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Asked by anon-191463 to Graeme, Agnes, Kathryn, Chris, Anne, Adam on 10 Nov 2018. This question was also asked by anon-191466, anon-190901.
Question: What is dark energy/matter?
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Agnes Wojtusiak answered on 10 Nov 2018: last edited 15 Nov 2018 3:20 pm
Good question – if only anyone knew!!
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We know that for the universe to look the way it does, there must be something making gravity strong enough to keep galaxies together, but we can’t see it – that’s dark matter. We also know that for the universe to keep speeding up how fast it expands, something that we can’t see must be pushing on it – that’s dark energy. We have no idea WHAT it actually is, but we’re trying to find out in places like the Boulby Underground Lab (it’s a really cool lab built deep underground in an old mine!).
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There is a bit more in this really good YouTube video: https://youtu.be/QAa2O_8wBUQ – it’s by “Kurzgesagt – In a Nutshell” and they do loads of fun videos about interesting science stuff, I recommend them 🙂
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Anne Green answered on 12 Nov 2018: last edited 12 Nov 2018 4:54 pm
See my comment below for my answer to this.
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Adam McGuinness answered on 13 Nov 2018: last edited 13 Nov 2018 12:43 pm
What Agnes said, well done! Ooh, and Anne!
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Graeme Poole answered on 13 Nov 2018:
85% of the Universe is made up of “dark matter”. The problem is we have no idea what it is!! Dark matter doesn’t interact with any experiments, so we can’t measure it. We know it exists because for galaxies to stay together and not fall apart, a lot more mass is needed than the stars and planets in the galaxies. So there is something else in them – this is dark matter.
The Universe is expanding and the expansion is getting faster and faster. We have no idea what is driving this accelerating expansion – we call this mysterious energy “dark energy”
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Anne commented on :
I’ll have a go at answering this one, since dark matter is what I’ve been working on for the past 15 years (and I’ve also worked on dark energy a little bit).
From looking at the way the things we can see (stars and galaxies) move and cluster together we’ve worked out that 80% of the matter in the universe is invisible dark matter. And it has to be some new particle we’ve not yet found which moves very slowly. Exactly what it is we don’t know. But we’ve got lots of good ideas that we’re trying to test. One of the most popular ideas is Weakly Interacting Massive Paricles, or WIMPs for short. They do exactly what their name says: they’re heavy (and therefore move slowly) and interact only weakly with each other and normal matter. They’re predicted to exist by theories, like string theory, which try to unify the fundamental forces. And if they exist they would be produced in the Big Bang in roughly the right amount. The LHC is trying to make WIMPs. You can also detect them lab experiments (like the ones at the Boulby mine that Agnes mentioned) when they very occasionally bump into normal matter. Or you can look for the high-energy gamma-rays that are produced when 2 WIMPs collide and annihilate. Another possibility is Primordial Black Holes: black holes that could be made just after the Big Bang.
Dark energy is even wierder than dark matter. If the Universe only contains matter the rate at which it’s expanding should slow down, because gravity tries to pull things together. However in the late 1990s we discovered that in fact, in the relatively recent past, the expansion started speeding up. For that to happen the Universe has to contain something with negative pressure. One possibility for dark energy is the cosmological constant, the energy which empty space has due to particle-anti-particle pairs coming into existance for a tiny fraction before disappearing again. However when we calculate how big the cosmological constant should be the answer we get is wrong by 1 with 120 zero on the end! Astronomers are trying to test theories of dark energy by looking at how galaxies cluster and bend light.
Agnes commented on :
Haha, WIMPs 😀
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Thanks so much Anne, that’s really interesting!