Friday, January 3, 2014

What Does the Future of the Universe Hold?

The collision of our galaxy with the Andromeda galaxy is billions of years away, but it’s never too early to wonder what will happen.

The Andromeda Galaxy 
It is remarkable how often the origin of things is tied to the very same phenomena that ultimately lead to their demise—a fact that is especially evident when we ponder cosmic endings, from the end of the Earth to the end of the universe.

For instance, planetary scientists increasingly suspect that comets (frozen balls of dust and ice) and ice-laden meteorites crashing into the primordial Earth probably provided most of the planet’s water—and perhaps much of the organic material—necessary for life. Organic molecules have been detected in comets such as the Hale-Bopp, and, in a recent study, researchers simulated those cosmic crash landings by using a gas gun to fire metal projectiles at 16,000 miles per hour into blocks of ice containing some of the same chemicals that make up comets. The shock wave and heat generated by the impact created molecules that formed amino acids, the building blocks of proteins.

Yet the very same objects that gave this planet life could also spell its demise. Astronomers predict that a comet or asteroid large enough to cause global devastation will smash into the Earth about every 100 million years or so. Fortunately, if such a comet or asteroid were to arrive sooner than expected, we are constructing observational systems to discover and track near-Earth objects, conceivably providing us with sufficient time to pre-empt catastrophe.

Other cosmic smash-ups, however, cannot be averted, no matter how much advance warning we have. The inexorable tug of gravity that enabled the formation of the Milky Way has also put us on a collision course with our neighboring galaxy, Andromeda. Recent observations confirm that Andromeda is heading straight toward us at about 60 miles per second, and will traverse the 2.5 million light-year distance currently separating our galaxies in about four billion years.

While the collision of two galaxies might conjure up images of mass devastation, the event will be largely imperceptible to our descendants, if any are still around. (They will have had to find another home: By that time, the increasing luminosity of our sun will have rendered Earth uninhabitable.) Galaxies are mostly empty space, so almost no stars or planets will actually collide.

Nonetheless, the Milky Way as we know it will cease to exist. Initially, the two galaxies will slide past each other and draw apart until gravity hits the brakes and pulls them back together. As Andromeda and the Milky Way merge, both will lose their disk-like structure, forming a single elliptical galaxy that some astronomers have dubbed “Milkomeda.”

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