# How well do we know galactic geography?

**URL:** <https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658>\
**Category:** Factual Questions\
**Created:** [September 11, 2009, 7:05am UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658 "2009-09-11T07:05:14Z")\
**Posts on this page:** 3\
**Page:** 4

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**Author:** ![Sitnam](https://avatars.discourse-cdn.com/v4/letter/s/6a8cbe/32.png) [@Sitnam](https://boards.straightdope.com/u/Sitnam)\
**Post date:** [September 15, 2009, 4:13am UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/61 "2009-09-15T04:13:02Z")

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> [@The\_Hamster\_King](#):
>
> Galaxy formation occurred only in the matter-dense regions on the walls of the voids.

All the more support for my assertion that seeing fall galaxies constitutes ‘seeing the edges of the universe’.

> [@The\_Hamster\_King](#):
>
> And it’s not true that farthest visible galaxy is the “edge of the universe”. There are almost certainly many, many more galaxies (perhaps an infinite number) beyond the range of what we can see.

They’ve been providing light since the moment of their creation, we should be able to see them now. Please tell me the why you disagree on both of both of those.

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**Author:** ![toadspittle](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@toadspittle](https://boards.straightdope.com/u/toadspittle)\
**Post date:** [September 15, 2009, 1:12pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/62 "2009-09-15T13:12:35Z")

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> [@Sitnam](#):
>
> They’ve been providing light since the moment of their creation, we should be able to see them now. Please tell me the why you disagree on both of both of those.

Because, after inflation, the universe may have expanded to, say, 500 billion light-years (arbitrary number) in radius (…if “radius” truly has any meaning in this sense…). But light can only have traveled around 13 or 14 billion light years since the time of last scattering. Therefore, any galaxy 500 billion light years away, or 100 billion light years away, or 15 billion light years away, at that point in time has still not had enough time to send a photon to us. Get it?

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**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\
**Post date:** [September 15, 2009, 4:48pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/63 "2009-09-15T16:48:06Z")

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> [@Sitnam](#):
>
> All the more support for my assertion that seeing fall galaxies constitutes ‘seeing the edges of the universe’.

How is that? Can you explain your reasoning?

> [@](#):
>
> They’ve been providing light since the moment of their creation, we should be able to see them now. Please tell me the why you disagree on both of both of those.

\*\*Toadspittle \*\*has already answered, but I feel compelled to put my two cents in.

It’s easy to imagine the Big Bang like an explosion. Everything starts off at a point in space and goes flying off in all directions. Since nothing can travel faster than light, something that happens in one part of the explosion should be visible to all the other parts.

But that’s not a particularly good way to imagine the Big Bang. The Big Bang wasn’t an explosion IN space, but a rapid expansion OF space. And since the expansion of space isn’t limited by the speed of light, it’s possible (indeed, almost certain) for most of the universe to be carried away from our local neighborhood so rapidly that it’s outside our “light cone”: The light from those very distant galaxies hasn’t had time to reach us yet. (And, because the rate of expansion seems to be accelerating, it probably never will.)

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