Einstein’s General Relativity provides an elegant description of how space, time and matter affect one another. It makes precise predictions of gravitational effects, which have been verified by many measurements.
But if we use the theory to try to understand the motion of galaxies, we get the wrong answer, unless we invent a new form of so-called ‘dark’ matter. This is not a small correction – there needs to be much more of the Dark Matter than normal matter, and what is more, it doesn’t seem to be made up of quarks and electrons like all other matter. In fact it doesn’t seem to be made up of any of the particles in the Standard Model of particle physics.
Furthermore, the universe is expanding at an increasing rate, rather than – as you might expect if it started of with a big bang and then gravity takes over – slowing down. This effect we ascribe to something we call ‘Dark Energy’. Dark Energy can be accommodated within General Relativity, but only by adding an absurdly precise ‘cosmological constant’, which looks very weird, or “unnatural” as a theoretical physicist would put it.
It gets worse. Dark Energy is a sort of ‘energy in empty space’, and particle physics also predicts this kind of energy, due to quantum fluctuations of the Standard Model – including and especially the Higgs boson. But these fluctuations would naively lead to so much Dark Energy that atoms themselves (never mind theoretical physicists) would never form in the first place.
That’s wrong, obviously.
And then there’s the nagging, possibly connected, fact that we don’t have a way of making General Relativity and quantum theory work together at very high energies.
Since gravity is a common thread here, all of these problems might seem to imply that General Relativity needs to be modified in some way. That’s a thought that has occurred to many physicists. However, General Relativity is so subtle, and so, well, General, that replacing it, or even successfully tweaking it, is a very hard thing to do.
Still, physicists are persistent, and there are new ideas coming forward all the time. One possible tweak is to postulate a new particle which carries a ‘fifth force’ (the other four forces being electromagnetism, the weak and strong interactions, and gravity).
To explain Dark Energy, this force has to affect all matter – as gravity itself does – and operate over large distances. Such forces have been looked for already, and if they affect the motion of the planets in the solar system, for example, they have to be enormously more feeble than gravity, otherwise we would have seen them already. But if they are enormously more feeble than the gravity between stars and galaxies, they won’t make any difference to the Dark Energy or Dark Matter problems, so that’s a waste of time.
One way potential way around this conundrum is a process called ‘screening’, in which the strength of a force depends upon the environment it is in. A recent paper from a group at the University of Nottingham describes a model in which the force is screened by matter itself. In dense regions of the universe (like the Earth, for instance) the force is hidden, while in empty space, the force can operate. In the case of the Dark Energy problem, which is what the theory was aiming for, this can provide exactly what the data need. The force can make the universe accelerate at large distances, while having no measurable effect on the orbit of the planets. As a bonus, this new force can also have a significant impact on the way galaxies rotate, which might at least partially solve the dark matter issue as well.
To a physicist, the way this new force works is reminiscent of the way the theory of Brout, Englert and Higgs gives mass to fundamental particles. It involves a scalar boson – a particle like the Higgs boson, which has no spin – and it involves the idea of symmetry breaking¹. But I am aware that using the Higgs as an analogy to explain something to a general audience is not a winning strategy, so here’s a better attempt, I hope. Consider a pointillist painting:
- When averaged over dense regions of space, a symmetry hides the fifth force. This is like looking at the picture above from a metre or so away. The dots are hidden in the colours and landscape of the painting.
- Close to the painting, the dots are visible. In the same way, for things the size of atoms or smaller, the averaging doesn’t happen, so the fifth force may show up.
- And on very long distance scales, space is empty, so the density is low and again the force reappears. Similarly, a very long way from the painting, it is just a single dot again.
All this is speculative, of course. Speculative ideas are one thing we expect from theorists. Another thing we expect is testable predictions, and this model seems to be testable in an excitingly wide range of experiments. Upcoming observatories, such as the European Space Agency EUCLID mission and the Dark Energy Spectroscopic Instrument, will characterise gravity and dark energy on astrophysical scales. Precise atomic physics experiments could measure the effect of the fifth force on atoms, and most interestingly to me personally (since I work on it), this is the first plausible theory I have come across in which the Large Hadron Collider can contribute to the understanding Dark Energy.
This article was prompted and informed by an outstanding seminar by Clare Burrage, at UCL on Friday. Here she is talking about some of the background to this work in one of the excellent Sixty Symbols videos:
¹ See equations 4 & 5 of their paper if you are keen. It’s neat.
Jon Butterworth’s book Smashing Physics is available as “Most Wanted Particle” in Canada & the US.
