FRESH battle lines are being drawn in the debate over whether dark matter is needed to explain the structure of galaxies.
Those physicists who would like to throw out dark matter in favour of a controversial new form of gravity are facing one of their biggest challenges yet. It seems that if their theory is valid, the Earth should have been swallowed up long ago by black holes that would regularly appear in our solar system. Dark matter鈥檚 opponents are not ready to give up just yet, though. A 鈥渄ark fluid鈥� sloshing around galaxies like ours might just allow them to fight another day.
Dark matter has been proposed to explain why spiral galaxies are not torn apart by their rapid rotation. The favoured view among cosmologists is that the gravitational force exerted by some kind of dark matter must be pulling the outer stars in. Though physicists are closing in on what this stuff might be, nobody has directly detected it (see 鈥淐osmic enlightenment鈥�), and that leaves the door open for alternative explanations.
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In the 1980s, Mordehai Milgrom, then at Princeton University, suggested that galaxies could stay intact if gravity exerted a slightly stronger than expected pull over distances of galactic proportions. Proponents of this alternative theory, called modified Newtonian dynamics (MOND), have been locked in battle with dark matter鈥檚 advocates ever since.
Both sides have used astronomical observations to support their theory. For example, in 2006, a spectacular collision between two galaxy clusters 鈥� producing what is known as the Bullet cluster 鈥� provided strong evidence for dark matter, because this stuff had apparently separated out from normal matter (New 杏吧原创, 9 September 2006, p 12). However, MOND鈥檚 proponents came up with their own explanations, and confusion increased the following year when astronomers found evidence of another galaxy collision with similar distributions of matter, dubbed the 鈥渢rain wreck鈥�, that had turned out completely differently from the Bullet smash.
Now Carlo Contaldi, Toby Wiseman and Ben Withers at Imperial College London say they have uncovered a serious flaw in theories based on modified gravity: it is a rather too good at forming black holes. They built a simulation based on the most successful MOND candidate 鈥� a theory known as TeVeS, proposed in 2004 by Jacob Bekenstein at the Hebrew University in Jerusalem, Israel. Bekenstein suggested that a set of fields are woven together across space-time, and that they interact to make gravity pull a little bit tighter around the outskirts of galaxies (New 杏吧原创, 22 January 2005, p 10).
The simulation showed that TeVeS space-time spontaneously bunches up, becoming infinitely dense at many different points, thus creating black holes all over the place (). This happens so often that a black hole would appear somewhere in our solar system every 10 hours or less. 鈥淚t鈥檚 really pathological behaviour,鈥� says Wiseman.
鈥淢ost alternatives to dark matter suggest black holes should spring up in our solar system once every 10 hours or less鈥�
Most other modified gravity theories put forward to explain galaxy formation would be vulnerable to the same problem, because they share the same mathematical foundation as TeVeS, adds Contaldi, who has submitted the work to Physical Review D.
HongSheng Zhao at the University of St Andrews in the UK agrees that at first sight this looks like bad news for MOND. 鈥淭he team have used a very clever new tactic to try and falsify these theories,鈥� he says. 鈥淚f black holes are popping up everywhere, this is a very big problem for their viability.鈥�
He suggests, however, that there is a way around the problem. He and his colleagues have constructed a MOND theory based on a 鈥渄ark fluid鈥� that mimics the effects of dark matter and dark energy 鈥� the even more mysterious stuff that is thought to be causing the expansion of the universe to accelerate. Their version of modified gravity avoids Contaldi鈥檚 black-hole trap because it contains a small repulsive component that stops space-time bunching up catastrophically ().
鈥淶hao鈥檚 鈥榙ark fluid鈥� theory avoids the black hole trap thanks to areas of negative gravity that stop space-time bunching up鈥�
His calculations, which are set out in a paper to be published in The Astrophysical Journal, show that in regions where there is visible matter the dark fluid becomes compressed. This enhances gravity, making it stronger than in other regions. Zhao claims that the model can already explain how more than 100 nearby spiral galaxies stick together.
In areas where there is little visible matter, by contrast, the repulsive component dominates. Zhao says that this creates a dark energy effect whose strength matches the observed rate of acceleration of the expansion of the universe.
The dark fluid can also reconcile the observations of what happened in the Bullet and train-wreck clusters. Whereas previous modified gravity theories cannot explain why the two collisions ended up differently, this is not a problem with Zhao鈥檚 theory because the distribution of the dark fluid 鈥� and therefore of the enhanced gravity effect 鈥� can vary.
Zhao likens the interaction between visible matter and the dark fluid to the interplay between wind and the surface of an ocean. Disturbances in one will affect the other, but depending on how the disturbances were created and the surrounding conditions, ocean turbulence can overshoot wind turbulence or lag behind it. In the case of the Bullet cluster, the dark fluid may have sloshed forward, piling up beyond the visible matter. In the train wreck, the fluid may have lagged behind visible matter.
Though Zhao鈥檚 model is much more mathematically complicated than other modified gravity theories, and so will not persuade cosmologists seeking a simple alternative to dark matter, Contaldi agrees that it resolves the black hole problems his team discovered. He likes it for other reasons too. 鈥淧utting dark matter and dark energy together is great,鈥� he says, 鈥渂ecause we all want to find a unified solution to these problems.鈥�
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