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MiffedMouse t1_j2njpvw wrote

Reply to comment by JonJackjon in How do galaxies move? by modsarebrainstems

A note on the “size” of the universe - you will see articles referencing the size of the “observable” universe. This is just the bit of the universe that we can see. As /u/mfb- says, there is currently no known “edge” to the universe.

However, the current “observable” universe is likely the most of the universe we will ever see. This is because the universe is expanding. After a certain distance, things are moving away from us faster than the speed of light. Those objects are unreachable to us now (and probably forever). You can think of it like a moving walkway that is moving faster than a person can run. Even if you run full tilt against the moving walkway, you will not reach the other side.

So there is no “edge” in the sense of a wall or something, but there is a limit to what we can see and (as far as current models predict) a limit to what we will ever see.

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mfb- t1_j2o8rpd wrote

> After a certain distance, things are moving away from us faster than the speed of light. Those objects are unreachable to us now

That's a common misconception. We see things where the distance to them always increased faster than the speed of light. The matter that emitted the CMB we see today is an example. The number of things we can see is still increasing as the universe gets older.

In the distant future, in a universe completely dominated by dark energy, your statement will be right.

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mahoagie t1_j2qe98p wrote

Please say more about dark matter- in its universe, why would that be true?

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mfb- t1_j2rbdcl wrote

Dark energy (which I discussed) and dark matter (which I didn't) are completely different things.

A universe with only dark energy (or where everything else is negligible) expands exponentially, i.e. if you follow the distance between two objects over time then this distance increases exponentially. It has a constant expansion rate. If you emit light at a distance where the distance increases at the speed of light then the light will always keep that distance - the expansion perfectly matches the speed of the light, and the expansion rate doesn't change so the light will never come closer.

In our universe, where matter still plays a role (~3/4 dark energy, 1/4 matter today), the expansion rate is decreasing a bit. Light emitted at the same distance of "light speed distance increase" doesn't get closer to us today, but it will start getting closer "tomorrow" (will take hundreds of millions of years before this is significant, of course).

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MiffedMouse t1_j349be8 wrote

The ant on a rubberband example does not work for the universe. Even if the Hubble constant was constant with time, the universe expands exponentially, not linearly (so the “universe” rubberband length goes 1,2,4,8; not 1,2,3,4). An ant on an exponentially growing rubberband cannot reach everywhere.

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mfb- t1_j34a80w wrote

For the past 10 billion years a linear expansion was a pretty decent approximation. The early universe slowed its expansion, which makes the relative reach of the ant even larger (or, equivalently, the early recession speeds were larger).

The Hubble rate is still decreasing. It's expected to approach a constant in the future. I covered that in the second paragraph:

> In the distant future, in a universe completely dominated by dark energy, your statement will be right.

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