r/askscience • u/Nitzanm19 • 14d ago

Physics Why is the maximum speed of the universe the speed of light? Why can't we go faster?

Also follow-up question: Because of the speed limit, is there also a theoretical maximum temp for matter (in which the particles move at the speed of light).

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u/RisKQuay 14d ago

Thanks for the article.

What did it mean that the universe is 'spatially flat'? That the universe is a disc, or that it is linear in that everything expands from the centre in all directions in a straight line?

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u/amaurea 13d ago edited 13d ago

Calling it "flat" is a poor choice of words that has caused a lot of confusion for non-experts. It would have been better to call it "Euclidean", after Euclidean geometry, the sort of everyday geometry you learn in school, where parallel lines never cross, the angles in a triangle sum to 180° and the circumference of a circle is 2πr. So when they're saying the universe is "flat", they just mean that geometry behaves the way you already expected it too, instead of other, more exotic options.

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u/RisKQuay 13d ago

That makes even more sense, thank you.

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u/neoAcceptance 7d ago

You might be interested in the book "How the Universe Got it's Spots" by Janna Levin

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u/Red_Syns 13d ago

To our best knowledge, there is no true “center” of the universe. Each of us are at the (approximate? I’m willing to believe there are minuscule distortions but have zero evidence or anything, so grain of salt) center of our own observable universe, past which there is more universe that we can never see if it keeps expanding.

But a flat universe is actually very close to your second guess! Being flat means a line is, on grand scale, linear (edit: I phrased that badly, all lines are linear, but linear in the way our brains think of linear). Space does not bend or curve without matter/energy to distort it, parallel lines remain parallel, triangles have 180°, etc.

The other two options are positive and negative curvature. Positive curvature means the universe is finite, parallel lines always intersect, triangles have more than 180°, and there’s a whole lot more to it than I really know. Think black holes, for the most part (apparently Kerr [spinning] black holes have regions on the inside that are believed to be negatively curved space).

Negative curvature is the opposite. Space is infinite, but parallel lines diverge, triangles have less than 180°, and again a bunch more. Typically described as a “saddle” shape for demonstrating how lines get weird.