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Dark Matter and the Universe’s Expansion

Nobody can see dark matter, but the mysterious stuff can be detected because it influences large-scale structures like galaxies and galactic clusters. As far as we know, galaxies wouldn’t look the way they do without it. And studies of the cosmic microwave background lead to the belief that dark matter is five times more common than the normal matter we see around us in the form of stars, gas and dust. But that’s about all we know, and we’re therefore left with a problem. How do we study the accelerating expansion of the universe without being able to measure its effects on dark matter?

For that expansion is considered to be the result of an equally mysterious ‘dark energy’ that may well interact with both visible and dark matter, an interaction we need to know more about. A solution that may allow us to study this effect is being developing by Marc Kamionkowski (California Institute of Technology) and Michael Kesden (University of Toronto), who are studying the way dark matter in galaxies disrupts the satellite galaxies near them. Kesden reported on this work at the AAS Calgary meeting today.

To get a handle on it, think about tidal forces, as in the Moon’s effects in raising tides on Earth’s oceans. In a similar way, the Milky Way is disrupting the stars and gas of nearby satellite galaxies, in some cases even pulling stars away from these smaller galaxies to create streams of stars that lead or trail the satellite in its orbit around the Milky Way.

These stellar streams can be useful indeed, if Kamionkowski and Kesden are correct. In their scenario, a satellite galaxy continues to be dominated by dark matter and the forces acting upon it, but the stars that have been disrupted begin to orbit the parent galaxy solely under the influence of gravity. The relative actions of each make for a provocative comparison. An attractive dark matter force should pull the satellite galaxy around its orbit faster than would be expected under the influence of gravity alone. A repulsive dark matter force should slow the satellite down.

So the team hopes to compare the leading and trailing stars with the movement of the satellite galaxies, contrasting observation to simulation in hopes of detecting a dark matter force even if it is only a fraction of that of gravity. “What we’re doing here is a twenty-first century equivalent of Galileo’s Leaning-Tower experiment,” says Kamionkowski. “Galileo demonstrated there that terrestrial materials all fall in the same way in a gravitational field, and we’re trying to figure out whether his conclusion applies to dark matter as well.”

Centauri Dreams‘ take: This is a long and demanding project. Kamionkowski and Kesden will use the Sagittarius dwarf spheroidal galaxy some 78,000 light years from Earth as the testbed, and their simulations indicate that dark matter forces just a few percent of that of gravity should be detectable with their methods. What we can say about the fundamental forces that govern our universe — and indeed about the ultimate fate of that universe — depends heavily on our learning how dark matter operates and what effects the so-called dark energy exerts on it. We can’t expect quick breakthroughs here, but the idea that there is a fifth fundamental force affecting matter we cannot see should put to rest the notion that we are anywhere near a true ‘theory of everything.’