Question of the Week

Does dark matter form dark planets?

Thu, 8th Aug 2013

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Daniel asked:

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


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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Does dark matter respond to gravity differently from how ordinary matter does, then? How does anyone know this to be true and who worked out the maths to describe this? Do you have a reference, please? woolyhead, Sat, 26th Oct 2013

It is thought that Dark Matter responds to gravity in the same way as "Normal" matter.
The difference is thought to be in how Dark Matter responds to electric & magnetic fields.

There are many theories about what Dark Matter may be made of; astronomical observations and Earth-based experiments have ruled out some of these candidates.

The common aspects of these theories are:

The Dark Matter particles are electrically neutral (like neutrons & neutrinos and a number of other, known particles). This means that:

They don't respond to electric and magnetic fields, so they don't form atoms & molecules & dust grains like the familiar protons & electrons

They don't absorb or emit electromagnetic radiation. Emitting heat is the main way that gas in an astronomical cloud can cool down (lose energy) to collapse into a protostar or protoplanetary disk, eventually forming a star or planet.

It does not even respond to the Strong nuclear force, so it does not even form aggregates with the complexity of an atomic nucleus (consisting of 2-400 nucleons).
Unable to lose energy or to form larger aggregations, it is thought that the Dark Matter stays in the form of galaxy-sized clouds of fundamental particles, rather than collapsing to form Dark Matter stars and planets.

evan_au, Sun, 27th Oct 2013

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