Planck density

In a tiny sliver of time just after the Big Bang, she knew, matter was thickly packed at the Planck density.

The Planck density is the unit of density, denoted by ρ, in the system of natural units known as Planck units.

At one unit of Planck time after the Big Bang, the mass density of the universe is thought to have been approximately one unit of Planck density.

The Planck density, the derived unit of density in the Planck unit system (1 Planck mass divided by 1 Planck volume)

According to theories, at some point later, the collapsing object will reach the maximum possible energy density for a certain volume of space or the Planck density (as there is nothing that can stop it).

The natural scale is naïvely the Planck scale so this small value could be seen as another form of fine-tuning (called a hierarchy problem): the energy density given by the scalar potential is down by 10 compared to the Planck density.

The ultra-high-energy cosmic rays observed in 1991 had a measured energy of about 50 joules, equivalent to about 2.5x10 E. Most Planck units are fantastically small and thus are unrelated to "macroscopic" phenomena (or fantastically large, as in the case of Planck density).