Wednesday 27 September 2017

Are black hole regarded as baryonic matter in cosmology?


Black holes do not have any property as baryonic or non-baryonic matter, as is stated by the no-hair theorem. Regarding conventional theories, in different models of cosmology, like Lambda-CDM, are black holes regarded as baryonic matter, and does the black hole matter significantly affect the baryonic density?



Answer



Baryonic doesn't literally mean baryonic. It's a label of convenience for something that behaves in a certain way. The main evidence for dark matter comes from big bang nucleosynthesis (BBN), and from the fact that we need it in order to reconcile the observed rotation curves of galaxies with the observed strength of the CMB fluctuations. In the BBN era, presumably we didn't have any black holes, so the issue doesn't arise. If we want to explain a galaxy's rotation curve, then a black hole is exactly the same as any other star, so it would presumably go on the census as baryonic matter, but in any case black holes are a negligible fraction of baryonic matter, and they always will be. (There is a common misconception that all matter will end up in black holes, which simply isn't true.)


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