# How do we know the mass of the universe

**URL:** <https://boards.straightdope.com/t/how-do-we-know-the-mass-of-the-universe/825826>\
**Category:** Factual Questions\
**Created:** [December 8, 2018, 11:28pm UTC](https://boards.straightdope.com/t/how-do-we-know-the-mass-of-the-universe/825826 "2018-12-08T23:28:05Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\
**Post date:** [December 8, 2018, 11:28pm UTC](https://boards.straightdope.com/t/how-do-we-know-the-mass-of-the-universe/825826/1 "2018-12-08T23:28:05Z")

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Apparently there is a visible event horizon to the universe which we cannot see past.

> [@Physicist](#):
>
> Within the limit of precision to which we can gauge the curvature of the Universe, it is infinitely large i.e. zero curvature is within the range of uncertainty. At the upper limit of curvature, the Universe must be at least several hundred times larger than what we can observe. That means that there are far more, maybe infinitely more, galaxies that we cannot see than that we can see.

If we cannot be sure how many more galaxies there are (and if there may be infinitely more), how do we determine the mass of the universe?  
.

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**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\
**Post date:** [December 8, 2018, 11:32pm UTC](https://boards.straightdope.com/t/how-do-we-know-the-mass-of-the-universe/825826/2 "2018-12-08T23:32:36Z")

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The quantity quoted for mass is the “observable universe”, the above statement is extrapolation on what the _minimum_ size of the part we cant see is.

There is less need to consider anything outside of the “observable universe” in most cases and so “observable universe” is shortened to “universe.”

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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:** [December 8, 2018, 11:37pm UTC](https://boards.straightdope.com/t/how-do-we-know-the-mass-of-the-universe/825826/3 "2018-12-08T23:37:14Z")

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We don’t know the mass of the Universe. Where did you read that we do?

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**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\
**Post date:** [December 9, 2018, 2:17am UTC](https://boards.straightdope.com/t/how-do-we-know-the-mass-of-the-universe/825826/4 "2018-12-09T02:17:03Z")

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> [@Chronos](#):
>
> We don’t know the mass of the Universe. Where did you read that we do?

This is not my area of expertice obviously so if I have confused some aspects of it please excuse my ignorance, but I thought the way that we judge how the universe will end is by comparing the rate expansion with how much mass there is and how it’s affected by gravity.

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**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\
**Post date:** [December 9, 2018, 2:28am UTC](https://boards.straightdope.com/t/how-do-we-know-the-mass-of-the-universe/825826/5 "2018-12-09T02:28:05Z")

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> [@Lucas\_Jackson](#):
>
> Apparently there is a visible event horizon to the universe which we cannot see past.
> 
> If we cannot be sure how many more galaxies there are (and if there may be infinitely more), how do we determine the mass of the universe?

You didn’t link to [the source](https://www.quora.com/The-universe-is-expanding-and-galaxies-are-disappearing-from-our-horizon-Do-you-think-that-we-have-already-missed-galaxies-that-have-surpassed-our-horizon-like-an-elephant-in-the-room-that-we-cant-see).

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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:** [December 9, 2018, 3:44am UTC](https://boards.straightdope.com/t/how-do-we-know-the-mass-of-the-universe/825826/6 "2018-12-09T03:44:42Z")

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It’s the density that matters for the fate of the Universe, not the total mass.

Though actually, it turns out that the density of mass in the Universe doesn’t seem to matter, either, because it’s dwarfed in its effect by the dark energy (whatever the heck that is).