Homology with local coefficients and flat bundles

algebraic-topologycovering-spacesfiber-bundleshomology-cohomology

I have a problem in understanding Prop.1.10 in
https://pages.uoregon.edu/sadofsky/691/sseq-local-coefficients.pdf
My interpretation is as follows:

Let $X$ be a smooth n-manifold and $U\to X$ be its universal cover. Notice that $U$ is a $\pi_1 X$ principal bundle. Let $\rho: \pi_1 X\to Aut(S) $ be a an action of $\pi_1 X$ on the group $S$. Then we can form the associated bundle $\tilde{X}=U\times_\rho S $.
The proposition 1.10 says that
$$H_*(X,S) \simeq H_*(\tilde{X},\mathbb{Z})$$
where the LHS is the homology with twisted coefficients in $S$.

Problem:
If $S=V$ is a vector space then $\tilde{X}=U\times_{\rho} V$ is the flat bundle associated to the flat connection induced by $\rho.$ This is a vector bundle over $X$ hence it retract over $X$ therefore
$$H_*(X,V) \simeq H_*(\tilde{X},\mathbb{Z})\simeq H_*(X, \mathbb{Z}).$$
Which is not true for any flat connection.

Best Answer

Your general statement is simply false. For instance, if the action is trivial, your statement would say that the homology of any space agrees with the homology of its universal cover, which is ridiculous.

What is true is that if $A = \Bbb Z[\pi_1(X)/H]$ as a $\pi_1(X)$-module, and $p: X' \to X$ is the covering space associated to the subgroup $H \subset \pi_1(X)$, then $\mathcal A = \tilde X \times_{\pi_1 X} A$ gives a local coefficient system over $X$ (if you like to think of a local coefficient system as being a bundle of abelian groups over $X$). Then we have $$H_*(X;\mathcal A) \cong H_*(X'; \Bbb Z).$$

What this says is that a very specific local coefficient system gives the homology of this covering space. Not that you can compute the homology of general coefficient systems by passing to a covering space.