Physical Significance of the Planck Density


Solution 1

This is the density at which the Universe can no longer be described without quantum gravity. People often loosely refer to the big bang starting with infinite density but more accurately we should start it at the Planck density as beyond that we don't have the equations to model the Universe's evolution.

Solution 2

Planck's density can also be interpreted as the matter density for which the spacetime torsion field induced by fermions fields becomes important, in General Relativity (or more precisely under the Einstein-Cartan-Sciama-Kibble theory). At Planck's density, torsion should kick-in, even in classical General Relativity.

Solution 3

Its main use is really just to be a unit based on absolutes. The kilogram is based off of a weight in Paris. This can change over time. Planck units are based off of constants. It doesn't have any physical significance in of itself, but it is useful.

Solution 4

Planck density - density of hypothetical Planck Star or central object of BH present there instead of singularity.


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Updated on August 01, 2022


  • Eriek
    Eriek over 1 year

    The Planck density is the Planck mass devided by the Planck volume, approximately 1093 g/cm3.

    Does this quantity have any known physical relevance? The Planck mass is believed to be the smallest black hole possible, and the Planck length is believed to be the smallest meaningful length. So what about the Planck density?

    • Shing
      Shing about 8 years
      probably no one knows.... to my knowledge it is in the realm of quantum gravity, which is still under development.
    • Admin
      Admin almost 8 years
      thought to approach mass ( energy ) density at the Planck time
    • valerio
      valerio about 7 years
      My guess? No meaning at all. This has probably more to do with numerology than with physics...
  • Kyle Kanos
    Kyle Kanos over 6 years
    I think you have it backwards: a Planck star is a star whose density is approximately the Planck density, which I don't think really helps much here.