Abstract:
First the title could be also understood as ``3-manifolds related by non-zero degree maps" or "Degrees of maps between 3-manifolds" for some aspects in this survey talk. The topology of surfaces was completely understood at the end of 19th century, but maps between surfaces kept to be an active topic in the 20th century and many important results just appeared in the last 25 years. The topology of 3-manifolds was well-understood only in the later 20th century, and the topic of non-zero degree maps between 3-manifolds becomes active only rather recently. We will survey questions and results in the topic indicated by the title, present its relations to 3-manifold topology and its applications to problems in geometry group theory, fixed point theory and dynamics. There are four aspects addressed: (1) Results concerning the existence and finiteness about the maps of non-zero degree (in particular of degree one) between 3-manifolds and their suitable correspondence about epimorphisms on knot groups and 3-manifold groups. (2) A measurement of the topological complexity on 3-manifolds and knots given by "degree one map partial order", and the interactions between the studies of non-zero degree map among 3-manifolds and of topology of 3-manifolds. (3) The standard forms of non-zero degree maps and automorphisms on 3-manifolds and applications to minimizing the fixed points in the isotopy class. (4) The uniqueness of the covering degrees between 3-manifolds and the uniqueness embedding indices (in particular the co-Hopfian property) between Kleinian groups. The methods used are varied, and we try to describe them briefly.

Abstract:
Thom-Pontrjagin constructions are used to give a computable necessary and sufficient condition when a homomorphism $\phi : H^n(L;Z)\to H^n(M;Z)$ can be realized by a map $f:M\to L$ of degree $k$ for closed $(n-1)$-connected $2n$-manifolds $M$ and $L$, $n>1$. A corollary is that each $(n-1)$-connected $2n$-manifold admits selfmaps of degree larger than 1, $n>1$. In the most interesting case of dimension 4, with the additional surgery arguments we give a necessary and sufficient condition for the existence of a degree $k$ map from a closed orientable 4-manifold $M$ to a closed simply connected 4-manifold $L$ in terms of their intersection forms, in particular there is a map $f:M\to L$ of degree 1 if and only if the intersection form of $L$ is isomorphic to a direct summand of that of $M$.

Abstract:
We prove a rigidity theorem for degree one maps between small 3-manifolds using Heegaard genus, and provide some applications and connections to Heegaard genus and Dehn surgery problems.

Abstract:
We construct a hyperbolic 3-manifold $M$ (with $\partial M$ totally geodesic) which contains no essential closed surfaces, but for any even integer $g> 0$ there are infinitely many separating slopes $r$ on $\partial M$ so that $M[r]$, the 3-manifold obtained by attaching 2-handle to $M$ along $r$, contains an essential separating closed surface of genus $g$ and is still hyperbolic. The result contrasts sharply with those known finiteness results for the cases $g=0,1$. Our 3-manifold $M$ is the complement of a simple small knot in a handlebody.

Abstract:
For given closed orientable 3-manifolds $M$ and $N$ let $\c{D}(M,N)$ be the set of mapping degrees from $M$ to $N$. We address the problem: For which $N$, $\c{D}(M,N)$ is finite for all $M$? The answer is known in Thurston's picture of closed orientable irreducible 3-manifolds unless the target is a non-trivial graph manifold. We prove that for each closed non-trivial graph manifold $N$, $\c{D}(M,N)$ is finite for all graph manifold $M$. The proof uses a recently developed standard forms of maps between graph manifolds and the estimation of the $\widetilde{\rm PSL}(2,{\R})$-volume for certain class of graph manifolds.

Abstract:
This paper shows that the Seifert volume of each closed non-trivial graph manifold is virtually positive. As a consequence, for each closed orientable prime 3-manifold $N$, the set of mapping degrees $\c{D}(M,N)$ is finite for any 3-manifold $M$, unless $N$ is finitely covered by either a torus bundle, or a trivial circle bundle, or the 3-sphere.

Abstract:
We construct a small, hyperbolic 3-manifold $M$ such that, for any integer $g\geq 2$, there are infinitely many separating slopes $r$ in $\partial M$ so that $M(r)$, the 3-manifold obtained by attaching a 2-handle to $M$ along $r$, is hyperbolic and contains an essential separating closed surface of genus $g$. The result contrasts sharply with those known finiteness results on Dehn filling, and it also contrasts sharply with the known finiteness result on handle addition for the cases $g=0,1$. Our 3-manifold $M$ is the complement of a hyperbolic, small knot in a handlebody of genus 3.

Abstract:
We give some applications of the Chern Simons gauge theory to the study of the set ${\rm vol}(N,G)$ of volumes of all representations $\rho\co\pi_1N\to G$, where $N$ is a closed oriented three-manifold and $G$ is either ${\rm Iso}_e\t{\rm SL_2(\R)}$, the isometry group of the Seifert geometry, or ${\rm Iso}_+{\Hi}^3$, the orientation preserving isometry group of the hyperbolic 3-space. We focus on three natural questions: (1) How to find non-zero values in ${\rm vol}(N, G)$? or weakly how to find non-zero elements in ${\rm vol}(\t N, G)$ for some finite cover $\t N$ of $N$? (2) Do these volumes satisfy the covering property in the sense of Thurston? (3) What kind of topological information is enclosed in the elements of ${\rm vol}(N, G)$? We determine ${\rm vol}(N, G)$ when $N$ supports the Seifert geometry, and we find some non-zero values in ${\rm vol}(N,G)$ for certain 3-manifolds with non-trivial geometric decomposition for either $G={\rm Iso}_+{\Hi}^3$ or ${\rm Iso}_e\t{\rm SL_2(\R)}$. Moreover we will show that unlike the Gromov simplicial volume, these non-zero elements carry the gluing information between the geometric pieces of $N$. For a large class 3-manifolds $N$, including all rational homology 3-spheres, we prove that $N$ has a positive Gromov simplicial volume iff it admits a finite covering $\t N$ with ${\rm vol}(\t N,{\rm Iso}_+{\Hi}^3)\ne \{0\}$. On the other hand, among such class, there are some $N$ with positive simplicial volume but ${\rm vol}(N,{\rm Iso}_+{\Hi}^3)=\{0\}$, yielding a negative answer to question (2) for hyperbolic volume.