If the universe is expanding, but the amount of matter in it remains constant(ish), does that mean the ‘average density of the universe’ is decreasing?

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Not sure this question makes a ton of sense period, let alone from an actual physics standpoint. But in general terms, is this a valid question and if so, what’s the answer and its effects?

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Yes.

Eventually the matter in the universe will become so diffuse it will lead to complete heat death.

Very good question! The short answer: Yes. Since it’s expanding, the point at which matter seems to flee faster than the speed of light will affect stars closer and closer to us. Our neighbors flee farther and farther until we’re pretty much alone. The far future is not gonna look anything like our night sky. No galaxies, no distant starts…. And future generations won’t be able to study them, even though the technology would probably allow them to do crazy experiments 🙁

Yes, the universe is becoming more diffuse, in mass and in energy. If you measure energy density as a temperature, it’s only a few degrees (Kelvin) on average now. It dropped very rapidly during the big bang and initial inflation of the universe and has been declining more slowly ever since.

Yes, it’s actually the beginning of a theory for the ultimate fate of the universe called “The Big Rip”. The idea that the universe keeps expanding until the distance between even sub atomic particles becomes infinite.