杏吧原创

THE Sun creates a 鈥渨ake鈥� in the dark matter flowing through the Solar System, just as a rock creates a wake in a stream of water, say two physicists in the US. They have calculated the structure of the wake and say it could help us understand the make-up of the Sun. 鈥淚n the future, when experiments are capable of detecting the wake, it could be used to take a dark matter 鈥榅-ray鈥� of the interior of the Sun,鈥� says Pierre Sikivie of the University of Florida in Gainesville.

Cosmologists believe that dark matter reveals itself only by its gravitational effect and outweighs the visible matter in the Milky Way 10 times over. In a paper submitted to Physical Review D, Sikivie and Stuart Wick describe their study of how dark matter ought to move through the Solar System.

Nobody knows the identity of dark matter, but it may consist of as-yet undiscovered subatomic particles such as 鈥渁xions鈥� or 鈥渘eutralinos鈥�. Sikivie and Wick assume only that the dark stuff will be moving differently from the visible stars and so will flow like a stream through the Solar System.

According to their model, the Sun鈥檚 gravity will disrupt the stream, bending the trajectory of the dark matter towards the Sun. As material passing on one side of the Sun is bent one way while material passing on the other side is bent the opposite way, two flows will be created downstream of the Sun from the original one. 鈥淭he two flows interact to create a line of enhanced density, which extends from the Sun to infinity,鈥� says Sikivie.

The two physicists have also found a second, more complex, structure in the wake. This arises because dark matter actually passes through the centre of the Sun, creating another flow downstream. 鈥淭he three flows interact to create what we call a skirt鈥攁 cone of enhanced density whose vertex is at the Sun,鈥� says Sikivie.

The skirt鈥檚 importance is that its properties depend on the way matter is distributed within the Sun. 鈥淚f you could measure those properties, you could effectively look inside the Sun,鈥� says Sikivie.

Sikivie and Wick鈥檚 current calculations are general. However, they hope to refine them for a particular model of the distribution of dark matter in the Galaxy. 鈥淭hen we will be able to predict the times in the year when the Earth will pass through regions of enhanced dark matter,鈥� says Sikivie.

Other researchers are impressed by the idea. 鈥淪ikivie and Wick have reawakened the community to the possibility that cold flows of dark matter may lead to strong signals from the wake of the Sun,鈥� says Paolo Gondolo of Case Western Reserve University in Cleveland.Marcus Chown