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JonJackjon t1_j2n7xd0 wrote

I'm confused about the concept of the universe. When I was a kid we were told the universe was "everything" now is not quite everything but currently expanding size. However logically even nothing has dimensions. Seriously, could we go to the edge of the universe then stick a meter stick a little bit further.

(not serious) or is the edge of the universe an ice wall guarded by some galactic guardians?

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mfb- t1_j2n9ang wrote

> the universe was "everything"

Correct. It's expanding, but that's an independent statement.

> Seriously, could we go to the edge of the universe then stick a meter stick a little bit further.

There is no edge and no center either. On a large scale, the universe is the same in every place and every direction.

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jiggiwatt t1_j2narkx wrote

I've heard the analogy used that you can think of 3D space as flattened onto the surface of a balloon which is expanding. No matter which direction you take, you always end up back where you started (after a relativity breaking amount of time.

Edit: as the balloon expands, the actual material expands similar to how spacetime is expanding. Interestingly, at some point eons into the future, this expansion will make the milky way an island where everything else is so far away, any future civilization will never receive its light and will think our galaxy is all that exists in the universe.

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BlinkOnceForYes t1_j2p1qkv wrote

Expansion-wise, sure. But we haven't concretely proven the 'shape' of the universe. Would we end up at the other side? Or would we keep going infinitely far?

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Wroisu t1_j2p747l wrote

The 3D universe can be thought of as the surface of an expanding hypersphere. If the universe weren’t expanding, you could go all the way around and come back to where you started.

But since it’s expanding, you’ll never be able to move fast enough to come all the way back around again.

A “finite but unbounded universe”

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TheMace808 t1_j2puavw wrote

It’s not proven to have positive curvature like a balloon, at the largest scales we could measure the universe has no base curvature. It expands in every direction all at once like dots on a balloon but isn’t shaped like it as far as we know

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Wroisu t1_j2pureb wrote

the argument I’d give in return is that it only appears locally flat (local as in the entire observable universe) because the total thing is much larger than 93 billion light years across. Like if your entire observable universe was Kansas, but you didn’t know Kansas was part of a globe.

The margin of error for positive curvature is 0.4% so… within the limits of things that are known and possible.

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TheMace808 t1_j2puvor wrote

Yeah that’s why I said as far as we know we can think up and theorize many a thing but the evidence we have suggests it’s flat

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koebelin t1_j2py05v wrote

There’s probably an infinite number of areas of space like what we call “the universe” for trillions of light years in every direction, some expanding, some colliding, some contracting. This is one idea some people have.

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[deleted] t1_j2nb8ko wrote

[removed]

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Wroisu t1_j2p7dn0 wrote

Yes, but in the case that the universe is just the 3D surface of a hypersphere, it would also be expanding, expanding faster than you could move to come back all the way around again.

This is what Carl Sagan meant by “finite but unbounded”

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MiffedMouse t1_j2njpvw wrote

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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LJofthelaw t1_j2o9obx wrote

The observable universe is not infinite. And it keeps expanding such that more and more of it is not visible. But that's just because we're limited by time and the speed of light with respect to what we can see. If you went to the edge of the earth's observable universe you'd probably just find more universe. You'd be at the centre of a new observable bubble with the earth and a portion of the earth's observable universe bubble in one corner. As you move anywhere the "observable universe" moves with you. You take a step left and now the observable universe is one step larger to the left (though it's expanding faster than that so you wouldn't see anything new). Make sense?

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JonJackjon t1_j2qa7qb wrote

Yes thank you. The concept is much clearer now.

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Lumpy-Dingo-947 t1_j2q5bhb wrote

The speed of light is fixed. So when we look at light that is emitted from something that is moving away from us very fast it gets red shifted. When it’s moving towards us it gets blue shifted. Same idea if we’re the ones moving and seeing.

Also because light moves at fixed speed the light we see that’s far away was emitted a very long time ago. So we see things as they were when the light was emitted.

We can’t see infra red, but our cameras can. And we can make sensors that can see much lower frequencies than that. However some stuff is so far/long ago away that it gets shifted beyond any ability to measure and becomes cosmic background radiation that essentially acts as noise.

Some things will simply never reach us as long as the universe is expanding, and somethings are so shifted that they cannot be observed meaningfully.

We can infer a lot about the parts we cannot observe because there is a general spherical symmetry to the Big Bang.

But the edge is just the farthest anything from the Big Bang has gone that we can observe . We haven’t seen/understood evidence that there are things that weren’t from the Big Bang yet.

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ScootysDad t1_j2nid7p wrote

This is a trick question. Our current understanding of the universe is this: The portion of the universe that is visible to us is about 93-ish billion light years in diameter or about 28.5 gigaparsecs. Space is expanding at a rate of about 45mi (73km) for every megaparsec. Consequently, beyond the observable universe there are things racing away from earth (frame of reference) faster than light speed thus are part of our particle universe but forever disconnected from our reality. We will never know because any information emitted will never reach us.

I'm hopeful that one day we will devise the necessary physics to dwell into the edges of the universe much like the edges between us and the point of singularity of the black holes.

An interesting thought experiment, as I said earlier, there are things racing away from us at faster than light speed. From their frame of reference, we are receding from them at equal rate. So technically we're both right. We are going through the universe on a roller coaster traveling faster than light. "Make it so"

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Mars_rocket t1_j2pabn2 wrote

Furthermore, from the perspective of somebody 93 billion light years away the universe extends equally in all directions, including the direction opposite to that pointing at us. Therefore it must be infinite.

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EinsteinWasVegan t1_j2pzam9 wrote

In addition to what others have said, "nothing" doesn't have dimension.

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JonJackjon t1_j2q8ly0 wrote

Perhaps I'm looking at it wrong. And these concepts are a big stretch to my ability to comprehend.

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[deleted] t1_j2swk1z wrote

>now is not quite everything but currently expanding size.

Well, the space between stuff is what is expanding/being created constantly,the galaxies are stationary (kinda, not really.)

>Seriously, could we go to the edge of the universe then stick a meter stick a little bit further.

No.

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Bloodwolv t1_j2nz725 wrote

That's the cool thing about infinity. There is no end. You can always add 1 one more.

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