Dark Matter on Mane Street · 12:58pm Oct 29th, 2023
The finale of Chapter 5 nicely leads us to Dark Matter Day (31 October, also known as Nightmare Night), the time of the year when we celebrate the invisible component of the universe. Time for an update on this darkest of contemporary physics research topics.
Dark matter is the part of the universe that we can’t see, as it doesn’t emit light, or absorb or reflect it. We know it exists from the effect of its gravity on stars, galaxies and other phenomena such as the cosmic microwave background radiation. The evidence that most of the universe is dark has steadily accumulated over the years. Few astronomers doubt that it exists as alternative theories struggle to explain all observations.
As the simple explanations have been ruled out, it is assumed dark matter must be a new type of matter, made from a new type of particle. This particle must fit certain criteria. It doesn’t interact with light. It does interact with gravity. It must exist in the universe in great numbers. And it must be stable enough to not have decayed into anything else. There are some more subtle hints that it interacts via the weak nuclear force, so enough of them could have been created in the early universe after the Big Bang.
How can you test such a hypothesis? By searching for this new particle of course, which can be done in three possible ways:
Create it: smash two proton beams together with enough energy to make a dark particle, and look for a sign of lots of energy and momentum apparently vanishing inside the detector. (We can’t directly see dark matter particles as they are too dark, so we are looking for a dark-matter-shaped hole in the data.) The ATLAS experiment has released a review of their searches for dark matter in the form of supersymmetric particles. They haven’t found any.
Direct detection: assembly a huge sensitive particle detector in a deep underground laboratory, and see if you can rule out every possible source of background, and tag the very rare dark matter interactions. In the last few years, we have had results from the LZ experiment, which use a large tank of liquid xenon, and the DarkSide experiment, using a large tank of liquid argon. Neither of them have found it.
Indirect detection: (this is probably the weirdest option). Look for a signal of high energy neutrinos coming from the centre of the Earth, which is a sign that dark matter particles passing through the earth have become tired and decided to stay. Over billions of years, they have accumulated in the centre, orbiting around inside the Earth’s core without interacting, until it has become so crowded that they bump into each other and annihilate into something else. If that something else is neutrinos then they can get out of the Earth and we can see them. The IceCube experiment at the South Pole have searched for such a signal, and they haven’t seen it either.
The search continues. For years, the focus has been on building bigger detectors, but as we are now reaching the limit of what is possible, there is more attention being paid to checking blind spots and investigating other approaches. Work in progress.
Hyped that this latest episode sees the return of my favorite character, the really big tank of liquid xenon.
How many neutrinos are extimated to exist, relative to how many atoms, and from any distance just what would be any difference between a single WIMP, and a dynamic density concentration of vastly more much lighter neutrinos?
A single wrecking ball is a wrecking ball, but its made of all those induvidual iron atoms that are in approximately the same place at approximately the same time?
Why is that what you're looking for is always in the last place you look?
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* I mean, aside from the fact that when you find it, you stop looking... Hmn... That would explain it, wouldn't it?
They are doing this experiment at the bottom of a gold mine not far from me... well the facility is built and the systems installed... now to see if they actually see something or nothing with the SABRE experiment.
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Dark matter and the return of the really big tank of liquid xenon. There's a Power Pony comic story there.
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The neutrinos we know account for only 1% or so of dark matter. They also move too fast to explain small scale structures like galaxies. There are theories of possible massive sterile neutrinos.
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Of course the theory suggests that it is not only in the last place you look, but also everywhere else. It's just sufficiently weakly interacting that you only see it in the last place that you look.
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That's an interesting one as they are using sodium iodide detectors, so it will be able to test the dark matter signal reported by the DAMA experiment, which no one believes, but has not yet been conclusively tested with a detector of the same material.