That's what einstein said. There is no fixed reference frame, but only relative ones. Every "inertial"(meaning, motion without any external force) frame of reference is equally valid as any other inertial frame movibg with respect to it.

But for sure we can tell earth's orbit is not inertial since circular motion occur, which is due to external force of gravity.

Edit:typo

[–] Ymer@feddit.dk 2 points 1 day ago (11 children)

Shouldn't it be (at least theoretically) possible to find some sort of geometric center where - on average - the rest of the universe is expanding away from?

[–] BrainInABox@lemmy.ml 6 points 1 day ago (3 children)

Turns out, no; every point is expanding away from every other point, so every point sees itself as the center of expansion.

[–] Ymer@feddit.dk 2 points 1 day ago (2 children)

That could sort of explain why it's inherently impossible to determine the center - but that doesn't rule out the existence of a geometric center of the universe, right?

[–] BrainInABox@lemmy.ml 3 points 23 hours ago

Kiiiind of. I suppose we haven't definitively ruled out a geometric centre, but it runs up against Occam's Razor in a really big way. A centre of the universe would require some kind of boundary or edge to the universe, and the physical dynamics of how that would even work are very much non-trivial.

Generally the universe is thought to either loop back on itself like the surface of a globe, or extend infinitely in every direction.

[–] mexicancartel@lemmy.dbzer0.com 2 points 23 hours ago

For a geometric center you would need a boundary of the universe

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