Redshift

How quickly is the Universe expanding?

In the 1920s, Edwin Powell Hubble was analyzing the distances between Earth and various galaxies using the Mount Wilson Observatory in California. In the process, he measured the electromagnetic radiation of the galaxies and discovered mostly what are known as 'redshifts' in their spectra.
The illustration shows the redshift of the spectral lines of a far-away super galaxy cluster (BAS11). Compared to those of the Sun (top), the spectral lines of the galaxy cluster (bottom) are shifted into the red.

The illustration shows the redshift of the spectral lines of a far-away super galaxy cluster (BAS11). Compared to those of the Sun (top), the spectral lines of the galaxy cluster (bottom) are shifted into the red.

Technically that means that the distribution of the various wavelengths shifts into the red. The Doppler effect, among other things, is responsible for this: when a galaxy moves away from us, the spectral lines shift to the red, long-wave end of the electromagnetic spectrum. If a galaxy moves towards us, the spectral lines are shifted to the blue, short wave end of the spectrum. We are aware of a similar effect with sound waves: the pitch (for example, of an ambulance siren) changes as the source of the sound moves towards or away from the listener.

In 1929, Hubble published the outcome of his investigation – almost all galaxies are moving away from us and their speed increases in a linear relationship with their distance from us. According to today’s calculations, the ‘Hubble constant’ – the constant of proportionality between the distance of a galaxy and its speed – has a value of around 46 miles per second and per megaparsec. This means that a galaxy at a distance of one megaparsec would move away from us at a speed of 46 miles per second.

The galaxies are not racing away – space is simply expanding

It is rather improbable that the Earth is at the center of the Universe and all of the galaxies are thus moving away from us. More likely there is another cause for the frequent redshifts – the Universe is expanding. The Hubble constant thus indicates the speed at which the Universe is currently expanding. The observed redshifts are, therefore, less frequently caused by the Doppler effect than by the fact that space itself is being stretched in an expanding Universe and so too are the wavelengths of the electromagnetic radiation traveling through it.

Source: German Aerospace Center
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