18 January 2011


It is a truism familiar to all students and enthusiasts of astronomy and cosmology -- our universe is numbingly vast. The distances between stars and galaxies are so immense that light itself, traveling at just over 186,000 miles per second, takes a long time to span the gulf of space. Nothing can travel faster than light. Here are a few facts to put speed and distance into perspective:

  • If a photon of light were able to bend its course into a curve, it would circle the Earth at the equator 7.75 times in one second.

  • That same light takes 8.3 minutes to travel the 93 million miles between the Sun to the Earth. This means that each time we look at the sun, we aren't seeing it as it appears now. We're seeing it as it appeared 8.3 minutes ago. We are seeing the past. We are witnessing history.

  • The more distant the object, the longer it takes light to travel the distance from that object to our eye, hence the further back in history we see. The nearest star to our sun, Alpha Centauri, is situated 4.37 light years away. A light year is a measure of distance (how far light travels in a year's time), not a measure of time. So when we look at the nearest star, we are seeing it as it appeared 4.37 years ago, not as it appears now. Even older history, but nothing like the history we observe when we view the most distant objects in the universe, whose light takes billions of years to reach us. Wrapping one's imagination around such distances could fry one's brain.

With this as background, I refer you to a NYTimes article titled Darkness on the Edge of the Universe. The article provides a clear explanation of the history of our understanding of not merely the size of the universe, but its evolution. For our universe is expanding, as discovered by Edwin Hubble in 1929. His finding upset Einstein's vision of a static universe, and in the decades since physicists have struggled to understand the balance between the force which holds the matter in the universe together, gravity, and the antigravity force which drives the matter in the universe apart, dark energy.

In recent years, two teams of astronomers have independently arrived at the same conclusion -- that the rate of expansion of the universe, far from slowing down, is actually accelerating. Standing beside the vital questions raised is "an unassailable conclusion, one that's particularly unnerving. If the dark energy doesn't degrade over time, then the accelerated expansion of space will continue unabated, dragging away distant galaxies ever farther and ever faster. A hundred billion years from now, any galaxy that's not resident in our neighborhood will have been swept away by swelling space for so long that it will be racing from us at faster than the speed of light. (Although nothing can move through space faster than the speed of light, there's no limit to how fast space itself can expand.)

Light from such galaxies will therefore fight a losing battle to traverse the rapidly widening gulf which separates us. The light will never reach Earth and so the galaxies will slip permanently beyond our capacity to see, regardless of how powerful our telescopes may become.

Because of this, when future astronomers look to the sky, they will no longer witness the past. The past will have drifted beyond the cliffs of space. Observations will reveal nothing but an endless stretch of inky black stillness. (Click on any image to enlarge.)

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