# Is the (Mass + Energy) of the Universe constant

**URL:** <https://boards.straightdope.com/t/is-the-mass-energy-of-the-universe-constant/637783>\
**Category:** Factual Questions\
**Created:** [October 15, 2012, 3:47pm UTC](https://boards.straightdope.com/t/is-the-mass-energy-of-the-universe-constant/637783 "2012-10-15T15:47:30Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\
**Post date:** [October 15, 2012, 3:47pm UTC](https://boards.straightdope.com/t/is-the-mass-energy-of-the-universe-constant/637783/1 "2012-10-15T15:47:30Z")

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From the big bang to the present - has the mass + energy of the universe changed ? I understand that there could be dark matter etc. etc. but as far as current Physics goes - is mass + energy essentially constant as the universe expands ?

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [October 15, 2012, 11:27pm UTC](https://boards.straightdope.com/t/is-the-mass-energy-of-the-universe-constant/637783/2 "2012-10-15T23:27:53Z")

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Probably not. Kinetic energy gets damped out by something called “cosmological drag”: Basically, if you’re moving at some speed, eventually you’ll end up in a region of the Universe where that speed is at rest. Meanwhile, the dark energy content of the Universe is continually increasing, since it has (or at least, appears to have) a constant energy _density_, but the volume of the Universe is increasing. One could contrive a universe where these two effects canceled each other out, but that wouldn’t be our Universe.

And no, this is not a violation of the Law of Conservation of Energy. That’s a local law, and need not apply globally.

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**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\
**Post date:** [October 16, 2012, 4:47am UTC](https://boards.straightdope.com/t/is-the-mass-energy-of-the-universe-constant/637783/3 "2012-10-16T04:47:18Z")

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Thanks Chronos for the reply. Since dark energy is a part of our universe and since it is continually added - is it fair to say that the parts of our universe did not start at the big bang (vis-a-vis the newly added dark energy) ?