The Official String Theory Web Site:--> Cosmology :--> How old is the Universe? (basic / advanced)
 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.
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How old is the Universe? // Structure
of the Universe // The Big Bang // Before
the Big Bang? // What about string theory?
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