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How Old is Old?
How old is the universe? How old is the human race? Cosmologists believe the universe to be 15 billion years old. It began with an immense explosion called the Big Bang, the echoes of which can still be detected by radio telescopes today. It expanded from an initial singularity and passed through several developmental stages. Today, it is still expanding. Astronomical observations of distant galaxies reveal tell-tale red shifts in their spectra. The further the galaxy, the greater the red shift. From these data the rate of expansion is inferred. The rate of expansion is expressed in a number called the Hubble Constant, after Dr. Edwin Hubble, who made the first observations of these red shifts and correctly assessed their meaning. The Hubble Constant today is estimated between 50 and 100 (km/s)/Mpc.
Our solar system is believed to be 4.6 billion years old. The solar system has existed for approximately the last third of cosmic history. A scale model may be built to help comprehend these numbers. If the history of the universe were represented by a 3,000 page book (for comparison, Webster's Unabridged New International Dictionary, Second Edition, holds a total of 3,208 pages) then the entire history of the solar system would be represented by the last 920 pages.
Those 920 pages include the formation of the sun and planets, the dawn and rise of life, the great geological ages, and the entire history of the human race:
If an average of 500 words per page is assumed then the entire book would contain 1,500,000 (1.5 million) words. Each word would represent 10,000 years of cosmic history. The last word of the last page would represent all of recorded human history.
Surely, humanity is a newcomer to the planet!
Please send suggestions/corrections to:
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Responsible NASA Official: Theresa.M.Scott (Acting)