We consider a quantum gas consisting of
N hard spheres with radius
a>0, obeying bosonic statistics and moving in the box
Λ=[0;L]3 with periodic boundary conditions. We are interested in the ground state energy per unit volume in the thermodynamic limit, with
N,L→∞ at fixed density
ρ=N/L3. We derive an upper bound for the ground state energy density, matching the famous Lee-Huang-Yang formula, up to lower order terms, in the dilute limit
ρa3≪1.