Answer
The doubling here is $1.3863$ when $t=0$.
Work Step by Step
The exponential growth model is the following:
$Q(t)=Q_0∗e^{t_s∗k}$
Where $t_s∗k=\ln2$, and $t_s$ is the doubling life at $t=0$ and k is constant.
Here, $k=0.5$
$t_s∗k=ln2$
$t_s∗0.5=ln2$
$t_s=\frac{ln2}{0.5}=1.3863$
Therefore the doubling here is $1.3863$ when $t=0$.