How old is the Universe? (1)

These days it is universally accepted that the Big Bang happened around 13.8 billion years ago but how do we know that? In a series of three or four posts I will outline the path which has led science to this fascinating insight.

As mentioned in a previous post on da-boss in the ancient times humans thought of the Universe as unchanging and everlasting. The scientific basis for this theory was the observation that the stars appear static, both in their position and brightness. However, in 19th century more accurate measurements showed that there are some minute variations in the relative position of some stars. These changes appeared to follow an annual cycle and soon the astronomers realised that it was not the stars but the Earth that was moving. The phenomenon, known as parallax, is demonstrated in the diagram below:


Essentially, the stars which are closer to us appear to move against the background of the more distant stars because our vantage point changes during the annual journey of the Earth around the Sun. Knowing the Earth’s orbit it is possible to work out the distance to a star, depending on how much it appears to move in the sky between summer and winter. This is how the distance to star 61 Cygni was first measured in 1838 by Friedrich Bessel. Because the angular changes in the observed position of stars are miniscule the parallax method of measuring celestial distances will only work for the stars which are closest to us.

In another interesting development Hippolyte Fizeau reported in 1848 that the light coming from some stars is “redshifted” – meaning that its spectrum is moved toward the long wave (red) end of the visual range. This is similar to the Doppler effect apparent when a car whizzes past us and the frequency of the engine noise lowers. The same way the light emitted by objects moving away from us will shift towards lower frequency – meaning red. The problem was that the stars whose light was redshifted were way too far for us to measure the distance to them using parallax.

To sum things up, at the end of 19th century we knew the distance to the closest stars outside the Solar System and understood that many of the more distant stars were moving away from us. However, the commonly held belief was still that the Universe is static – meaning neither contracting nor expanding. The next post will outline the startling developments which shook the astronomers’ world in the first half of 20th century so stay tuned!

How old is the universe? (2)


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