4 ms·
"as the Universe expands, photons lose energy. But that doesn't mean energy isn't conserved; it means that the energy goes into the Universe's expansion itself,
by invalidator 2y ago
"as the Universe expands, photons lose energy. But that doesn't mean energy isn't conserved; it means that the energy goes into the Universe's expansion itself, in the form of work."
-- https://www.forbes.com/sites/startswithabang/2015/12/19/ask-ethan-when-a-photon-gets-redshifted-where-does-the-energy-go/ https://www.forbes.com/sites/startswithabang/2015/12/19/ask-...
- btilly 2y agoThat only works if radiation density and expansion are connected. General Relativity says that they aren't.
- mnky9800n 2y agoHow does general relativity say that?
- btilly 2y agoThe term for the stress-energy tensor has the wrong impact on expansion for a simple relationship of any kind. You can rescue it with pseudo-tensors, but then you force the existence of a preferred reference frame. Which undermines the principles of GR. Some still argue for that. Others don't.
- deleted 2y ago[deleted]