of David Mumford‘s concept of stability in geometric invariant theory (King 94).
There are in general different stability conditions on one and the same triangulated category, in fact their moduli space $Stab(\mathcal{D})$ forms a complex manifold (Prop. below). The collection of stable objects in $\mathcal{D}$ in general depends on the choice of stability conditions, hence of the choice of point in this moduli space: There are in general codimension-1 submanifolds of $Stab(\mathcal{D})$, called walls, such that the set of stable objects is constant close to both sides of these walls, but changes as the wall is crossed. This phenomenon is hence known as wall crossing.
The purely mathematical motivation for these definitions is, to a large extent, just their intrinsic richness. The concept finds its meaning in the concept of stability of D-branes in string theory and was abstracted from informal considerations about B-branes of the B-model topological string due to (Douglas-Fiol-Romerlsberger 00) and followups (“$\Pi$-stability”).
This string-theoretic interpretation also sheds light on the older notion of slope-stability of coherent sheaves, we discuss this below.
We explain here how to understand the mathematical formulation of stability conditions as a formalization of the concept of stable BPS states of D-branes, hence of extremal black branes. This yields a very simple and beautiful picture, which may be hard to extract from the original proposal due to Douglas-Fiol-Romerlsberger 00 (as witnessed by various reviews of the concept, see for instance Stellari 15, slide 9).
A key point to notice is that coherent sheaves and with them most of the kinds of objects in triangulated categories of relevance here, may be thought of as models for D-branes carrying D-brane charge.
Specifically, for $X$ a space, to be thought of as spacetime, a coherent sheaf $E$ over $X$ may be thought of as an abelian sheaf of sections of a kind of complex vector bundle over $X$, whose fiber dimension is allowed to jump in some controlled way. Hence coherent sheaves are a slight generalization of complex vector bundles.
When considering D-branes on $X$, such coherent sheaves/vector bundles $E$ appear as the Chan-Paton gauge fields on the D-brane.
From this perspective, the rank
of $E$ is the number of coincident D-branes (there may be fractional D-branes, and, once we pass to the derived category, there may be anti D-branes, so that this number need not be a positive integer).
The D-branes have a fixed tension and hence a fixed mass-density, so that the total mass of the D-branes corresponding to $E$ is proportional to this rank. Ignoring the constant proportionality factor, we make this explicit by re-writing the rank as
In addition to their mass, the D-branes carry charge, called RR-charge. This is a generalization of the classical magnetic charge known from Dirac charge quantization. As explained there, magnetic charge reflected in a complex line bundle, as sourced by magnetic monopoles (D0-branes) is measured by the first Chern class $c_1(E)$. In generalization of this, the total D-brane charge reflected in a Chan-Paton gauge field $E$ is proportional its Chern character $ch(E)$.
For coherent sheaves this is essentially what is classically called the degree of a coherent sheaf. Interpreting this as the charge carried by the D-brane, we may write
Accordingly, D-branes $E$ have a charge density proportional to
This quotient is what is classically called the slope, as shown on the right. The terminology comes from thinking of the pair (mass, charge), hence the pair (rank, degree) as specifying a point in the plane
Under this identification the charge density is the slope of a line of the line in the Cartesian plane which goes through zero and through $(M(E), Q(E))$, whence the term slope of a coherent sheaf. But understanding this not as a slope but as a charge density reveals why this has anything to do with “stability”, as we proceed to explain now.
First notice that, alternatively, we may identify the real plane with the complex plane and thus unify the mass and charge of D-branes into a single complex number
with an absolute value $m(E)$ and a complex phase (modulo $\pi$) $\phi(E)$.
(Such unification of two different quantities into a single complex quantity appears all over supersymmetric theory, for instance also in the definition of complex volume or the complex coupling constant appearing in the context of S-duality.)
Finally to see what all this has to do with “stability”:
In a supersymmetric theory such as superstring theory the stable states are supposed to be the BPS-states. When thinking of D-branes as black branes, being higher dimensional generalizations of charged black holes, the BPS states correspond to the higher dimensional analog of the extremal black holes, namely those that carry maximum charge for given mass, hence that maximize their charge density
Now it is plausible that a D-brane $E$ maximizes its potential charge density if removing any part $e$ of it reveals that $e$ by itself has lower charge density.
More formally, $E$ should maximize its charge density $Q(E)/M(E)$ and hence be stable if for all sub-parts
hence for all subobjects in the relevant category (such as that of coherent sheaves) we have that the charge density of the part is smaller than the charge density of the whole:
Conversely, this means that a D-brane state $e$ can increase its charge density, hence get close to being BPS and hence stable, by forming a bound state to be come an $E$.
In the case of coherent sheaves, where the D-brane charge density is called the slope of a coherent sheaf $slope(E) = Q(E)/M(E)$, as above, this says that $E$ is a stable coherent sheaf precisely if for all subobjects $e \subset E$ we have
This is the classical formulation of slope stability or $\mu$-stability of coherent sheaves.
Of course we may equivalently re-express in terms of the complex phase $\phi(E)$ induced by the complexified mass/charge from (1). Since the slope of a complex number (regarded as a vector in the plane) equals the tan of its complex phase we have
But since $tan((-)/2)$ is a monotone function on the domain $(-\pi/2, \pi/2)$, then if we agree to regard $\phi$ as taking values in that interval, then the above stability condition becomes equivalently the statement that for all subobjects $e \hookrightarrow E$ we have
This is the form in which Bridgeland 02 phrases the stability condition, see Def. below.
But the upshot is that this is still equivalent to saying that $E$ is stable precisely if it maximizes its charge- over mass-density, as befits an extremal black hole and, more generally, a BPS black brane.
$\,$
(stability function)
Let $\mathcal{A}$ be an additive category.
A stability function (sometimes also called a central charge) is a linear map
from Grothendieck group $K(\mathcal{A})$ to the additive group of complex numbers, such that for all non-zero objects $E \in \mathcal{A}$, the image $Z(E)$ lies in the semi-upper half plane
The phase
of an non-zero object $E \in \mathcal{A}$ is just the complex phase $\phi$ that occurs in the representation from $H$. Alternatively, by plotting $Z(E)$ in the complex plane, the phase is the argument (slope) divided by $\pi$.
((semi-)stable objects)
For $\mathcal{A}$ an abelian category equipped with a stability function $Z = r exp(i\pi \phi)$ (Def. ). Then a non-zero object $E \in \mathcal{A}$ is called
a semi-stable object if for all non-zero subobjects $F\subset E$ the phase (3) of $F$ is smaller or equal that of $E$
a stable object if for all non-zero, proper subobjects $F \subset E$ the phase (3) of $F$ is strictly smaller than that of $E$:
(Harder-Narasimhan property)
A stability function $Z \;\colon\; K(\mathcal{A})\to \mathbb{C}$ (Def. ) is said to have the Harder-Narasimhan property if for any non-zero object $E$ there exists a finite filtration by subobjects
such that the quotients
are all semi-stable (4) and satisfy
(slicing)
Let $\mathcal{D}$ be a triangulated category (usually arising as the derived category of some abelian category).
A slicing $\mathcal{P}$ on $\mathcal{D}$ is a choice of additive full subcategories $\mathcal{P}(\phi) \subset \mathcal{D}$ for each $\phi \in \mathbb{R}$ satisfying
$\mathcal{P}(\phi +1)=\mathcal{P}(\phi)[1]$
If $\phi_1 \gt \phi_2$ and $A_j\in \mathcal{P}(\phi_j)$, then $Hom(A_1, A_2)=0$.
Any object has a finite filtration by the slicing:
If $E\in \mathcal{D}$, then there exists $\phi_1 \gt \cdots \gt \phi_n$ and a sequence $0=E_0\to E_1 \to \cdots \to E_n = E$ such that the mapping cone $E_{j-1}\to E_j \to F_j \to E_{j-1}[1]$ satisfies $F_j\in \mathcal{P}(\phi_j)$.
(stability condition)
A stability condition on a triangulated category $\mathcal{D}$ is a pair $\sigma = (Z, \mathcal{P})$ consisting of
satisfying the relation that given a non-zero object $E\in \mathcal{P}(\phi)$, then there is a positive real number $m(E)$ such that the value $Z(E)$ of the stability function (Def. ) is
This justifies the repeated notation of $\phi$, since this says that if an object lies in a particular slice $\mathcal{P}(\phi)$, then it must also have complex phase $\phi$.
$\,$
Bridgeland proves that an equivalent way to give a stability is to specify a heart of a bounded t-structure on $\mathcal{D}$ and give a stability function the heart that satisfies the Harder-Narasimhan property.
Let $\mathcal{A}$ be an abelian category equipped with a stability condition (Def. ).
For $E \in \mathcal{A}$ a stable object (Def ), every endomorphism of $E$ is either the zero morphism or is an isomorphism; in particular if $\mathcal{A}$ is enriched in vector spaces over an algebraically closed field $k$, then $End(E) \simeq k$.
More generally, for $E_1, E_2 \in \mathcal{A}$ two stable objects of the same slope/phase, $\phi(E_1) =\phi(E_2)$, any morphism $E_1 \to E_2$ is either the zero morphism or is an isomorphism.
e.g. (Bayer 11, 2.3 Exercise 1, Martinez 13, prop. 3)
(space of stability conditions)
Under reasonable hypotheses (…), one can put a natural topology on the space $Stab(\mathcal{D})$ of all stability conditions (Def. ) , under which the space becomes a complex manifold.
(Bridgeland 02, theorem 1.2, reviewed in Bridgeland 09, 3.2)
Most work using this fact has been done in the case of the bounded derived category of coherent sheaves $\mathcal{D}=D^b(Coh(X))$ where $X$ is a smooth, projective variety over $\mathbb{C}$ so that $\mathcal{D}$ is $\mathbb{C}$-linear and $K(\mathcal{D})$ is finitely generated.
The space of stability conditions $Stab(X)$ has a locally finite wall and chamber decomposition. The philosophy is that if one fixes a numerical condition $v$ and considers the moduli space of $\sigma$-stable sheaves as $\sigma$ varies through $Stab(X)$, then the moduli spaces $M_\sigma(v)\simeq M_{\sigma '}(v)$ should be isomorphic if $\sigma$ and $\sigma'$ are in the same chamber. Bayer and Macri have shown this is true for K3 surfaces, and upon crossing a wall the moduli spaces are related by a birational map.
A motivating example for the concept of Bridgeland stability is the following classical notion.
(slope-stability of coherent sheaves)
Let $X$ be a non-singular, projective curve over $\mathbb{C}$. Let $\mathcal{A}=Coh(X)$ be the category of coherent sheaves on $X$.
In this case the standard stability function (Def. ) is
where $deg$ is the degree and $rk$ the rank of a coherent sheaf $E$.
The classical notion of the slope of a vector bundle is
When constructing a moduli space of vector bundles using GIT one needs to consider only slope (semi-)stable vector bundles (see e.g. Reineke 08, sections 3 and 4).
One can immediately see that a vector bundle/coherent sheaf is slope (semi-)stable if and only if it is (semi-)stable with respect to this stability function (6).
Thus Bridgeland stability generalizes the classical notions of stability of vector bundles.
For $G_{ADE} \subset SU(2)$ a finite subgroup of SU(2), let $\tilde X$ be the resolution of the corresponding ADE-singularity.
Then the connected component of the space of stability conditions (Def. ) on the bounded derived category of coherent sheaves over $\tilde X$ can be described explicitly (Thomas 02, Bridgeland 05, Brav-Thomas 09).
Specifically for type-A singularities the space of stability conditions (Prop. ) is in fact connected and simply-connected topological space (Ishii-Ueda-Uehara 10). In fact spaces of stability structures over Dynkin quivers are contractible (Qiu-Woold 14)
Brief review is in Bridgeland 09, section 6.3.
$\,$
The general definition is due to
generalizing the classical concept of slope-stability of vector bundles and of modules as in
and motivated by informal arguments in string theory about the “$\Pi$-stability” for B-branes of the B-model topological string, due to Douglas-Fiol-Römerlsberger 00 and expanded on in Douglas 01, Douglas 02, Aspinwall 04 and other articles.
Further developments include
R. Pandharipande, R.P. Thomas, Stable pairs and BPS invariants, arXiv:0711.3899
Maxim Kontsevich, Yan Soibelman, Stability structures, motivic Donaldson-Thomas invariants and cluster transformations, arXiv:0811.2435
Rina Anno, Roman Bezrukavnikov, Ivan Mirković, A thin stringy moduli space for Slodowy slices, arxiv/1108.1563
Arend Bayer, Emaneule Macri, Projectivity and Birational Geometry of Bridgeland Moduli Spaces (arXiv:1203.4613)
Tom Bridgeland, Ivan Smith, Quadratic differentials as stability conditions, arxiv/1302.7030
Cristian Martinez, Duality, Bridgeland wall-crossing and flips of secant varieties (arXiv:1311.1183)
Tom Bridgeland, Spaces of stability conditions, Proc. of symposia in pure math. 80, 2009 (math/0611510)
Markus Reineke, Moduli of representations of quivers (arXiv:0802.2147)
Arend Bayer, A tour to stability conditions on derived categories, 2011 (pdf)
Daniel Huybrechts, Introduction to stability conditions (arXiv:1111.1745)
Jan Engenhorst, Bridgeland Stability Conditions in Algebra, Geometry and Physics, 2014 (pdf)
Paolo Stellari, A tour on Bridgeland stability, 2015 (pdf)
Discussion of examples of stability conditions
over resolutions of ADE-singularities:
R. P. Thomas, Stability conditions and the braid group, Communications in Analysis and Geometry 14, 135-161, 2006 (arXiv:math/0212214)
Tom Bridgeland, Stability conditions and Kleinian singularities, International Mathematics Research Notices 2009.21 (2009): 4142-4157 (arXiv:0508257)
Akira Ishii, Kazushi Ueda, Hokuto Uehara, Stability conditions on $A_n$-singularities, Journal of Differential Geometry 84 (2010) 87-126 (arXiv:math/0609551)
Christopher Brav, Hugh Thomas, Braid groups and Kleinian singularities (arXiv:0910.2521)
Yu Qiu, Jon Woolf, Contractible stability spaces and faithful braid group actions (arXiv:1407.5986)
Yu Qiu, Def. 2.1 Stability conditions and quantum dilogarithm identities for Dynkin quivers, Adv. Math., 269 (2015), pp 220-264 (arXiv:1111.1010)
Tom Bridgeland, Yu Qiu, Tom Sutherland, Stability conditions and the $A_2$ quiver (arXiv:1406.2566)
The proposal that slope-stability of vector bundles should generalize to a notion of stability (“$\Pi$-stability”) of B-branes/D-branes originates with
In terms of stability ($\Pi$-stability) of B-branes of the B-model topological string:
Michael Douglas, D-branes, categories and $N=1$ supersymmetry, J.Math.Phys. 42 (2001) 2818–2843;
Michael Douglas, Dirichlet branes, homological mirror symmetry, and stability, Proc. ICM, Vol. III (Beijing, 2002), 395–408, Higher Ed. Press, Beijing, 2002
Paul Aspinwall, D-Branes on Calabi-Yau Manifolds (arXiv:hep-th/0403166)
Aaron Bergman, Stability Conditions and Branes at Singularities, Journal of High Energy Physics 2008.10 (2008): 07 (arXiv:hep-th/0702092)
Dmitry Malyshev, Herman Verlinde, D-branes at Singularities and String Phenomenology, Nucl.Phys.Proc.Suppl.171:139-163, 2007 (arXiv:0711.2451)
On marginally stable branes: