Planck density

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

$\rho_\text{P} = \frac{m_\text{P}}{l_\text{P}^3} = \frac{c^5}{\hbar G^2}$ ≈ 5.1 × 10⁹⁶ kg/m³

where:

m_P is the Planck mass

l_P is the Planck length

c is the speed of light in a vacuum

$\hbar$ is the reduced Planck constant

G is the gravitational constant

This is a unit which is very large, about equivalent to 10²³ solar masses squeezed into the space of a single atomic nucleus. 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.