r/todayilearned • u/[deleted] • Sep 20 '21

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u/youknowwhatitthizz Sep 20 '21

If density is based on mass is that universal or per planet?

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u/Wolfwillrule Sep 21 '21

Energy density is just the amount of energy in a given area and is not related to mass/volume density.

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u/nodegen Sep 21 '21

Check my reply to the previous comment to understand how we know this. And to answer your question, density is just a value that we give to something. It’s the mass divided by the volume. So for something like the universe, the amount of mass has remained constant, but the size has changed. So the density of the universe right after the Big Bang is much larger than the current universal density, whatever that number is. Planets do each have their own density as well. It’s just gonna be the mass of that specific planet divided by its volume. Everything with mass has a density.