# Edge of our universe

**URL:** <https://boards.straightdope.com/t/edge-of-our-universe/989164>\
**Category:** Factual Questions\
**Created:** [August 26, 2023, 11:31pm UTC](https://boards.straightdope.com/t/edge-of-our-universe/989164 "2023-08-26T23:31:44Z")\
**Posts on this page:** 3\
**Page:** 4

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**Author:** ![Left\_Hand\_of\_Dorkness](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/left_hand_of_dorkness/32/7156_2.png) [@Left\_Hand\_of\_Dorkness](https://boards.straightdope.com/u/Left_Hand_of_Dorkness)\
**Post date:** [September 1, 2023, 7:06pm UTC](https://boards.straightdope.com/t/edge-of-our-universe/989164/61 "2023-09-01T19:06:57Z")

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My question may be wholly ignorant, substituting metaphor for physics, and if so, I apologize in advance.

There’s a flat pool of water. A drop falls into it, and ripples begin to radiate outward from the drop. Little bugs living on the surface can’t travel faster than those ripples; the expanding ripples comprise their entire universe.

1. If we submit that the ripples comprise space, does that function as a two-dimensional metaphor for our universe?
2. Is it coherent with the metaphor to imagine that other drops would fall into the pool elsewhere and create their own expanding ripples?
3. Is it possible that eventually the ripples will intersect?

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**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\
**Post date:** [September 1, 2023, 7:55pm UTC](https://boards.straightdope.com/t/edge-of-our-universe/989164/62 "2023-09-01T19:55:46Z")

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I guess it is not a good analogy? Because the unstated cosmological assumption here is of a homogeneous, isotropic universe

> **[Cosmological principle](https://en.wikipedia.org/wiki/Cosmological_principle)**
>
> Is the universe homogeneous and isotropic at large enough scales, as claimed by the cosmological principle and assumed by all models that use the Friedmann–Lemaître–Robertson–Walker metric, including the current version of the ΛCDM model, or is the universe inhomogeneous or anisotropic? In modern physical cosmology, the cosmological principle is the notion that the spatial distribution of matter in the universe is equally distributed and isotropic when viewed on a large enough scale, since the fo...

so no ripples; there is just a scale factor

> **[Scale factor (cosmology)](https://en.wikipedia.org/wiki/Scale_factor_(cosmology))**
>
> The relative expansion of the universe is parametrized by a dimensionless scale factor 
>   
>     
>       
> a
>       
>     
> {\\displaystyle a}
>   
> . Also known as the cosmic scale factor or sometimes the Robertson Walker scale factor, this is a key parameter of the Friedmann equations.
> In the early stages of the Big Bang, most of the energy was in the form of radiation, and that radiation was the dominant influence on the expansion of the universe. Later, with cooling from the expansion the ...

measuring the expansion (so doubling it stretches distances by a factor of 2, for instance).

I guess by “ripple” you in fact mean the entire universe… if you had, let’s say, two disconnected pieces (“ripples”), off the top of my head they could not at some point merge together (or, in the other direction, the universe expand in such a way that it split in two pieces).

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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:** [September 2, 2023, 1:22pm UTC](https://boards.straightdope.com/t/edge-of-our-universe/989164/63 "2023-09-02T13:22:15Z")

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It is possible that two planets, widely separated in the Universe, might have two different observable universes, that start off separate but which eventually, after some amount of time, overlap. In fact, this inevitably happens all the time. But each of those “boundaries” would have no significance whatsoever for anyone but a resident of the planet at its center. In particular, if you were at the point where those two spheres were intersecting (and everyone is at such a point, for some pair of spheres), you wouldn’t notice anything at all about it.

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