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.

 A very massive, very old cluster of galaxies, as photographed by the Hubble Space Telescope

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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