THE tale of Galileo dropping two cannonballs of unequal weights from the Leaning Tower of Pisa laid the cornerstone for a theory that explains the way matter behaves in a gravitational field. Now a test on a far grander scale, involving stars and galaxies, could determine how the unseen mass that makes up dark matter behaves under the influence of the unseen field known as dark energy.
Mike Kesden of the Canadian Institute for Theoretical Astrophysics in Toronto has a model for how the dark matter thought to make up 90 per cent of the mass of a galaxy interacts with dark energy, the force thought to be causing the expansion of the universe to accelerate. Because of this interaction, he thinks dark matter acquires an extra “mass” that attracts other dark matter. Normal matter, however, is immune to this attraction.
Kesden, of course, can’t lob balls of dark matter off the nearest tower to test how it responds to this extra force. Instead, he is proposing an astronomical alternative. He wants to study stars that belong to the Sagittarius dwarf galaxy, which orbits the Milky Way and is being torn apart by the tidal forces of our galaxy’s much stronger gravitational field. The dwarf’s central core of stars is intact, but the stars in its outer regions have been teased out into two streams, one leading the dwarf’s core and the other trailing it.
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If Kesden’s model is correct, the invisible dark-matter core of the Sagittarius dwarf should experience not only the gravity of the Milky Way’s combined matter and dark matter, but also an additional force due to the Milky Way’s dark matter alone. This extra force would accelerate the dwarf galaxy’s dark-matter core in its orbit around the Milky Way, dragging the stars in the core along with it. Meanwhile, stars in the two streams, which are no longer tightly bound to the dark-matter core, would move in orbits that are far less affected by the extra force.
According to Kesden’s calculations, the effect of the extra dark-matter force would appear in an unequal distribution of stars in the leading and trailing streams. “It would look like the trailing stream is longer and has more stars in it,” he says.
At this week’s meeting of the American Astronomical Society in Calgary, Alberta, Kesden suggested that existing data from large-scale sky surveys may be sufficient to test for the presence of an additional dark-matter force to within “a few per cent”.
“Sagittarius should experience the gravity of the Milky Way’s matter and dark matter, plus another force due to dark matter alone”
“The approach makes a lot of sense,” says cosmologist Jim Peebles of Princeton University. Peebles has previously proposed a similar mechanism to explain why the large voids between superclusters of galaxies appear to be emptier than current theories predict. Most astronomers still disagree with the idea of a unique dark-matter force, so if Kesden’s test reveals such a force “that would be very important”, says Peebles.
