In the Fastest Mixing Markov Chain problem, we are given a graph $$G = (V, E)$$
G
=
(
V
,
E
)
and desire the discrete-time Markov chain with smallest mixing time $$\tau $$
τ
subject to having equilibrium distribution uniform on V and non-zero transition probabilities only across edges of the graph. It is well-known that the mixing time $$\tau _\textsf {RW}$$
τ
RW
of the lazy random walk on G is characterised by the edge conductance $$\Phi $$
Φ
of G via Cheeger’s inequality: $$\Phi ^{-1} \lesssim \tau _\textsf {RW} \lesssim \Phi ^{-2} \log |V|$$
Φ
-
1
≲
τ
RW
≲
Φ
-
2
log
|
V
|
. Analogously, we characterise the fastest mixing time $$\tau ^\star $$
τ
⋆
via a Cheeger-type inequality but for a different geometric quantity, namely the vertex conductance $$\Psi $$
Ψ
of G: $$\Psi ^{-1} \lesssim \tau ^\star \lesssim \Psi ^{-2} (\log |V|)^2$$
Ψ
-
1
≲
τ
⋆
≲
Ψ
-
2
(
log
|
V
|
)
2
. This characterisation forbids fast mixing for graphs with small vertex conductance. To bypass this fundamental barrier, we consider Markov chains on G with equilibrium distribution which need not be uniform, but rather only $$\varepsilon $$
ε
-close to uniform in total variation. We show that it is always possible to construct such a chain with mixing time $$\tau \lesssim \varepsilon ^{-1} ({\text {diam}} G)^2 \log |V|$$
τ
≲
ε
-
1
(
diam
G
)
2
log
|
V
|
. Finally, we discuss analogous questions for continuous-time and time-inhomogeneous chains.