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'The Universe is expanding'. You have probably heard that said many times. It may sound unbelievable but it is true. This has been proven by scientists looking at the Doppler Effect, otherwise known as the Redshift Effect, and Fraunhofer Lines.
All stars, including the Sun, emit electromagnetic (EM) radiation at many different wavelengths; only some of these are visible to the human eye. If you were to shine light through a prism you would see the spectrum of colours. This was first done by the great English mathematician, Sir Isaac Newton (1642 - 1727); although he did not follow up the discovery of the spectrum with any further research.
The different colours seen are different wavelengths of visible light. In 1802, the English chemist William Hyde Wollaston (1766-1828) discovered that the solar spectrum was not uniform. It contained dark lines at certain wavelengths. He mistook these lines as the boundaries between colours. We now know that the colours are continuous.
It was not until 1814 that German optician, Josef Fraunhofer (1787-1826), studied these lines in greater detail. He noted the positions of 324 them; although he still couldn't explain them.
Finally, in 1859, two more Germans, Gustav Kirchhoff (1824-1887) and Robert Bunsen1 (1811-1899), invented an instrument for separating light using prisms, which we know as the Bunsen-Kirchhoff spectroscope; founded the modern science of spectroscopy; and discovered the true reason for what were by then called Fraunhofer Lines. They found that certain elements absorb some wavelengths, which means that the elements found in the sun create black lines in the spectrum.
When an object is moving, any waves it emits are either stretched or squashed, so the wavelength either increases or decreases. In sound this makes something moving towards you sound higher, and something moving away sound lower. In light, an object such as a star, will be shifted towards the red end of the spectrum when moving away from you and towards the blue (or violet) end when coming towards you. The faster the object is moving, the greater the shift.
How This Proves Expansion Theory
Scientists knew that if the universe was expanding, then space would be stretched. This meant that anything in the space would also be stretched. This stretching is too small to be noticed in matter because gravity counteracts it; but in EM waves this is noticeable as a redshift.
When this redshift in the Fraunhofer lines of distant stars and galaxies was noticed it was clear proof that both the Expansion and Big Bang theories were correct. All other theories, such as Steady-State Theory2, were generally disregarded.
Another useful consequence of this is that the further away an object is, the more space the light has travelled through so it will be more redshifted, this allows astronomers to accurately calculate the distance of other galaxies.