  We study the fluctuation of the Atlas model, where a unit drift is assigned
to the lowest ranked particle among a semi-infinite ($ \mathbb{Z}_+ $-indexed)
system of otherwise independent Brownian particles, initiated according to a
Poisson point process on $ \mathbb{R}_+ $. In this context, we show that the
joint law of ranked particles, after being centered and scaled by $t^{-1/4}$,
converges as $t \to \infty$ to the Gaussian field corresponding to the solution
of the additive stochastic heat equation on $\mathbb{R}_+$ with Neumann
boundary condition at zero. This allows us to express the asymptotic
fluctuation of the lowest ranked particle in terms of a $ \frac{1}{4}
$-fractional Brownian motion. In particular, we prove a conjecture of Pal and
Pitman (2008) about the asymptotic Gaussian fluctuation of the ranked
particles.
