How do we know that quasars are not white holes?

Question

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.

Answer 1

White holes cannot exist in a universe with a finite age. As it happens black holes cannot exist either, though we expect there will be objects that are so similar to black holes that they cannot be distinguished from a true black hole.

So the simplest answer to your question is that quasars cannot be white holes since white holes don't exist. However there are other objections as well. The emission from a white hole is fundamentally unpredictable so we have no way of knowing what would come out of them. By contrast the emission from quasars is well predicted by models that describe them as supermassive black holes in galactic nuclei. While we cannot prove quasars are not white holes Occam's razor suggests this is unlikely.