Where does a black hole lead to12/27/2023 ( Credit: Andrew Hamilton/JILA/University of Colorado) All the mechanisms by which normal matter can wind up creating a black hole fail when applied to dark matter.Įven for a complicated entity like a massive, rotating black hole (a Kerr black hole), once you cross the (outer) event horizon, even if you’re made of dark matter, you’ll fall towards the central singularity and add to the black hole’s mass. And there are no stellar remnants or other comparably dense objects made out of dark matter, meaning that there’s no way to have a collision between dark matter-rich entities that lead to a black hole. There are no regions that ever collect enough dark matter to pull light within that region back on itself, meaning that you can’t have dark matter undergo direct collapse. There is no such thing as a dark matter star, so you can’t have dark matter undergo core collapse. Without a mechanism by which it can shed angular momentum, dark matter can never even approach the densities necessary to create an event horizon and hence, a black hole. Dark matter can only form sparse, diffuse clumps, mostly on very large scales. Normal matter forms individual, dense, small-scale clumps. Together, all of these phenomena result in a major difference that can be summed up as follows: normal matter sheds momentum and angular momentum and sinks to the core of the galaxy, where it clumps together, while dark matter always remains diffusely distributed in an enormous halo around the galaxy, unable to shed linear or angular momentum. Although dark matter substructure is present within the halo, no region ever gets dense enough to even remotely approach the densities needed to give rise to a black hole. The dark matter, experiencing none of that, remains distributed sparsely and diffusely throughout the halo. The normal matter present within a galaxy gets concentrated in the central region of the gravitational potential, owing to processes like friction, heating, and collisions. Astrophysically, this makes a tremendous difference in what happens to each one. Normal matter experiences electromagnetic interactions. Why, then, would normal matter be so effective at forming black holes, while dark matter wouldn’t be? The key lies not in the gravitational force, but in the other dissipative forces: things like friction and collisions, which rely on the electromagnetic interaction. In every environment where there’s a copious amount of normal matter, such as a galaxy like the Milky Way, there’s significantly more dark matter overall, with ratios of 5:1 in favor of dark matter, at minimum.Normal matter and dark matter both experience the gravitational force equally, obeying Newton’s and Einstein’s laws of gravitation in precisely the same fashion.Normal matter makes up just one-sixth of the total mass in the Universe, with dark matter constituting the remaining five-sixths.This might be a bit of a puzzle to you, when you consider the following facts. Kochanek (OSU))Īs you’ve likely noticed, all of these rely on normal matter: matter that’s made out of atoms and their constituent components. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time. The visible/near-IR photos from Hubble show a massive star, about 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation.
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