The Ultimate Energy Density of Observable Cold Matter
  We demonstrate that the largest measured mass of a neutron star establishes an upper bound to the energy density of observable cold matter. An equation of state-independent expression satisfied by both normal neutron stars and self-bound quark matter stars is derived for the largest energy density inside stars as a function their masses. The largest observed mass sets the lowest upper limit to the density. Implications from existing and future neutron star mass measurements are discussed. 
James M. Lattimer Madappa Prakash