This topic has come up in other physics threads, but I don’t see that it has its own dedicated thread. My knowledge on this topic is having taken 2 semesters of undergraduate level physics, reading various popular physics books, and listening to various physics podcasts such as the former Daniel and Jorge Explain the Universe, now Daniel and Kelly’s Extraordinary Universe.
As I understand it, according to Noether’s theorem all the conservation laws have a corresponding symmetry. In the case of the law of conservation of energy, the corresponding symmetry is time, meaning that an experiment conducted now should yield the same results as one conducted 12 billion years ago, or 12 trillion years from now, and so on. But isn’t it the case that both of these are in fact not true? Meaning that as best as I can understand, the laws of physics have changed, at least in the sense that the rate of expansion of the universe is changing. In particular I have read that the rate of expansion sped up significantly around 5 billion years ago. On the flip side, it seems like energy is constantly being created by the expansion of the universe, since the dark energy density stays the same while the total volume increases, meaning we have more dark energy as time goes on. We also seem to lose energy with the red shifting of light as time goes on. As best as I can tell from what I’ve read, those two numbers don’t offset each other. If this is the case, doesn’t that mean that time is asymmetric and that energy is not conserved?
I am not a physicist, but as I understand it, the rate of expansion is changing because the properties of the universe are changing (mostly that property being size, because the universe is expanding), and is it changes different forces become predominant in their interactions, but the same forces were acting on it all along.
An object falling towards the Earth from space has the speed at which it falls change over time; even the acceleration it experiences imperceptibly changes as it comes closer to the Earth and feels the pull of gravity more strongly. Then, when it gets very close to Earth, the acceleration changes radically again when it slams into the atmosphere. This isn’t happening because the laws of physics changed, but because the object is in different situations over time.
My understanding is that this isn’t because the laws of physics changed, but because before this point the universe was closely packed together enough that gravity held matter somewhat closer together; but past a critical point, dark energy’s input (which grows with distance, while gravity falls off quadratically) overtook gravity.
There was the same amount of dark energy per volume of space, but the volume of space was much smaller.
Energy is neither created nor destroyed in a closed system, and the universe seems to behave as if it is not a closed system, when it comes to dark energy. I don’t think we understand it well enough to say that it’s being “created”, as opposed to coming from somewhere else, or just being a measurement artifact that represents the space of the universe itself expanding.
As I understand it the expansion of the universe is negative potential energy, and in the right circumstances that can result in a balancing amount of positive energy being created. Such as in the early universe, when it underwent rapid inflation and filled with the energy that condensed into particles then into the matter of the present universe.
As for the asymmetry of time, also known as “the arrow of time” that’s an unsolved question in physics.