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Orbifold
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It is a topological space (called an ''underlying space'') with an orbifold structure (see below).
The underlying space locally looks like a Quotient Space of a
Euclidean Space under the action of a Finite Group of Isometries .

In String Theory , the word "orbifold" has additional meaning, discussed below.

The main example of underlying space is a quotient space of a manifold under the action of a finite Group of Diffeomorphism s, in particular a ''manifold with boundary'' carries natural
orbifold structure, since it is the Z2-factor of its
Double .
A factor space of a manifold along a smooth S^1-action without fixed points carries the structure of an orbifold (this is not a partial case of the main example).

Orbifold structure gives a natural Stratification by open manifolds on its underlying space, where one stratum corresponds to a set of singular points of the same type.

It should be noted that one topological space can carry many different
orbifold structures.
For example, consider the orbifold ''O'' associated with a
factor space of the 2-sphere along a rotation by \pi^{}_{} ; it is homeomorphic to the 2-sphere, but the natural orbifold structure is different.
It is possible to adopt most of the characteristics of manifolds to orbifolds and
these characteristics are usually different from correspondent characteristics of underlying space.
In the above example, the ''orbifold Fundamental Group '' of ''O'' is Z2
and its ''orbifold Euler Characteristic '' is 1.


FORMAL DEFINITION

Like a manifold, an orbifold is specified by local conditions; however, whereas
a manifold locally looks like \mathbb{R}^n , an orbifold
locally looks like a quotient of \mathbb{R}^n . Hence
an orbifold need not be a manifold.

A (topological) orbifold O, is a
Hausdorff topological space X with countable Base , called the underlying space, with an orbifold structure, which is defined by an orbifold atlas (see below).

An orbifold chart is an open subset U\subset X together with open set V \subset \mathbb{R}^n and
a continuous map \phi : V o U which satisfy the following property:
there is a finite group \Gamma acting by Linear Transformation s on V and a homeomorphism artheta : V/\Gamma o U such that \phi= artheta\circ\pi, where \pi\, denotes the projection V o V/\Gamma.

A collection of orbifold charts \{\phi_\alpha:V_\alpha o U_\alpha\} is called an orbifold atlas if it satisfies the following properties:

# \displaystyle\bigcup_\alpha U_\alpha=X,
# if \phi_\alpha(x)=\phi_\beta(y) \, then there is a neighborhood x\in V_x\subset V_\alpha and y\in V_y\subset V_\beta and a homeomorphism \psi:V_x o V_y such that \phi_\alpha=\phi_\beta\circ\psi.

The orbifold atlas defines the orbifold structure completely and we
regard two orbifold atlases of X to give the same orbifold
structure if they can be combined to give a larger orbifold atlas.

One can add differentiability conditions on the gluing map \psi^{}_{}
in the above definition and get a definition of differentiable orbifolds in the same way as it was done for manifolds.

2-DIMENSIONAL ORBIFOLDS

In two dimensions, there are three singular point types of an orbifold:
  • A boundary point

  • An elliptic point of order ''n'', such as the origin of R2 quotiented out by a cyclic group of order ''n'' of rotations.

  • A corner reflector of order ''n'': the origin of R2 quotiented out by a dihedral group of order 2''n''.


A compact 2-dimensional orbifold has an Euler characteristic Χ
given by
:Χ = Χ(''X''0) − Σ(1 − 1/''n''''i'')/2 − Σ(1 − 1/''m''''i'')
where Χ(''X''0) is the Euler characteristic of the underlying topological manifold ''X''0, and ''n''''i'' are the orders of the corner reflectors, and ''m''''i'' are the orders of the elliptic points.

A 2-dimensional compact connected orbifold has a hyperbolic structure if its Euler characteristic is less than 0, a Euclidean structure if it is 0, and if its Euler characteristic is positive it is either bad or has an elliptic structure (an orbifold is called bad if it does not have a manifold as a covering space). In other words, its universal covering space has a hyperbolic, Euclidean, or spherical structure.

The compact 2-dimensional connected orbifolds that are not hyperbolic are listed in the table below. The 17 parabolic orbifolds are the quotients of the plane by the 17 Wallpaper Group s.


ORBIFOLDS IN STRING THEORY

In String Theory , the word "orbifold" has a slightly new meaning. For mathematicians, an orbifold is a generalization of the notion of Manifold that allows the presence of the points whose neighborhood is Diffeomorphic to a quotient of R^n by a finite group, i.e. R^n / \Gamma. In physics, the notion of an orbifold usually describes an object that can be globally written as an orbit space M/G where M is a manifold (or a theory), and G is a group of its isometries (or symmetries) - not necessarily all of them. In string theory, these symmetries do not have to have a geometric interpretation.

A Quantum Field Theory defined on an orbifold becomes singular near the fixed points of G. However string theory requires us to add new parts of the Closed String Hilbert Space - namely the twisted sectors where the fields defined on the closed strings are periodic up to an action from G. Orbifolding is therefore a general procedure of string theory to derive a new string theory from an old string theory in which the elements of G have been identified with the identity. Such a procedure reduces the number of states because the states must be invariant under G, but it also increases the number of states because of the extra twisted sectors. The result is usually a perfectly smooth, new string theory.

D-branes propagating on the orbifolds are described, at low energies, by gauge theories defined by the Quiver Diagram s. Open strings attached to these D-branes have no twisted sector, and so the number of open string states is reduced by the orbifolding procedure.

More specifically,
when the orbifold group G is a discrete subgroup of spacetime isometries, then if it has no fixed point, the result is usually a compact smooth space; the twisted sector comprises of closed strings wound around the compact dimension, which are called winding states.

When the orbifold group G is a discrete subgroup of spacetime isometries, and it has fixed points, then these usually have Conical Singularities , because Rn/ Zk has such a singularity at the fixed point of Zk . In string theory, gravitational singularities are usually a sign of extra Degrees Of Freedom which are located at locus point in spacetime. In the case of the orbifold these Degrees Of Freedom are the twisted states, which are strings "stuck" at the fixed points. when the fields related with these twisted states acquire a non-zero Vacuum Expectation Value , the singularity is deformed, i.e. the metric is changed and become regular at this point and around it. An example for a resulting geometry is the is Eguchi-Hanson spacetime.

From the point of view of D-branes in the vicinity of the fixed points, the effective theory of the open strings attached to these D-branes is a supersymmetric field theory, whose space of vacua has a singular point, where additional massless degrees of freedom exist. The fields related with the closed string twisted sector couple to the open strings in such a way as to add a Fayet-Illiopolys term to the supersymmetric field theory Lagrangian, so that when such a field acquire a non-zero Vacuum Expectation Value , the Fayet-Illiopolys term is non-zero, and thereby deforms the theory (i.e. changes it) so that the singularity no longer exists [http://www-spires.fnal.gov/spires/find/hep/www?j=NUPHA,B342,246 .


HISTORY

Orbifolds and related concepts are implicit in the work of pioneers such as Henri Poincaré . The first formal definition of an orbifold-like object was given by Ichiro Satake in 1956; he defined the ''V-manifold'', which had a codimension 2 singular locus, in the context of Riemannian geometry. William Thurston , who was unaware of Satake's work, later in the mid 1970s defined and named the more general notion of orbifold as part of his study of hyperbolic structures.


FURTHER READING

William Thurston, ''The Geometry and Topology of Three-Manifolds'' (Chapter 13), Princeton University lecture notes (1978-1981).