White holes are speculative in nature. Black holes are known to exist (supermassive black holes in galactic nuclei and stellar-mass black holes in binaries).
My question: if white holes are expected to be very luminous objects, how do we know theoretically that quasars are not actually powered by white holes?
And observationally, is this possibility definitively ruled out for all (or any) quasars and/or are there constraints for the possibility?
This question seems related, but the questions asked there are different than here, I think.