Answer
The doubling time here is $0.693$ 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, $k=1$
$t_s∗k=ln2$
$t_s∗1=ln2$
$t_s=\frac{ln2}{1}=0.693$
Therefore the doubling time here is $0.693$ when $t=0$.