RINGS of invisible matter are spreading out from the centre of our Galaxy
like ripples on a pond, a physicist in Florida claims. He says the last one
passed the Sun about 900 million years ago, and the next is due in 350 million
years. 鈥淭he rings could shake up the comet cloud around the Sun, causing
cataclysmic events on Earth,鈥� says Pierre Sikivie of the University of Florida
in Gainesville.
Most of the mass of the Universe is dark鈥攊t cannot radiate energy as
light鈥攖hough its precise nature has yet to be identified. As the gravity
of our Galaxy pulls in matter from surrounding space, dark matter would suffer a
different fate from visible matter. 鈥淐ollisions between particles of ordinary
matter allow it to convert its energy of motion into heat and settle into a thin
disc at the centre of the Galaxy,鈥� says Sikivie. 鈥淏ut dark matter cannot shed
this energy and flies back out again like a pendulum.鈥�
What determines how many times a shell of dark matter鈥攔oughly spherical
in shape鈥攆alls into the centre of the Galaxy and flies back out again is
how far away it was when the Galaxy started forming. Material that was millions
of light years from the centre would now be falling in for the first time,
whereas material that was very close could have gone in and out a hundred
times.
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Calculations by Sikivie show that in the midst of all this complexity there
are stable, doughnut-shaped structures he calls 鈥渃austic rings鈥�, which spread
outwards from the centre of the Galaxy. 鈥淭he rings are rather like vortices in a
fluid,鈥� says Sikivie. 鈥淎lthough matter constantly moves in and out of them, they
retain their integrity.鈥�
The model suggests the rings are confined to a plane, probably the disc of
our Galaxy. They also move more slowly than the shells, and only four have
passed our part of the Galaxy since it formed about 10 billion years ago,
according to Sikivie鈥檚 calculations. 鈥淲e鈥檙e currently between the
fourth and fifth rings,鈥� he says.
He claims there is evidence of rings 6 to 13 in the rate at which stars orbit
in the Milky Way. He says the observed orbit speeds fit with the idea that stars
on the outside of a ring speed up because of the extra gravity of the ring,
while their counterparts on the inside are slowed down. He also claims there is
evidence of rings in another galaxy, called NGC 3198.
In the dark matter rings, Sikivie adds, the density of matter is boosted
extremely sharply鈥攑erhaps by as much as a factor of 10. This could shake
loose comets on the fringes of our Solar System, he speculates, threatening the
Earth with major impacts. The passage of the first ring past our region of the
Galaxy 5.9 billion years ago might even have triggered the formation of the
Solar System, he says. The work will appear in Physics Letters B.
鈥淚 like the approach,鈥� says Joseph Silk of the University of California at
Berkeley. However, he believes it鈥檚 an extremely idealised model. 鈥淚 feel it鈥檚
still a little premature to take caustic rings even as a serious speculation,鈥�
he says.
