We consider the stochastic heat equation
\[
∂
t
Z
=
∂
x
2
Z
−
Z
W
˙
\partial _tZ= \partial _x^2 Z - Z \dot W
\]
on the real line, where
W
˙
\dot W
is space-time white noise.
h
(
t
,
x
)
=
−
log
Z
(
t
,
x
)
h(t,x)=-\operatorname {log} Z(t,x)
is interpreted as a solution of the KPZ equation, and
u
(
t
,
x
)
=
∂
x
h
(
t
,
x
)
u(t,x)=\partial _x h(t,x)
as a solution of the stochastic Burgers equation. We take
Z
(
0
,
x
)
=
exp
{
B
(
x
)
}
Z(0,x)=\exp \{B(x)\}
, where
B
(
x
)
B(x)
is a two-sided Brownian motion, corresponding to the stationary solution of the stochastic Burgers equation. We show that there exist
0
>
c
1
≤
c
2
>
∞
0> c_1\le c_2 >\infty
such that
\[
c
1
t
2
/
3
≤
Var
(
log
Z
(
t
,
x
)
)
≤
c
2
t
2
/
3
.
c_1t^{2/3}\le \operatorname {Var}(\operatorname {log} Z(t,x) )\le c_2 t^{2/3}.
\]
Analogous results are obtained for some moments of the correlation functions of
u
(
t
,
x
)
u(t,x)
. In particular, it is shown there that the bulk diffusivity satisfies
\[
c
1
t
1
/
3
≤
D
bulk
(
t
)
≤
c
2
t
1
/
3
.
c_1t^{1/3}\le D_\textrm {bulk}(t) \le c_2 t^{1/3}.
\]
The proof uses approximation by weakly asymmetric simple exclusion processes, for which we obtain the microscopic analogies of the results by coupling.