Tuesday, 14 March 2017

Is dark matter disappearing altogether?

A few days ago at a talk, I mentioned in passing that in a few years' time we may no longer think that dark matter exists. (In the unlikely event you've not heard of it, dark matter is a hypothetical kind of stuff that only interacts with ordinary matter through gravity, which is thought to exist because large collections of matter, such as galaxies act as if they have more matter in them than they should have.)
A galactic cluster that provides more
gravitational lensing that its ordinary matter
predicts. (Image from Hubble via Wikipedia)
After the talk, a handful of teenage physics enthusiasts collared me and said 'Surely you don't think dark matter doesn't exist?' After asking them not to call me Shirley, I admitted I was a dark matter sceptic. But I felt their pain. When I was their age, the steady state theory of cosmology was still an accepted challenger to the big bang, but its star was fading fast. I preferred steady state in part because it seemed to be a more elegant theory and in part because one of its originators was my teenage physics hero, the remarkable Fred Hoyle. I was genuinely upset when steady state was pushed out of consideration. Science may be objective, but it doesn't stop us from having emotional attachments. 

I'd say for the first time since it became widely acknowledged, dark matter is in danger of being replaced as the best accepted theory within a decade. There has been one challenger for a while in the form of MOND (Modified Newtonian Dynamics), but critics are quick to point out it doesn't explain all the phenomena ascribed to dark matter. (To be fair dark matter isn't 100 per cent either, but that's by the by.) However, there are now at least two other alternatives that explain the behaviour of large collections of matter to some degree without the need for a new type of stuff.

One suggestion is painfully simple - it takes a microscope to those innocent words in the first paragraph 'should have'. The existence of dark matter is based on guestimate of the amount of conventional matter in galaxies and other large collections of space stuff. It is, without doubt, a good guestimate, based on best current knowledge. But the reality is that the calculation has to involve estimation based in part on theory, and that leaves room for error. It only takes a small correction to make dark matter disappear. Again, there are of holes left by making this assumption, but it's a potential line of thought.

The second suggestion is a lot more sophisticated (so some theoreticians may prefer it). The concept of emergent gravity, where gravity is somewhat like thermodynamics in emerging from statistical behaviour, rather than being a true underlying fundamental force, has been put forward by some as a way of providing a mechanism to do away with dark matter. As this semi-technical article by Sabine Hossenfelder shows, there are still significant problems for this explanation, but it is without doubt another strand.

At the moment, then, nothing has knocked dark matter from its 'best accepted theory' perch. But it has never been so strongly challenged. We always need to remember that science is not about black and white, absolute fact, but establishing the best theory we can given the current evidence. Dark matter could recover from its wobble, just as the big bang did with modifications that brought it into line with current data. But there is no doubt that we exist in cosmological (and particle theory) interesting times.

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