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Laws developed by Edwin Hubble.


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In 1912, Vesto M. Slipher, an American Astronomer who studied the spectra of galaxies, had noticed that, except for a few systems such as the Andromeda galaxy, the spectral lines were shifted toward longer wavelengths. This shift in wavelength, caused by the Doppler effect, showed that most galaxies were receding from our Galaxy at several hundred kilometres per second. Originally it was thought that these velocities were orientated randomly.

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The speed of recession of a Galaxy and its present distance from us can be used to estimate the length of time required for the universe to reach its present state. This will be a maximum figure, as the present expansion has already been slowed by the mutual gravitional attraction of galaxies.

In 1929 Edwin Hubble compared the distances he had estimated for various galaxies with the red shifts determined by Slipher for the same galaxies. He found that the more distant the galaxy, the higher was its recession velocity. This important relationship has become known as Hubble's law. It also showed that the universe was not static as most people had previously assumed.

Many Cosmologists had realised that gravity would cause the stars and galaxies to attract each other and that the universe would therefore contract, some, like Newton, had claimed, wrongly, that in an infinite universe the attraction of bodies 'outside' any particular body would give an outward force to balance the inward one. Even Einstein, when formulating general relativity in 1915, invented a repulsive force to balance gravity and keep the universe static.

This he called 'the cosmological constant', and later "the biggest mistake of my life" when Hubble announced his discovery.

In symbols Hubble's law is;

v = H0 r

The ratio of the recession velocity (v) of a Galaxy to its distance (r) is called the Hubble constant, H0 It has been very difficult to get an accurate figure for this constant as the distance to Galaxies has had several revisions over the years. Originally Hubble's own estimate was 800 but now it is estimated to be between 50 and 100 km/sec per megaparsec with a most likely figure of 86.

1/H0 is the ratio distance/velocity which is equal to time. In this case the time since the galaxies were all touching, assuming their velocities have been constant. This figure s thus a rough guide to the age of the universe. As the estimates of H0 have been revised downwards, so our estimated Age of the universe has gone up. This has taken away the paradox of some stars appearing to be older than the Universe itself, (and removed one of the reasons for the Steady State model being adopted).

Because Galaxies in all directions seem to recede from our galaxy, it might appear that our Galaxy is the centre of the universe. This is not the case, however. While Einstein and others were trying to wrestle with general relativity and force it to give a static universe, Alexander Friedmann took a different approach. He made two assumptions; the universe looks the same in all directions, and this would be true wherever anyone looked from. From these assumptions he solved the equations for general relativity and showed, in 1922, that the universe would not be static but would be expanding at a rate that was proportional to distance, exactly what Hubble later discovered.

One can imagine a balloon with evenly spaced dots painted on it. As the balloon is blown up, an observer on each spot would see all the other spots expanding away from it, just as observers see all the galaxies receding from our Galaxy. The analogy also provides a simple explanation for Hubble's law; the universe is expanding like a balloon.




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