Hubble Law

I often read about galaxies, quasars and other such distant things in the Universe. When I say distant I really do mean distant; they are millions to billions of lightyears away from us. Not only are they far, far away (curse George Lucas for making that sound clichéd), but most galaxies seem to be heading off as if our galaxy had a really bad case of body odour. The first hints of this came around 1912 when Vesto Slipher first measured the velocity of the Andromeda Nebula (now known as the Andromeda Galaxy). Unfortunately, galaxies don't come with handy speedometers, so Slipher had to make use of a technique called spectroscopy to work out their speeds.

Every chemical element in the Universe gives off specific colours of light, from the colours you see in a rainbow to radio waves, UV rays and X-rays. These are called emission lines. In fact, each chemical has its own unique set of emission lines which allow us to tell it apart from all the rest. This gives astronomers evidence equivalent to fingerprints used by the police to identify criminals. So measuring the light lets us work out the chemical composition of distant objects, but it can do much more. When the distant galaxy is moving, with respect to us on the Earth, all the colours are changed in a nice, predictable way. If the galaxy is moving away from us they get redder (redshifted) and if it is moving towards us they get bluer (blueshifted). This change in the colour is called the Doppler effect after the Austrian scientist Christian Doppler. You've most likely heard this effect with sound waves as a police car speeds by you. It is also used by police radar to work out that you were driving at 37 m.p.h in a 30 zone.

So Vesto Slipher was able to measure the colours of the emission lines and work out how fast a galaxy was moving by how much redder or bluer it was than would be expected. He determined that the Andromeda galaxy was coming towards us rather quickly - about 300 km/s. This seemed a tad fast, so he checked the value by observing the Sombrero galaxy. That was even worse; it was moving at 1000 km/s away from us! Over the following years he measured the velocities of another 24 galaxies and was puzzled to find that all but four were moving away.

This remained a puzzle until several years later and a lot of work by Milton Humason and Edwin Hubble. Between them they measured the distances and redshifts of 46 galaxies and plotted the values on a graph (you can do this yourself). The graph showed that there was a definite link between redshift and distance; generally the further the galaxy from us, the more redshifted it was. This became known as the Hubble Law and we now know that it is due to the expansion of the Universe.

So for really huge distances, that would otherwise have way too many zeros on the end of them, astronomers tend to use the redshift value as a measure of the distance. Redshifts are usually represented by the letter 'z' and start at zero for something that isn't moving. The redshift increases as the distance increases, but this isn't linear so it quickly gets quite big. A distance of 5 billion light years corresponds to a redshift of about 0.5, 10 billion light years is about 2.0 and 13 billion light years is around 8.0. This can be confusing, so luckily Ned Wright at UCLA has created a cosmological calculator to help work it out.

The direct link between speed and distance has done a fairly good job for the last 80 years or so, but in the last decade observations of distant supernove show that it doesn't quite work at the very largest distances. That, as they say, is a story for another day.

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Posted in astro blog by Stuart on Thursday 25th Aug 2005 (14:51 UTC) | Permalink
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