On the Hunt for Dark Matter

This month we sat down with Famelab runner-up Ben McAllister to discuss one of the biggest mysteries in science, Dark Matter.

Q: Dark matter theory seems to be the holy grain of physics at the moment, but does it tie in with string theory?

Good question – the composition of dark matter is certainly one of the big questions right now, so it would be nice if it tied in with some of the candidates for Grand Unified Theories, like string theory. As with lots of this fundamental physics stuff, it kind of depends who you ask, and what you mean by “dark matter” specifically. Certainly, axions (or particles similar to axions) are compatible with string theory. What I mean to say is, many string theorists have found that particles that look an awful lot like axions on paper arise naturally in their models.

Q: There are many ‘WIMP’ (weakly interacting massive particle) projects going on around the world right now. Has there been one instance of a detection yet?

Short answer, probably not (I’ll elaborate in a minute). Certainly, if we had a detection, that would be an enormous Nobel Prize-winning discovery. In part, this lack of WIMP detection is why some focus in the dark matter community is shifting away from WIMPs and onto the lower mass dark matter particle regime – which is where axions come in. Axions are much lighter than WIMPs, motivated by different theories, and detected in different ways. Axion searches are also comparatively young when compared with WIMP searches, so there is still a lot of parameter space to search through in the hopes of finding them.

The reason I say “probably not” when referring to WIMP detections are that there is actually one kind of intriguing result. An experiment called DAMA/LIBRA has claimed to see a signal consistent with WIMP detection, but it’s kind of odd because it is in a region of the WIMP parameter space which has been thoroughly excluded by other experiments. At any rate, we should get to the bottom of this soon; an Australian experiment (known as SABRE) is currently under construction in Victoria, with the aim of directly testing the claims of DAMA/LIBRA. SABRE should help us determine if this DAMA/LIBRA signal is an anomaly or an actual hint of WIMP detection.

Q: Sometimes the general public finds it hard to get behind big projects or ‘Blue Sky Research’ because there isn’t a foreseeable benefit. What is the one point you would make to someone to validate dark matter research?

This is a great question – it is certainly true that it is sometimes hard to rustle up support for more fundamental research – which is kind of a shame, because if you ask me fundamental research is just about the most important kind! To me, it’s as simple as this: think about everything humans have achieved so far. Electricity, computers, medicine, satellites, space travel etc. It’s incredible. We’ve accomplished all of this using only regular matter – using only the tiny fraction of the matter in the universe that we understand.

Dark matter makes up more than 5/6 of all matter in the universe and we know almost nothing about it. Imagine what we could do with if we could harness it. To me, the idea that it is useless, or pointless is patently absurd. Even if we don’t find life-altering, paradigm-shifting applications for dark matter after discovery, it is absolutely critical that we investigate it, because how else would we know?

In a broader sense fundamental research (which is to say: researchers digging around in areas where they aren’t immediately sure of the outcomes and applications but are simply exploring a glaring, nagging question or hole in our understanding) invariably leads to enormous societal advancement.

James Clerk Maxwell was not trying to lay the foundations for the entire modern world’s understanding of electricity and magnetism when he discovered the laws of electromagnetism that bear his name, and Max Planck was not trying to discover the first quantum theory (which would go on to give us lasers, semiconductors and superconductors – all critical components in modern technology) when he set out investigating the relationship between light intensity and colour.

These things just happened as a result of looking at the big, unanswered questions and getting down to work trying to answer them. So yes, this is something I’m quite passionate about – it’s absolutely critical that we try and tackle these enormous, seemingly arcane questions. It’s how we advance as a species.

Q: In your sting analogy during your talk for Famelab you did not mention the gravitational effect of the Super Massive Blackhole that resided in the centre of all galaxies. Isn’t the ‘pull’ from this massively dense object enough to keep everything in place?

To be clear, the Supermassive Black Hole (SMBH) is definitely included in the modelling of what the rotation curves should look like and amazingly we still find that much more mass is required.

However, it’s a little more subtle than just modelling more or less mass at the centre of the galaxy. Gravitational attraction decreases as the square of the distance between objects. Even for a massive object like the SMBH at the galactic centre, the matter in the outer spiral arms of the galaxy will feel a much, much weaker pull from it than the matter near the centre, as it is much further away.

This is why we expect the galactic rotation curves to look the way we expect them to, which is to say, we expect the matter on the outer arms to be moving slower than the matter near the centre, as it feels a much weaker gravitational attraction regardless of how much matter we stick in the centre. This would mean, in the string analogy, that the outer matter cannot move as quickly as the inner matter, as its “string” would break.

Q: Favourite element on the periodic table?

Probably one of the ones named after a cool scientist, like Curium, for Marie Curie, or Rutherfordium, for Ernest Rutherford. Failing that it’ll be one of the really, really heavy ones that don’t have a proper name yet, like “ununennium” – element 119, just because it sounds cool.

Q: If you had unlimited funding, describe the ultimate experiment you would create to verify dark matter?

Three words: Bigger, Stronger Magnets. In most kinds of axion detection, a lot comes down to the strength (and physical size) of the magnetic field you can apply in the laboratory. This is because most axion detection experiments rely on the conversion of axions into photons, and the amount of conversion depends on the strength of the applied magnetic field and the volume of the detector. Large, superconducting magnets with high fields can get very, very pricey.

Q: You were part of the Famelab finalists this year in Australia and to illustrate the amount of Dark Matter vs regular matter you made some wool puffballs. Is sewing a skill physicists require or did you have some help?

I actually learned how to make those pom-pom tactile, 3D data representation things whilst volunteering at Scitech, a science museum in Perth (if you haven’t been, go check it out. it’s awesome). So, I kind of knew how to make them, but I’d be lying if I said I didn’t have a lot of help from my girlfriend Lily, and our friends Grace, Nuala and Thomas. Thanks guys.

Q: What inspired you as a teen to pursue science as your career?

A couple of things! I was lucky enough to have access to some pretty great extra-curricular programs in school. I was involved in an after-school code and cypher club (super cool, right?) inspired by Alan Turing and the team at Bletchley Park cracking the Enigma Code in WWII (see the recent film “The Imitation Game” for more on this).

Perhaps more impactful, we were really fortunate to have Professor Peter Quinn from WA’s own International Centre for Radio Astronomy Research (ICRAR) and some of his students come and visit for some after-school sessions on astronomy and astrophysics. This was really inspiring, and a major reason I chose physics as a major at uni.

Of course, I also had a great physics teacher in school, Dr Holly Rose, and it will surprise absolutely nobody to learn that I am and always have been an enormous, unapologetic Sci-Fi nerd.

Q: Can you recommend a book or doco that would help
people get their heads around dark matter?

One of the best resources I’ve found is actually a short video from one of my favourite webcomics PhD Comics. The video is called “Dark Matters” and it’s great. You can check it out here: https://vimeo.com/22956103

Q: Is there another competing theory?

There are a few, but the most popular competing theories all rely on modifying our understanding of gravity in some way. This makes sense, seeing as all of our current observational evidence for dark matter is from gravitational interactions, eg galactic rotation curves, cluster interactions, gravitational strong lensing, etc.

The dark matter hypothesis essentially says “we require more gravity for these things to behave the way they do, so there must be a lot more matter providing this gravity.”The modified gravity hypothesis says “we require more gravity for these things to behave the way they do, so our theory of gravity must be wrong.”

This is a sensible approach to the problem, no doubt, but so far nobody has been able to write down a theory of modified gravity that is consistent with all of the observations, whereas the dark matter hypothesis IS consistent with all the

Not to mention that fact that we are pretty sure our model of gravity (being Einstein’s General Relativity) is quite good, given that it was able to predict things like planetary orbits within the solar system to ridiculous levels of precision – it seems a little hard to swallow that it could be so precise for some things but so so wildly wrong for others, like galactic rotation curves.

Suffice to say, if someone can write down a modified gravity theory that is consistent with General Relativity for things like our solar system, and consistent with all the observational evidence for dark matter, people will start to pay attention. Until then, it’s all about the hunt for new particles, and the hunt is on.

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