"The two galaxies are gravitationally bound to each other," Loeb said. "Eventually they will come together."
The model is also based on the probable distribution of mass within the "Local Group"—a region some 10 million light-years across that includes the Milky Way, Andromeda, and a number of smaller galaxies.
Scientists believe the space between the galaxies is occupied by a diffuse medium of gas and invisible "dark matter."
Cox and Loeb say friction from this medium should act like a cosmic braking system, causing the galaxies to lose momentum and ensuring their eventual merger. (Related: "Dark Matter Ring Detected by Hubble" [May 15, 2007].)
"This is a very interesting piece of work," said astronomer Gregory Laughlin of the University of California, Santa Cruz. "They've taken care to incorporate the best available estimate of Andromeda's [sideways] velocity, although that's still the most uncertain ingredient in the simulations."
The University of Toronto's John Dubinski noted that the model can be improved with better estimates once the Space Interferometry Mission (SIM) or the European mission GAIA can measure Andromeda's sideways velocity, in the coming five to ten years.
Welcome to Milkomeda
The new model shows the likelihood of various possible fates for our own solar system as the enormous galaxies bump and merge.
For example, there is about a 10 percent chance that the sun and its planets will be ejected to the trailing edge of the Milky Way by the first close encounter two billion years from now.
There is even a slight chance—about 3 percent—that the solar system will be scooped up by Andromeda on the second passage, leaving the Milky Way completely until the galaxies re-unite.
Such cosmic spectacles would certainly be visible to any future earthbound observers of the night sky.
The aging and warming of the sun may make Earth uninhabitable before the merger is complete.
But Loeb likes to speculate that our descendents might still be around to witness the birth of Milkomeda.
In two billion years, he said, "in addition to the strip of the Milky Way, there will be another dense band of stars. That's the Andromeda galaxy approaching us."
As the galaxies hurtle past and through one another, he added, near-passing stars may unleash spectacular showers of comets.
Ultimately, however, Cox and Loeb's calculations show that the most likely outcome for the solar system is exile to the far periphery of Milkomeda.
"The sun will be kicked to a region with fewer stars," Loeb said. "The chance of having other stars nearby will be far smaller than today."
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