It seems to me that extra gravitational potential energy is created as the universe expands and the distance between massive objects such as galaxy clusters increases; this implies that energy is not conserved in the universe. Is that right?
Answer
That's actually a tricky question.
The short answer to the title question is yes, it does. But the answer to the follow up question about conservation is, it is still conserved.
In a much simpler universe, what hwlau said would be true - as the gravitational potential energy increases, the kinetic energy decreases. But we do know through the Hubble telescope that this is not the case. As the universe expands, the planets and dust and stars accelerate away from each other.
The answer is only hypothetical for now, which is dark energy. Here's the wikipedia page on it. The problem is, dark energy is very unknown. I'm not sure how this force will be able to counteract gravity. However, dark energy only manifests itself through gravity, and not through the other 3 fundamental forces, so there is definitely some relationship there. Dark energy can possibly negate the increased potential energy through some mechanism, but how it does that exactly is also unknown.
There's little I can give you other than the wikipedia link above, but notice when you open it, how the only explanations available are evidence arguing for dark energy's existence, what properties it should have, and other explanations for the expansion aside from dark energy (very interesting part). There is no explanation as to its mechanism, except that it causes things to accelerate against each other.
It's a good question. There is simply no good, accurate answer at the moment.
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