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Comment by winter_blue | Hacker News Reader

> matter in the universe isn't slowing down in spreading apart, but accelerating

How come this does not violate the law of conservation of energy which says there's a fixed amount of energy in the universe? For matter to accelerate would require energy right? Where is the energy fueling this universal acceleration coming from? According to Newton's laws, doesn't there also need to be an external actor causing this acceleration? (Sorry if this sounds like a basic question--I don't know much physics besides some required intro courses I took for my CS major).

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Energy conservation is a consequence of time-translation symmetry (by Noether’s theorem), which is broken in general relativity. Hence, broadly speaking, energy is not conserved in general relativity.

https://www.preposterousuniverse.com/blog/2010/02/22/energy-...

"How come this does not violate the law that says there's a fixed amount of energy in the universe?"

It may. It is currently not clear if conservation of energy holds at the cosmological scale.

There is no matter being accelerated but space itself is expanding, which is why distant objects may accelerate beyond the speed of light, atleast that is what it looks like.

Wow, that's new to me. My understanding has always been that space is just the absence of matter. The ideally of null/void expanding is strange, to say the least.

Do you have recommendations for any books or resources online that break this down, and explain it in an ELI5 way?

I don't know any good resources/books on the topic, I've learned most of it via the education system with some additional background in internet.

There isn't much of a good analog from daily life, but essentially as you said, the null/void itself is becoming more. Two objects with a certain distance between eachother will find this distance growing even if they are not in relative motion in reality, though from either's perspective, the other is moving away at increasing speeds (due to more space inbetween expanding). Eventually the distance will be large enough that the space created between the two objects will exceed the distance that light could have travelled; the object becomes superluminal, or faster-than-light. This is allowed because it's not actually the object moving but the coordinate system they are in moving. With some luck you can still observe these objects.

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