The Cosmological Redshift

Astronomers routinely analyse the light from stars using the technique of spectroscopy. Spectroscopy subdivides light into its constituent colours — violet to red — and identifies from the presence of spectral lines, the chemical elements which are present in the star. These are the elements which are involved in nuclear reactions in the star. Each element is identified by a characteristic pattern of spectral lines.

As long ago as 1914 astronomers began to make a series of observations which led to the conclusion that distant stars had spectral lines which were shifted from their expected wavelengths in the spectrum, to positions which were at higher wavelengths towards the red end of the spectrum. Using observations well known from physics on the effects of relative motion on wavelength — the Doppler effect — the red shift in starlight is interpreted as the motion of the galaxy away from the observer.

The more distant a galaxy, the greater the redshift. Until recently the greatest redshift was 5.64. However, new data reported in the journal Science in 1999 reports galaxies with a redshift of 10. These formed when the Universe was only 9% of its current size and probably just a few hundred million years old.

Edwin Hubble made the very important observation that there is a simple relationship between the amount of red shift of a galaxy and its distance from earth. [See http://www.astro.ucla.edu/~wright/cosmo_01.htm.] This discovery, published in 1929 and now known as Hubble's law, can be expressed by the relationship:

v = rH0

The velocity (of a galaxy away from Earth) = the distance x a constant (known as the Hubble constant).

The precise value of the Hubble constant has been the subject of much debate, but has been determined with increasing precision since the 1920s. Recently the value was reported in the top American journal Science (1999, vol. 284, pages 1438-1439) as 70 km/sec per megaparsec (a megaparsec is 3.26 million light years).

The importance of the Hubble constant is that it expresses the rate at which the Universe is expanding. For this reason it is possible to calculate the age of the Universe from the Hubble constant. Clearly astronomers are keen to determine the Hubble constant with the greatest of precision.