Wonders of the Universe

HUBBLE EXPANSION

So, how do we know that the irregular, messy galaxies in the Hubble image are billions of light years away? The picture below shows some of the most distant galaxies we have observed. The most obvious thing about them is that they are all red. Why is this so? To answer this question correctly, we need our friend Edwin Hubble, the astronomer, again.

During the 1920s, Edwin Hubble was using what was then the world’s most powerful telescope at the Mount Wilson Observatory in Pasadena, California, to observe stars called Cepheid variables. These Cepheid variables are stars whose brightness varies regularly over a period of days or months, and they are astonishingly useful to astronomers because the period of their brightening and dimming is directly related to their intrinsic brightness. In other words, it is a simple matter to work out exactly how bright a Cepheid variable star actually is just by watching it brighten and dim for a few months. If you know how bright something really is, then measure how bright it looks to you, you can work out how far away it is. Edwin Hubble’s research project was simply to search for Cepheid variables in the sky and measure their distance from Earth. During his observations, he discovered two remarkable things: firstly, he quickly determined that the Cepheid variables he found in the so-called spiral nebulae (which at the time were thought to be clouds of glowing gas within the Milky Way) were in fact well outside our galaxy. For the first time, Hubble showed that there are other galaxies in the Universe, millions of light years away.

This image shows some of the most distant galaxies that we have observed – and all appear in a bright, sharp, red colour.

Hubble’s second observation was of even greater scientific importance. While he and others were also busy measuring the spectrum of the light from the stars in the spiral nebulae, which thanks to Hubble were now understood to be other galaxies beyond the Milky Way, they quickly observed that many of the galaxies appeared to be emitting light that was redder than it should be. Hubble quantified the amount of reddening in each galaxy as a number called redshift. Remember that red light has a longer wavelength than blue light, so seeing light redshifted simply means the wavelength is longer than expected. Hubble made his second great discovery by plotting a graph of the redshift of the light from the distant galaxies against their distance, which he had calculated from his observations of the Cepheid variables.

To his great surprise, Hubble noticed that his graph was approximately a straight line. This is because the further away a galaxy is, the greater its redshift – i.e. the more its light is stretched, and there is a very simple relationship between the distance and the redshift. Why is this? Well, the interpretation of Hubble’s result is quite remarkable. The more distant the galaxy, the further the light has travelled across the Universe to reach us. Also, the further it has travelled, the more it has been stretched. This relationship between distance travelled and amount of stretching occurs when something very simple but surprising is happening to the Universe. It is expanding! In other words, over the hundreds of millions of years during which the light has been travelling, space itself has been stretching at a relatively constant rate, and this has stretched the wavelength of the light in direct proportion to the distance it has had to travel. This is why the most distant galaxies have the largest redshift – their light has travelled through our expanding universe for longer and has therefore become more stretched. Hubble’s discovery of this so-called ‘cosmological redshift’ was one of the great intellectual moments in twentieth-century science, because he discovered that we live in an expanding universe.

HUBBLE’S LAW:

This diagram illustrates Hubble’s Law; the redshift of the light from distant galaxies is plotted against their actual distance, resulting in a straight line on the graph.

Stephan’s Quintet is a cluster of five galaxies in the constellation Pegasus, two of which, in the centre, appear to be intertwined. Studying the individual redshifts reveals that one of the galaxies is an interloper: the larger, bluer one at upper left is in fact a foreground galaxy seven times closer to us than the others. So redshifts allow us to create a three-dimensional model of the Universe.

NASA