Skip to main content

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.

Comments

Popular posts from this blog

Why I hate opera

If I'm honest, the title of this post is an exaggeration to make a point. I don't really hate opera. There are a couple of operas - notably Monteverdi's Incoranazione di Poppea and Purcell's Dido & Aeneas - that I quite like. But what I do find truly sickening is the reverence with which opera is treated, as if it were some particularly great art form. Nowhere was this more obvious than in ITV's 2010 gut-wrenchingly awful series Pop Star to Opera Star , where the likes of Alan Tichmarsh treated the real opera singers as if they were fragile pieces on Antiques Roadshow, and the music as if it were a gift of the gods. In my opinion - and I know not everyone agrees - opera is: Mediocre music Melodramatic plots Amateurishly hammy acting A forced and unpleasant singing style Ridiculously over-supported by public funds I won't even bother to go into any detail on the plots and the acting - this is just self-evident. But the other aspects need some exp

Is 5x3 the same as 3x5?

The Internet has gone mildly bonkers over a child in America who was marked down in a test because when asked to work out 5x3 by repeated addition he/she used 5+5+5 instead of 3+3+3+3+3. Those who support the teacher say that 5x3 means 'five lots of 3' where the complainants say that 'times' is commutative (reversible) so the distinction is meaningless as 5x3 and 3x5 are indistinguishable. It's certainly true that not all mathematical operations are commutative. I think we are all comfortable that 5-3 is not the same as 3-5.  However. This not true of multiplication (of numbers). And so if there is to be any distinction, it has to be in the use of English to interpret the 'x' sign. Unfortunately, even here there is no logical way of coming up with a definitive answer. I suspect most primary school teachers would expands 'times' as 'lots of' as mentioned above. So we get 5 x 3 as '5 lots of 3'. Unfortunately that only wor

Why backgammon is a better game than chess

I freely admit that chess, for those who enjoy it, is a wonderful game, but I honestly believe that as a game , backgammon is better (and this isn't just because I'm a lot better at playing backgammon than chess). Having relatively recently written a book on game theory, I have given quite a lot of thought to the nature of games, and from that I'd say that chess has two significant weaknesses compared with backgammon. One is the lack of randomness. Because backgammon includes the roll of the dice, it introduces a random factor into the play. Of course, a game that is totally random provides very little enjoyment. Tossing a coin isn't at all entertaining. But the clever thing about backgammon is that the randomness is contributory without dominating - there is still plenty of room for skill (apart from very flukey dice throws, I can always be beaten by a really good backgammon player), but the introduction of a random factor makes it more life-like, with more of a sense