I was trying to understand this interesting question by example.

Let me follow their previous discussion and ask: Let a generic nontrivial 2-cocycle $\omega_2^G(g_1,g_2) \in H^2(G,\mathbb{R}/\mathbb{Z})$ in the cohomology group of $G$ with $U(1)=\mathbb{R}/\mathbb{Z}$ coefficient. In otherwords, here the 2-cocycle $\omega_2^G$ is a complex $U(1)=\mathbb{R}/\mathbb{Z}$ function with the norm $|\omega_2^G|=1$ but with a $U(1)$ complex phase satisfying the cocycle condition.

I like to ask how can we trivialize the 2-cocycle $\omega_2(g_1,g_2)$ of $G$ into 2-coboundary if we lift $G$ into a larger group $J$, and given that we know the group homomorphism $r$: $$J \overset{r}{\rightarrow} G.$$ In particular I like to focus on:

$$SU(2) \overset{r}{\rightarrow} SO(3).$$

So that

$$\omega_2^J(j_1,j_2)=\omega_2^G(r(j_1),r(j_2))=\omega_2^G(g_1,g_2) \text{ is trivial in } H^2(J,U(1)).$$
Namely $\omega_2^G(r(j_1),r(j_2))$ becomes a 2-coboundary in $H^2(J,\mathbb{R}/\mathbb{Z})$ for the cohomology group of $J$, but $\omega_2^G(g_1,g_2)$ originally was not a 2-coboundary but was a 2-cocycle for the cohomology group of $G$. We can explicitly write
$$
\omega_2^G(g_1,g_2)=\omega_2^G(r(j_1),r(j_2))=
\frac{\beta_1^J(j_2)\beta_1^J(j_1)}{\beta_1^J(j_1 j_2)}.
$$
Here $\beta_1^J(j_1)$ is a 1-cochain for $j_1, j_2 \in J$, and that
$g_1=r(j_1)$, $g_2=r(j_2)\in G$.

Is that true that the 2-cocycle in $$H^2[SO(3),\mathbb{R}/\mathbb{Z}]=\mathbb{Z}/2\mathbb{Z}$$ for $G=SO(3)$ can be trivialize in $J=SU(2)$? How can it be shown? Here we consider the cohomology group of the Lie group $SO(3)$ and $SU(2)$.

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