 Homogeneous space

In mathematics, particularly in the theories of Lie groups, algebraic groups and topological groups, a homogeneous space for a group G is a nonempty manifold or topological space X on which G acts continuously by symmetry in a transitive way. A special case of this is when the topological group, G, in question is the homeomorphism group of the space, X. In this case X is homogeneous if intuitively X looks locally the same everywhere. Some authors insist that the action of G be effective (i.e. faithful), although the present article does not. Thus there is a group action of G on X which can be thought of as preserving some "geometric structure" on X, and making X into a single Gorbit.
Contents
Formal definition
Let X be a nonempty set and G a group. Then X is called a Gspace if it is equipped with an action of G on X.^{[1]} Note that automatically G acts by automorphisms (bijections) on the set. If X in addition belongs to some category, then the elements of G are assumed to act as automorphisms in the same category. Thus the maps on X effected by G are structure preserving. A homogeneous space is a Gspace on which G acts transitively.
Succinctly, if X is an object of the category C, then the structure of a Gspace is a homomorphism:
into the group of automorphisms of the object X in the category C. The pair (X,ρ) defines a homogeneous space provided ρ(G) is a transitive group of symmetries of the underlying set of X.
Examples
For example, if X is a topological space, then group elements are assumed to act as homeomorphisms on X. The structure of a Gspace is a group homomorphism ρ : G → Homeo(X) into the homeomorphism group of X.
Similarly, if X is a differentiable manifold, then the group elements are diffeomorphisms. The structure of a Gspace is a group homomorphism ρ : G → Diffeo(X) into the diffeomorphism group of X.
Geometry
From the point of view of the Erlangen programme, one may understand that "all points are the same", in the geometry of X. This was true of essentially all geometries proposed before Riemannian geometry, in the middle of the nineteenth century.
Thus, for example, Euclidean space, affine space and projective space are all in natural ways homogeneous spaces for their respective symmetry groups. The same is true of the models found of nonEuclidean geometry of constant curvature, such as hyperbolic space.
A further classical example is the space of lines in projective space of three dimensions (equivalently, the space of twodimensional subspaces of a fourdimensional vector space). It is simple linear algebra to show that GL_{4} acts transitively on those. We can parameterize them by line coordinates: these are the 2×2 minors of the 4x2 matrix with columns two basis vectors for the subspace. The geometry of the resulting homogeneous space is the line geometry of Julius Plücker.
Homogeneous spaces as coset spaces
In general, if X is a homogeneous space, and H_{o} is the stabilizer of some marked point o in X (a choice of origin), the points of X correspond to the left cosets G/H_{o}.
In general, a different choice of origin o will lead to a quotient of G by a different subgroup H_{o′} which is related to H_{o} by an inner automorphism of G. Specifically,
 H_{o'} = gH_{o}g ^{− 1} (1)
where g is any element of G for which go = o′. Note that the inner automorphism (1) does not depend on which such g is selected; it depends only on g modulo H_{o}.
If the action of G on X is continuous, then H is a closed subgroup of G. In particular, if G is a Lie group, then H is a closed Lie subgroup by Cartan's theorem. Hence G/H is a smooth manifold and so X carries a unique smooth structure compatible with the group action.
If H is the identity subgroup {e}, then X is a principal homogeneous space.
One can go further to double coset spaces, notably Clifford–Klein forms Γ\G/H, where Γ is a discrete subgroup (of G) acting properly discontinuously.
Example
For example in the line geometry case you can identify the the homozygeous we can identify H as a 12dimensional subgroup of the 16dimensional general linear group
 GL_{4},
defined by conditions on the matrix entries
 h_{13} = h_{14} = h_{23} = h_{24} = 0,
by looking for the stabilizer of the subspace spanned by the first two standard basis vectors. That shows that X has dimension 4.
Since the homogeneous coordinates given by the minors are 6 in number, this means that the latter are not independent of each other. In fact a single quadratic relation holds between the six minors, as was known to nineteenthcentury geometers.
This example was the first known example of a Grassmannian, other than a projective space. There are many further homogeneous spaces of the classical linear groups in common use in mathematics.
Prehomogeneous vector spaces
The idea of a prehomogeneous vector space was introduced by Mikio Sato.
It is a finitedimensional vector space V with a group action of an algebraic group G, such that there is an orbit of G that is open for the Zariski topology (and so, dense). An example is GL_{1} acting on a onedimensional space.
The definition is more restrictive than it initially appears: such spaces have remarkable properties, and there is a classification of irreducible prehomogeneous vector spaces, up to a transformation known as "castling".
Homogeneous spaces in physics
Cosmology using the general theory of relativity makes use of the Bianchi classification system. Homogeneous spaces in relativity represent the space part of background metrics for some cosmological models; for example, the three cases of the FriedmannLemaîtreRobertsonWalker metric may be represented by subsets of the Bianchi I (flat), V (open), VII (flat or open) and IX (closed) types, while the Mixmaster universe represents an anisotropic example of a Bianchi IX cosmology.^{[2]}
A homogeneous space of N dimensions admits a set of N(N − 1) / 2 Killing vectors.^{[3]} For three dimensions, this gives a total of six linearly independent Killing vector fields; homogeneous 3spaces have the property that one may use linear combinations of these to find three everywhere nonvanishing Killing vector fields ,
where the object , the "structure constants", form a constant orderthree tensor antisymmetric in its lower two indices (on the lefthand side, the brackets denote antisymmetrisation and ";" represents the covariant differential operator). In the case of a flat isotropic universe, one possibility is (type I), but in the case of a closed FLRW universe, where is the LeviCivita symbol.
See also
References
 ^ We assume that the action is on the left. The distinction is only important in the description of X as a coset space.
 ^ Lev Landau and Evgeny Lifshitz (1980), Course of Theoretical Physics vol. 2: The Classical Theory of Fields, ButterworthHeinemann, ISBN 9780750627689
 ^ Steven Weinberg (1972), Gravitation and Cosmology, John Wiley and Sons
Categories: Topological groups
 Lie groups
 Homogeneous spaces
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