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Linné on line arrow Physics and the Cosmos arrow Macrocosmos arrow Big Bang arrow The cosmic microwave background

The cosmic microwave background

The discovery of the cosmic microwave background has been called one of the most important scientific discoveries of the 20th century. This is probably an overstatement, but nevertheless it is one of the more important discoveries for understanding the evolution of the Universe. What is the cosmic microwave background? It is the radiation that has survived as a remnant from the Big Bang.

When the temperature in the Universe has decreased so much that the energy of the photons is too low to be able to ionise atoms they are decoupled. (You can read more about this here!) Decoupling means that they evolve independently of other matter in the Universe. As the Universe expands the energy of the photons decreases and their energy spectrum now corresponds to the "heat" radiation from a black body with the temperature 2.726 K. The figure shows the intensity of the radiation as a function of the frequency (wavelength). The measurements show very good agreement with the theoretical calculations of the spectrum of an ideal black body.

One important aspect of the background radiation is that it is isotropic, in other words the same in all directions. This shows that the Universe looks the same in all directions, which is necessary in an expanding Universe. It should be pointed out that there are also other models than the Big Bang-model, which also give an isotropic background. However, there are some small variations, which depend on inhomogeneities in the early Universe, that all models have problems in explaining.

The figure shows the deviations in background temperature from the ideal value. The top figure shows the recorded data. Here you can see the Doppler effect caused by the motion of the earth giving rise to an anisotropy. When this effect is subtracted you get the middle figure where you see a clear band across the picture. This band is caused by radiation from our own galaxy, the Milky Way. If you also subtract this effect you get the lower picture. Now we are left with the real fluctuations or irregularities in the background radiation which have their origin in the early Universe. It should be pointed out that the scale for the fluctuations is different in the three pictures, and that the resolution is increased in each step.