Enlightened by the Darkness

By Diane K. Fisher
(Space Place)

On the clearest of nights, I may see a dozen stars from my suburban backyard near Los Angeles.  Unfortunately, my studies of space and astronomy have been confined to books and the pictures taken by others.  Seldom have I experienced for myself a truly dark, clear, moonless sky. 

One of those rare times was a summer camping trip in Bryce Canyon, Utah. I lay on my sleeping bag in an open area away from trees. I saw millions of stars (so it seemed) and the cloud of the Milky Way streaking across the sky.  Nothing of planet Earth was in my view.  It was then I glimpsed my true situation in the universe, a speck of dust clinging to a tiny stone hurtling through the darkness of a cold, infinite universe.  I was awestruck by the beauty of the stars and the darkness-and terrified!

In the light of day and a more "down-to-Earth" state of mind, I wondered:  With around 100 billion galaxies out there, why is it still so dark out there? 

Until the 20th century, astronomers thought the universe was infinite.  They were perplexed though, because in an infinite universe, no matter where you look in the night sky, you should see a star.  Stars should overlap each other and the sky should be blazing with light and hot as the sun. This problem became known as "Olber's Paradox." 

Astronomers now realize that the universe is not infinite.  A finite universe-that is, a universe of limited size-even one with trillions of stars, just wouldn't have enough stars to light up all of space. 

Although a finite universe is enough to explain the darkness, the expansion of the universe also contributes. As light travels from a distant galaxy to us, the space through which the light is traveling is expanding.  Therefore, the amount of energy reaching us dwindles all the time, thus causing the color of the radiation to be "redshifted." (The wavelength is stretched out due to cosmic expansion.) The more distant the galaxy, the more redshifted the light.  The largest redshift astronomers have measured comes from radiation that was emitted when the Universe was only 300,000 years old.  This radiation has taken over 12 billion years to reach us and although it began as infrared radiation, it is now seen as the microwave background radiation.

GALEX (Galaxy Evolution Explorer) is a NASA space telescope that will survey the universe, including galaxies with redshifts that indicate their light has been traveling for up to 10 billion years (or 80% of the history of the universe).  Read about GALEX at www.galex.caltech.edu/ .  For budding astronomers, print out The Space Place New Millennium Program calendar at spaceplace.nasa.gov/calendar.htm to identify great sky watching opportunities.
Diane K. Fisher is the developer and writer for The Space Place web site.

This article was provided by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.


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