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.
- 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.
Credit: Pieter van Dokkum, Roberto Abraham, Gemini Observatory/AURA
Evidence for dark matter was first spotted in the 1930s, but it wasn't until the 1980s that astronomers started really searching for it. And they're still searching today.
The particles are believed to make up a large percentage of the Universe's mass but have remained 'invisible.' Instead, astronomers have been studying the force they have on other parts of the Universe that we can see, to try to learn more about them.
Now, physicists have used elaborate computer calculations to come up with at least an outline of the particles of this unknown form of matter.
"Dark matter is an invisible form of matter which until now has only revealed itself through its gravitational effects. What it consists of remains a complete mystery," explained co-author Dr Andreas Ringwald from Deutches Elektronen-Synchotron (Desy).
The adjective 'dark' does not simply mean it doesn't emit visible light. "It does not appear to give off any other wavelengths either - its interaction with photons must be very weak indeed," Ringwald explained.
What is clear, however, is that these particles must lie beyond the Standard Model of particle physics, and while that model is extremely successful, it currently only describes the conventional 15 percent of all matter in the cosmos.
Direct searches for heavy dark-matter particles using large detectors in underground labs and the indirect search for them using large particle accelerators are still ongoing, but have not turned up any dark matter particles so far. However, extremely light particles, dubbed axions, may be easier to spot, and it could even be possible to detect direct evidence of them.
To carry out the computer calculations, Ringwald – along with Professor Zoltán Fodor from the University of Wuppertal, Eötvös University in Budapest and Forschungszentrum Jülich – extended the Standard Model of particle physics which helped them to predict the mass of these axions. They used Jülich's supercomputer JUQUEEN.
The results show that if axions do make up the bulk of dark matter, they should have a mass of 50 to 1500 micro-electronvolts, making them be up to ten billion times lighter than electrons.
This would mean every cubic centimetre of the Universe should contain, on average, ten million such ultra-lightweight particles. Dark matter is not spread out evenly in the Universe, however, but forms clumps and branches of a weblike network. Because of this, our local region of the Milky Way should contain about one trillion axions per cubic centimetre.
These calculations will now provide physicists with a concrete range in which their search for axions is likely to be most promising.
"The results we are presenting will probably lead to a race to discover these particles," said Fodor. Their discovery would not only solve the problem of dark matter in the Universe, but at the same time answer the question why the strong interaction is so surprisingly symmetrical.
The scientists expect it will be possible within the next few years to either confirm or rule out the existence of axions experimentally.
The research is published in the journal Nature.
The Institute for Nuclear Research of the Hungarian Academy of Sciences in Debrecen, the Lendület Lattice Gauge Theory Research Group at the Eötvös University, the University of Zaragoza in Spain, and the Max Planck Institute for Physics in Munich were also involved in the research.