Why are distant galaxies redshifted


Absorption lines in the spectrum

If you analyze the spectrum of distant stars and galaxies, you will find various specific absorption lines, just like in the spectrum of the sun. The absorption lines arise, for example, from the absorption of light of the respective wavelength in the star's atmosphere. However, some lines are also created when passing through the earth's atmosphere. By observing the stars with satellites, such absorption lines can be avoided today.

Stronger redshift in distant galaxies

If one compares how far individual, characteristic absorption lines in the sun and in various more distant galaxies are "shifted into the red", one finds that the redshift usually increases with the distance of the galaxies. Fig. 1 shows the redshift of the absorption lines of the solar spectrum compared to the redshift of the identical absorption lines of the galaxy BAS11, which is 1 billion light years away from us. The lines of the distant galaxy are clearly shifted further into the red.

Expansion of the cosmos as the cause of the redshift

The cause of this stronger redshift is not the relative movement of the galaxy in comparison to the earth. The cause of the cosmological redshift is the expansion of space itself: If light is emitted from a distant galaxy at a certain point in time, the light needs a long time to arrive on earth. During this time the universe continues to expand and with the universe also the wavelength of light. This means that the light is “stretched”, so to speak, with the expansion of the universe. This effect follows automatically from the equations of general relativity and is called cosmological redshift.

Size of the redshift

In astronomy, the redshift is often given with the dimensionless quantity \ (z \). The redshift \ (z \) is the ratio of the change in wavelength to the original wavelength \ (\ lambda_0 \), i.e. \ [z = \ frac {\ lambda _ {\ rm {observed}} - \ lambda_0} {\ lambda_0} = \ frac {\ lambda _ {\ rm {observed}}} {\ lambda_0} -1 \]

If, for example, the wavelength of the light has doubled on the way from the emitting galaxy to the earth due to the expansion of space, then \ (\ lambda _ {\ rm {observed}} = 2 \ cdot \ lambda_0 \). The redshift is thus \ (z = \ frac {2 \ cdot \ lambda_0} {\ lambda_0} -1 = 1 \).

With the help of the redshift \ (z \) one can also make statements about the size of the universe. If the measured redshift is e.g. \ (z = 1 \), then the universe was only half of its current size at the time the light was emitted. How this expansion took place, i.e. whether it was linear, accelerated, oscillating (= moving back and forth), etc., cannot be viewed as the redshift, but has to be discussed elsewhere.

Interplay with relative movement and gravitational red and blue shift

The red or blue shift due to a relative movement of a galaxy in comparison to the earth (analogous to the Doppler effect) only plays a role for very close galaxies. For example, the light of the Andromeda Galaxy, which is 2.5 million light years away, is shifted into the blue, since it is moving towards the Milky Way at about \ (114 \, \ rm {km / s} \). From a distance of about 100 megaparsecs, the proportion of the Doppler effect in the redshift is vanishingly small compared to the cosmological redshift.

Furthermore, the red or blue shift can still be caused by gravity. If light moves away from a center of gravity, it becomes less energetic and thus redshifted. If light falls on a center of gravity, it becomes more energetic and thus blue-shifted. This behavior can be derived directly from the conservation of energy and was predicted by Einstein as early as 1911.