Homology sphere

In algebraic topology, a homology sphere is an "n"-manifold "X" having the homology groups of an "n"-sphere, for some integer "n" ≥ 1. That is, we have

:"H"₀("X",Z) = Z = "H"_"n"("X",Z)

and

:"H"_"i"("X",Z) = {0} for all other "i".

Therefore "X" is a connected space, with one non-zero higher Betti number: "b_n". It does not follow that "X" is simply connected, only that its fundamental group is perfect (see Hurewicz theorem).

A rational homology sphere is defined similarly but we use homology with rational coefficients.

Poincaré homology sphere

The Poincaré homology sphere (also known as Poincaré dodecahedral space) is a particular example of a homology sphere. It is the only homology 3-sphere (besides the 3-sphere itself) with a finite fundamental group. Its fundamental group is known as the binary icosahedral group and has order 120. This shows the Poincaré conjecture cannot be stated in homology terms alone.

A simple construction of this space, which makes clear the term "dodecahedral space", begins with a dodecahedron. Each face of the dodecahedron can be identified with its opposite face by using the minimal clockwise twist to line up the faces. Glue each pair of opposite faces together using this identification. After this gluing, the result is a closed 3-manifold.

The Poincaré homology sphere is a spherical 3-manifold. See Seifert-Weber space for a similar construction (using a different amount of "twist") that results in a hyperbolic 3-manifold.

Alternatively, the Poincaré homology sphere can be constructed as the quotient space SO(3)/I where I is the icosahedral group (i.e. the rotational symmetry group of the regular icosahedron and dodecahedron, isomorphic to the alternating group "A"₅). Less technically, this means that the Poincaré homology sphere is the space of all possible positions of an icosahedron. Alternatively, one can pass to the universal cover of SO(3) which can be realized as the group of unit quaternions and is homeomorphic to the 3-sphere. In this case, the Poincaré homology sphere is isomorphic to "S"³/Ĩ where Ĩ is the binary icosahedral group, the perfect double cover of I living in "S"³.

Another approach is by Dehn surgery. The Poincaré homology sphere results from +1 surgery on the right-handed trefoil knot.

The Poincaré homology sphere in cosmology

In 2003, an apparent periodicity in the cosmic microwave background led to the suggestion, by Jean-Pierre Luminet of the Observatoire de Paris and colleagues, that the shape of the Universe is a Poincaré sphere. [ [http://physicsweb.org/articles/news/7/10/5 "Is the universe a dodecahedron?"] , article at PhysicsWeb.] During the following year, astronomers searched for more evidence to support this hypothesis, finding a tentative "hint" from observations by the WMAP satellite supporting the hypothesis.cite journal
last =Roukema
first =Boudewijn
authorlink =
coauthors = Bartosz Lew, Magdalena Cechowska, Andrzej Marecki, Stanislaw Bajtlik
title =A Hint of Poincaré Dodecahedral Topology in the WMAP First Year Sky Map
journal = Astronomy and Astrophysics
volume =423
issue =
pages =821
publisher =
date = 25 February 2004
url =http://arxiv.org/abs/astro-ph/0402608
doi =
id =
accessdate = 2007-02-09 ]

Constructions and examples

*Surgery on a knot in the 3-sphere "S"³ with framing +1 or − 1 gives a homology sphere.

*More generally, surgery on a link gives a homology sphere whenever the matrix given by intersection numbers (off the diagonal) and framings (on the diagonal) has determinant +1 or −1.

*If "p", "q", and "r" are pairwise relatively prime positive integers then the link of the singularity "x"^"p" + "y"^"q" + "z"^"r" = 0 (in other words, the intersection of a small 5-sphere around 0 with this complex surface) is a homology 3-sphere, called a Brieskorn 3-sphere Σ("p", "q", "r"). It is homeomorphic to the standard 3-sphere if one of "p", "q", and "r" is 1, and Σ(2, 3, 5) is the Poincaré sphere.

*The connected sum of two oriented homology 3-spheres is a homology 3-sphere. A homology 3-sphere that cannot be written as a connected sum of two homology 3-spheres is called irreducible or prime, and every homology 3-sphere can be written as a connected sum of prime homology 3-spheres in an essentially unique way. (See Prime decomposition (3-manifold).)

*Suppose that "a"₁, ..., "a"_"r" are integers all at least 2 such that any two are coprime. Then the Seifert fiber space

:: $\{b,\; (o\_1,0);(a\_1,b\_1),dots,(a\_r,b\_r)\},$

:over the sphere with exceptional fibers of degrees "a"₁, ..., "a"_"r" is a homology sphere, where the "b"'s are chosen so that

:: $b+b\_1/a\_1+cdots+b\_r/a\_r=1/(a\_1cdots\; a\_r).$

:(There is always a way to chose the "b"′s, and the homology sphere does not depend (up to isomorphism) on the choice of "b"′s.) If "r" is at most 2 this is just the usual 3-sphere; otherwise they are distinct non-trivial homology spheres. If the "a"′s are 2, 3, and 5 this gives the Poincaré sphere. If there are at least 3 "a"′s, not 2, 3, 5, then this is an acyclic homology 3-sphere with infinite fundamental group that has a Thurston geometry modeled on the univeral cover of "SL"₂(R).

Invariants

*The Rokhlin invariant is a Z/2Z valued invariant of homology 3-spheres.
*The Casson invariant is an integer valued invariant of homology 3-spheres, whose reduction mod 2 is the Rokhlin invariant.

Applications

If "A" is a homology 3-sphere not isomorphic to the standard 3-sphere, then the suspension of "A" is an example of a 4-dimensional homology manifold that is not a topological manifold. The double suspension of "A" is homeomorphic to the standard 5-sphere, but its triangulation (induced by some triangulation of "A") is not a PL manifold. In other words, this gives an example of a finite simplicial complex that is a topological manifold but not a PL manifold. (It is not a PL manifold because the link of a point is not always a 4-sphere.)

Galewski and Stern showed that all compact topological manifolds (without boundary) of dimension at least 5 are homeomorphic to simplicial complexes if and only if there is a homology 3 sphere Σ with Rokhlin invariant 1 such that the connected sum Σ#Σ of Σ with itself bounds a smooth acyclic 4-manifold. The existence of such a homology 3-sphere is an unsolved problem (as of 2006).

External links

* [http://www.eg-models.de/models/Simplicial_Manifolds/2003.04.001/_preview.html A 16-Vertex Triangulation of the Poincaré Homology 3-Sphere and Non-PL Spheres with Few Vertices] by Anders Björner and Frank H. Lutz
*Lecture by David Gillman on [http://www.pims.math.ca/science/2002/cascade/gillman/ The best picture of Poincare's homology sphere ]

References

* Emmanuel Dror, "Homology spheres", Israel Journal of Mathematics 15 (1973), 115–129. MathSciNet|id=0328926
* David Galewski, Ronald Stern [http://links.jstor.org/sici?sici=0003-486X%28198001%292%3A111%3A1%3C1%3ACOSTOT%3E2.0.CO%3B2-N "Classification of simplicial triangulations of topological manifolds"] , Annals of Mathematics 111 (1980), no. 1, pp. 1-34.
* Robion Kirby, Martin Scharlemann, "Eight faces of the Poincaré homology 3-sphere". Geometric topology (Proc. Georgia Topology Conf., Athens, Ga., 1977), pp. 113–146, Academic Press, New York-London, 1979.
* Michel Kervaire, " [http://links.jstor.org/sici?sici=0002-9947%28196910%29144%3C67%3ASHSATF%3E2.0.CO%3B2-G Smooth homology spheres and their fundamental groups] ", Transactions of the American Mathematical Society 144 (1969) 67–72. MathSciNet|id=0253347
* Nikolai Saveliev, "Invariants of Homology 3-Spheres", Encyclopaedia of Mathematical Sciences, vol 140. Low-Dimensional Topology, I. Springer-Verlag, Berlin, 2002. MathSciNet|id=1941324 ISBN 3-540-43796-7