# How do we know their is anything outside the Observable universe if we can't see it. **URL:** \ **Category:** Factual Questions\ **Created:** [August 6, 2009, 8:41am UTC](https://boards.straightdope.com/t/how-do-we-know-their-is-anything-outside-the-observable-universe-if-we-cant-see-it/505514 "2009-08-06T08:41:06Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![ChrisBooth12](https://avatars.discourse-cdn.com/v4/letter/c/e95f7d/32.png) [@ChrisBooth12](https://boards.straightdope.com/u/ChrisBooth12)\ **Post date:** [August 6, 2009, 8:41am UTC](https://boards.straightdope.com/t/how-do-we-know-their-is-anything-outside-the-observable-universe-if-we-cant-see-it/505514/1 "2009-08-06T08:41:06Z")
We gaze off into space and can see light, detect radition and red shift and all that great stuff, but how do we know that their is stuff in the unobservable universe? ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [August 6, 2009, 9:09am UTC](https://boards.straightdope.com/t/how-do-we-know-their-is-anything-outside-the-observable-universe-if-we-cant-see-it/505514/2 "2009-08-06T09:09:16Z")
There are some recent results that [mass outside the observable universe](http://www.newscientist.com/article/dn14814-galaxy-flow-hints-at-huge-masses-over-cosmic-horizon.html)is affecting the mass inside it. But mostly it’s the standard scientific presumption that we are not in a privileged position; that if we see stars and galaxies as far as we can see that means that there’s probably more we can’t see. Although some theories hold that far outside the observable universe there might be other things; exactly what depends on the theory. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [August 6, 2009, 1:26pm UTC](https://boards.straightdope.com/t/how-do-we-know-their-is-anything-outside-the-observable-universe-if-we-cant-see-it/505514/3 "2009-08-06T13:26:35Z")
The observable universe is the set of all points from which a ray of light could have travelled to the observer in the time given by the age of the universe. So, obviously, the observable universe is observer-dependent, a sphere centered on his location; points within one observer’s observable universe thus lie ‘outside’ of the observable universe of another observer. Of course, it might be that there exists a point that, during the lifetime of the universe, could have been reached by light rays from every other point within the universe, but it seems rather unlikely that if that point existed, we should be exactly there. On the other hand, it could also be the case that the observable universe is _larger_ than the actual universe – in that case, light rays from some points would have had the time to reach the observer on several different paths, circumnavigating the whole universe. We then would see multiple copies of astronomical structures as they appeared at the time the light was emitted. There’s [been a search](http://arxiv.org/abs/astro-ph/0310233) for such repetitions in the cosmic microwave background, which concluded that the universe must be larger than 78 billion light years in diameter (which is somewhat smaller than the diameter of the observable universe, which is around 93 billion light years); it is hoped that future data (currently being gathered by the Planck observatory) may push this boundary further up. ---
**Author:** ![EdwardLost](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@EdwardLost](https://boards.straightdope.com/u/EdwardLost)\ **Post date:** [August 6, 2009, 5:41pm UTC](https://boards.straightdope.com/t/how-do-we-know-their-is-anything-outside-the-observable-universe-if-we-cant-see-it/505514/4 "2009-08-06T17:41:12Z")
How did we know there was a back side of the moon before 1959? There was an interesting SF short story about that, but I don’t remember the title. ---
**Author:** ![Dahu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dahu/32/3416_2.png) [@Dahu](https://boards.straightdope.com/u/Dahu)\ **Post date:** [August 6, 2009, 7:03pm UTC](https://boards.straightdope.com/t/how-do-we-know-their-is-anything-outside-the-observable-universe-if-we-cant-see-it/505514/5 "2009-08-06T19:03:40Z")
> [@Half\_Man\_Half\_Wit](#): > > Of course, it might be that there exists a point that, during the lifetime of the universe, could have been reached by light rays from every other point within the universe, but it seems rather unlikely that if that point existed, we should be exactly there. This is the thing that I’ve never been able to understand. If the universe started from a point, then why can’t every point in the universe ‘see’ every other point? If they couldn’t, then wouldn’t that mean that during the expansion, it would have been expanding faster than the speed of light? ---
**Author:** ![Superfluous\_Parentheses](https://avatars.discourse-cdn.com/v4/letter/s/8edcca/32.png) [@Superfluous\_Parentheses](https://boards.straightdope.com/u/Superfluous_Parentheses)\ **Post date:** [August 6, 2009, 7:23pm UTC](https://boards.straightdope.com/t/how-do-we-know-their-is-anything-outside-the-observable-universe-if-we-cant-see-it/505514/6 "2009-08-06T19:23:40Z")
> [@Dahu](#): > > This is the thing that I’ve never been able to understand. If the universe started from a point, then why can’t every point in the universe ‘see’ every other point? If they couldn’t, then wouldn’t that mean that during the expansion, it would have been expanding faster than the speed of light? It’s my understanding that that’s exactly what’s current theories claim has happened. IOW, space can expand faster than the speed of light. [Expansion of the universe - Wikipedia](http://en.wikipedia.org/wiki/Metric_expansion_of_space) ---
**Author:** ![Spatial\_Rift\_47](https://avatars.discourse-cdn.com/v4/letter/s/a8b319/32.png) [@Spatial\_Rift\_47](https://boards.straightdope.com/u/Spatial_Rift_47)\ **Post date:** [August 6, 2009, 7:26pm UTC](https://boards.straightdope.com/t/how-do-we-know-their-is-anything-outside-the-observable-universe-if-we-cant-see-it/505514/7 "2009-08-06T19:26:19Z")
Just to add to what **Superfluous Parentheses** said: If you’re thinking “but nothing can travel faster than the speed of light!” then you are thinking of matter and energy. There is no such speed limit on spacetime, which is the actual agent of expansion. ---
**Author:** ![HorseloverFat](https://avatars.discourse-cdn.com/v4/letter/h/8e8cbc/32.png) [@HorseloverFat](https://boards.straightdope.com/u/HorseloverFat)\ **Post date:** [August 6, 2009, 8:01pm UTC](https://boards.straightdope.com/t/how-do-we-know-their-is-anything-outside-the-observable-universe-if-we-cant-see-it/505514/8 "2009-08-06T20:01:21Z")
Because we can see orbits and other indirect signs of large invisible objects out there. Our models only make sense if there are black bodies and things we cant see affecting everything else. I guess someone could argue that there’s some undiscovered form of matter or some exotic force causing this, but that seems unlikely/occam’s razor. ---
**Author:** ![smoke](https://avatars.discourse-cdn.com/v4/letter/s/c5a1d2/32.png) [@smoke](https://boards.straightdope.com/u/smoke)\ **Post date:** [August 7, 2009, 1:07am UTC](https://boards.straightdope.com/t/how-do-we-know-their-is-anything-outside-the-observable-universe-if-we-cant-see-it/505514/9 "2009-08-07T01:07:47Z")
> [@HorseloverFat](#): > > Because we can see orbits and other indirect signs of large invisible objects out there. Our models only make sense if there are black bodies and things we cant see affecting everything else. I guess someone could argue that there’s some undiscovered form of matter or some exotic force causing this, but that seems unlikely/occam’s razor. I’m not at all an expert, so maybe I’m getting whooshed here, but isn’t that exactly what dark matter and dark energy is postulated to be? ---
**Author:** ![Wendell\_Wagner](https://avatars.discourse-cdn.com/v4/letter/w/8491ac/32.png) [@Wendell\_Wagner](https://boards.straightdope.com/u/Wendell_Wagner)\ **Post date:** [August 7, 2009, 1:58am UTC](https://boards.straightdope.com/t/how-do-we-know-their-is-anything-outside-the-observable-universe-if-we-cant-see-it/505514/10 "2009-08-07T01:58:04Z")
How do I know that the light turns off in my refrigerator when I shut the door? I can’t see it happen. Similarly, how can we know that there is anything outside the observable universe, since if we could know of any such thing, it would be observable? ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [August 7, 2009, 10:56am UTC](https://boards.straightdope.com/t/how-do-we-know-their-is-anything-outside-the-observable-universe-if-we-cant-see-it/505514/11 "2009-08-07T10:56:14Z")
> [@Wendell\_Wagner](#): > > Similarly, how can we know that there is anything outside the observable universe, since if we could know of any such thing, it would be observable? Because as said, it affects that which IS observable. And because theory predicts it.