What is the difference between dark matter and dark energy?

29 September 2015

Question

What is the difference between dark matter & dark energy? How do they react with each other?

Answer

We put Bhavesh's question to Andrew Pontzen from UCL... Andrew - The thing about dark energy is it's got a really bad name because I think one of the most famous equations in physics is Einstein's e=mc2, which makes it sound like energy equals mass times the speed of light squared whatever, but it sets up an equivalence between energy and mass. So, it would be very fair to think dark matter should be the same as dark energy. However, it's not. It's just badly named. Dark energy is the name we give to something beyond dark matter. We seem to need something extra; even more weird than dark matter if you like which seems to be pushing the universe apart. So, long after we realise that we need something to keep say galaxies glued together which is what dark matter is, we then realised in much more recent times that we actually need something forcing it apart to account for observations like a supernova for instance.

Kat - So, this is another thing to make the maths of the universe work.

Andrew - Yeah. It's beginning to look a bit like we've just patched everything up, isn't it?

Kat - Yes. There's a hole, quick shove a new thing in!

Andrew - Yeah. I mean, I think it's kind of fair actually in the case of dark energy. It is something that we've just shoved into the equations to say, "Okay, these observations have shown us that the universe is expanding at an ever increasing rate." It's just not something that should happen. So, we have kind of crowbarred this stuff in a bit. I think that's fair. In the case of dark matter which is the stuff that keeps stuff glued together, it's on a much firmer footing. It seems like it makes many, many predictions that we've been able to go and verify about stuff we should find in the universe, how it should be behaving. With dark energy, we're much less far on firm ground.

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