Density

[tali1]

Any physicists out there who can explain/quantify in "easy peasy" terms - critical density and planck density?- i understand planck density is the most extreme form of density (i thought it was jade goody ! boom boom !)- what would planck density be equivalent to?

[Eurox]

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.
Whereas critical density determines whether the universe if open, flat or closed. We don't know this exact density yet, though.

[Clanad]

The Planck density is enormous. It corresponds to the mass of 100 billion galaxies being squeezed into a space the size of an atomic nucleus. If we could extrapolate general relativity all the way back to the big bang the universe would have gone from infinite density to the Planck density in roughly 10-43 seconds (which is, of course, Planck Time (Thanks to Inflation and the Early Universe)
As to Critical density, "...The average density of matter in the universe today that would be needed exactly to halt, at some point in the future, the cosmic expansion. A universe that has precisely the critical density it is said to be flat or Euclidean. If the density of the universe is greater than the critical density, then not only will the expansion be stopped but there will be a collapse of the universe in the distant future..." (Source: Encyclopedia of Science).

By the way, measurements of the density of the observable universe suggest that it does not contain sufficient density to result in the Big Crunch... but will continue to expand indeffinitely...

[Clanad]

Apologies, Eurox... you're post wasn't displayed when I clicked "submit"...

[jake-the-peg]

Hmmm

You want to be careful about the way you think about concepts like the Planck density.

We do not yet have the science to properly describe what was going on this early. The science of the very small (quntum mechanics) and that of the very massive (General Relativity) don't work properly together.

Certainly when we're talking about the mass of the Universe that early we are not talking about galaxies squeezed into a tiny space.

There was no matter as you and I would recognise it. No atoms, no nucleii, no protons quite possibly no quarks either - just a seething mess of fundamental particles popping in and out of existance. All in an entire universe billions of times smaller than an atom.

To describe such a situation with a word like density is to my mind at least rather misleading

[tali1]

Could planck density (assuming acceptance of theory)be anolagised as ,say, "a flea giving birth to a blue whale?" in my simplified understanding of it?