Does dark matter form dark planets?

08 August 2013

Question

How come dark matter doesn't clump up into black holes / singularities or sun shaped objects?

Answer

Hannah - So, after the Big Bang, there was lots of matter around and gravity plus pressure caused it to clump together to form planets and moons, and suns. But how come dark matter doesn't cluster in the same way to form dark suns or dark planets, or does it? We turn to Dr. Andrew Pontzen, Cosmologist at University College London

Andrew - There's quite a lot to unpack in this question, the first thing to say is that dark matter is this substance that we're pretty sure is out there, shaping the visible contents of the universe, but the dark matter itself is invisible. So, we can't see directly what it does. Now, the question itself is getting at the idea of gravitational collapse which is a critical part of the way that we think the universe has evolved and familiar objects within it were born. Actually, dark matter does undergo gravitational collapse, so you can take an initially large volume of dark matter and shrink it down to something smaller, just because of the gravity of the dark matter itself. But the assumptions behind the dark matter tell us that unlike in the case where you have normal gas, the dark matter particles can't get rid of their energy, they you continue flying around at very high speeds. Although dark matter particles don't actually feel pressure in quite the same way that normal matter would, you can imagine they just moving so fast that they can't be concentrated into a small volume. If you tried packing them into a small box, they'd be moving so fast, they'll just fly straight out again. So, there's actually a limit to how small you can make a cloud of dark matter. You can't make dark matter collapse into a black hole for instance because you just can't get rid of the energy to make it that small. We actually think that typical clouds of dark matter are just the right size to be lurking around galaxies in what we call a dark matter halo.

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