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The age of
the Universe has been a subject of religious, mythological and scientific
importance. On the scientific side, Sir Isaac Newton's guess for the age of
the Universe was only a few thousand years. Einstein, the developer of the
General Theory of Relativity, preferred to believe that the Universe was
ageless and eternal. However, in 1929, observational evidence proved his
fantasy was not to be fulfilled by Nature.
 In order to understand this
evidence, let's think about how a train sounds to a person standing on the
platform. An arriving train makes a noise that starts low and gets higher
pitched as the train approaches the listener, sounding like oooooohEEEEEEEE.
A departing train makes a noise that gets lower pitched as the train goes
away from the listener, sounding like EEEEEEEEoooooooh. This change in the
sound of the pitch of the train noise depending on whether it is arriving or
departing the listener is called the Doppler shift. The Doppler shift happens
with light as well as with sound. A source of light that is approaching the
viewer will seem to the viewer to have a higher frequency than a source of
light that is receding from that viewer. In 1929, observations of distant
galaxies showed that the light from those galaxies behaved as if they were
going away from us. If all the distant galaxies are all receding from us on
the average, that means that the Universe as a whole could be expanding. It
could be blowing up like a balloon. If the Universe is
expanding, then what did it expand from? This is what tells us that
the Universe probably does have a finite age, it probably is not eternal and
ageless as Einstein wanted to believe. But then, okay, how old is
the Universe? We know from studies of
radioactivity of the Earth and Sun that our solar system probably formed
about 4.5 billions years ago, which means that the Universe must be at least
twice that old, because before our solar system formed, our Milky Way galaxy
had to form, and that probably took several billions years by itself. It would be reasonable to
guess that the Universe is at least twice as old as our Sun and Earth.
However, we can't do radioactive dating on distant stars and galaxies. The
best we can do is balance a lot of different measurements of the brightness
and distance of stars and the red shifting of their light to come up with
some ballpark figure. The oldest star clusters whose age we can estimate are
about 12 to 15 billions years old. So it seems safe to
estimate that the age of the Universe is at least 15 billion years old, but
probably not more than 20 billion years old. This matter is far from
being settled by astrophysicists and cosmologists, so stay tuned. There could
be radical new developments in the future. |
