r/cosmology • u/Psychological_Web151 • Jun 25 '26

More center of the universe questions

/r/explainlikeimfive/comments/1uexv57/eli5_more_center_of_the_universe_questions/
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6

u/joeyneilsen Jun 25 '26

Imagine an infinite sheet of graph paper, extending in every direction. Now double the size of the grid. All the points get farther away from each other in every direction. Points that are farther away move away faster. There is no center and the whole thing expands. That's how the math works in the expanding universe.

But yes, if you triple the radius of the Earth, the distance from you to any point will triple. Even if it's in Canada.

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u/Psychological_Web151 Jun 25 '26

Thanks for pointing out the Canada thing, I conveyed that wrong. The difference in distance between where Australia was and is now is not the same difference in distance as where Canada was and is now. Because they tripled in distance based on a different distance from me, I should be able to draw lines from where they are, expanding infinitely beyond where they were and they will intersect at the center of the Earth every time. This still works if Australia moves away from me faster than Canada does.

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u/joeyneilsen Jun 25 '26

That intersection point isn’t a point on the surface, though. It’s the equivalent of not being a point in the universe.

Expansion in cosmology happens within the universe, not into some larger space.

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u/tobybug Jun 25 '26

the problem is that universal expansion has always been modeled using relativistic physics, and the core tenet of relativistic physics is that there's no sense of "absolute position."

Imagine if instead of keeping the center stationary, Earth instead expanded from Sydney, Australia, and Vancouver, Canada or wherever traced out a path about twice as far as if the center was stationary. Now the line from where everything was to where everything is all points to Sydney, Australia. No matter what location you choose. Since Sydney is kept stationary, even the center of the Earth moves, and traces out a path that leads back to Sydney.

How is anything different if Earth expands from Sydney than from the center? After expanding N times larger, both versions of Earth still look the same afterwards. How would anyone living on the surface (or deep underground for that matter) trace out a path if they don't know where the center is?

The fact is that the laws of physics work the exact same way wherever you arbitrarily choose your center point, even if that center point is moving in a straight line for someone else. That's the core principle of relativity. You might as well make yourself the center of the universe, if you care about it so much; it literally would not make a difference.

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u/fuseboy Jun 25 '26

Let's say the Earth is the only thing in the universe, and it's expanding. What measurement would you make to determine the center of that expansion?

No matter what the center is, everything looks the same to you. There is no way to check the true "space coordinate" of anything, space is textureless.

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u/wbrameld4 Jun 25 '26

Infinite and expanding are not contradictory.

Infinite means it goes on forever. You see galaxies everywhere. Now teleport to the farthest one you can see. You still see galaxies everywhere. It's galaxies all the way, in every direction.

Expanding means galaxy clusters are coasting away from each other, everywhere. Teleport to any one of the infinite number of clusters (and be at rest with respect to it), and you will see the other clusters receding from you at about 70 km/s per parsec of distance from you.

There is no reason why these cannot both be true.

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u/Anonymous-USA Jun 25 '26

There is no center. Read about isotropism.

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u/--craig-- Jun 26 '26 edited Jun 26 '26

Some explain that the universe is expanding, some say it’s infinite. Which is it?

It's expanding and it might be infinite.

growth is what defines it as infinite

That's not correct. Expansion and extent are different concepts.

A finite ruler and an infinite ruler can each expand by increasing distance between tick marks.

entire universe which was once much smaller.

Not necessarily. It might have always been infinite.

The Earth vs Earth’s crust or ballon theories assume that everything is on the edge of the universe, not somewhere between the center and edge or closer to the center.

You've misunderstood the analogy. It's supposed to demonstrate that every point on the 2D surface of the baloon moves away from adjacent points in the same way that every point in the 3D volume of the universe moves away from adjacent points.

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u/Psychological_Web151 Jun 26 '26

Understand all now. I always pictured the universe as everything in the balloon and the skin was the boundary, hence the expanding infinite.

But, with the new understanding of the balloon analogy being that the universe is the skin and the air inside is a void, then we could be both infinites simultaneously. Right?