The Astronomical Unit


This page last modified 1998 July 16

Redshift and the Scale of the Universe

One of the endeavours of astronomy has been to try to ascertain the scale of the universe. Recall Bradley's attempt to triangulate the distance to Eltanin. In 1837, Friedrich Wilhelm Bessel found the parallax of 61 Cygni to be 0.3136 arc seconds; i.e. 10.3 ly distant. (A light year is the distance light travels in a year. A parsec, or parallax second, is 3.2616 ly) His apparatus was accurate to about 0.05 arcsec (a pinhead at 2 miles). This was a near star and the results were staggering. The nearest star, Proxima Centauri, is 4.3 ly distant.

In 1908, Henrietta Leavitt discovered the period-luminosity relationship of Cepheid variables, by studying those in the Small Magellanic Cloud — all the stars there can be deemed to be the same distance from us. Brighter Cepheids have longer periods. The Cepheid scale was calibrated in 1913 when Hertzprung measured the parallax of Cepheids in our own galaxy. This was revised in 1950 when Walter Baade determined that there were two distinct families of Cepheids, each with its own period-luminosity relationship. Recent observations have suggested that the scale needs to be recalibrated again. A period-luminosity relationship was also found for RR Lyrae variables.

In 1914, Vesto Slipher discovered that the spectral lines of several nebulae were shifted to the red end of the spectrum. Edwin Hubble interpreted this as the consequence of an expanding universe, as predicted by the equations of Albert Einstein. (Einstein didn't know that the universe was expanding, so he introduced a "fiddle factor", the Cosmological Constant, into his equations in order that they kept the universe stationary — he later called this his "biggest blunder"). Hubble reasoned that redshift is proportional to distance(Hubble's Law). The factor of proportionality, once known as the Hubble Constant, has been frequently re-evaluated — as a consequence, it is now known as the Hubble Parameter. For large redshifts this proportionality falls down and the Special Theory of Relativity has to be taken into account. It should also be noted that redshift can result from gravitational fields.

One consequence of an expanding universe is that it must once have been smaller. Taken to it's limit, this reasoning points towards a time when the universe was a singularity, a point of zero size (Einstein's theories confirm this). The initial eruption of this singularity in a fireball of super-hot, super-dense, energetic radiation is referred to as the Big Bang. The term was a derisory one coined by Fred Hoyle, who remains an opponent of the theory. Theories published in 1998 also contest the idea of a singularity. The reality is that we do not know how the universe came into existence.


This topic is covered to some extent in most books on cosmology. An easily understandable account is given in John Gribbin's excellent Companion to the Cosmos, which covers a number of other topics as well.