Volume estimates and the asymptotic behavior of expanding gradient Ricci solitons

We study the asymptotic volume ratio of non-steady gradient Ricci solitons. Moreover, a local estimate of the volume ratio is obtained for expanding solitons which satisfy $\lim_{dist(O,x)\rightarrow\infty} |Sect|\cdot dist(O,x)^2=0$. Therefore, for such a soliton, we can show that it must have $\mathbb{R}^n$ as one of its tangent cone at infinity. (Here we assume that the soliton is simply connected at infinity, has only one end and $n\geq 3$.)