Why is the cosmic microwave background crooked

Cosmic background radiation

An interference signal in the microwave range proved to be particularly persistent. It seemed to come from all directions in space with equal intensity. Assuming that the source of this radiation radiated like a black body, the emission of which can be described by Planck's law of radiation, a temperature of three Kelvin could be assigned to the interference radiation.

Without looking for it, Penzias and Wilson had discovered something that decisively intervened in the heated debate among cosmologists at the time about the origin of the universe. Did the universe have a beginning? Or has it always existed? Two conflicting ideas faced each other. The Big Bang model stated that the universe was initially so hot and dense that only radiation and elementary particles were present; as it developed further, it expanded and cooled down, gradually forming stars and galaxies. The competing doctrine assumed an everlasting universe; In order to explain the observed expansion movement of the galaxies, the continuous generation of matter had to be postulated in this hypothesis.

The three Kelvin radiation that Penzias and Wilson had discovered now brought the decision. Because the Big Bang model predicted exactly such radiation: When the hot primordial soup of particles and radiation had cooled down so far that the free protons and electrons came together to form stable atoms (which happened at a temperature of around 3000 Kelvin), space became transparent, and from then on the radiation could propagate unhindered by the matter. The hot heat radiation from this recombination phase, as this era is also called by physicists, cooled down by around a factor of 1000, i.e. to around three Kelvin, in the course of the further expansion of the cosmos.

Penzias and Wilson's measurements had shown three Kelvin radiation at a single wavelength. Follow-up measurements - as most recently with the European Planck satellite - have confirmed the spectral course at all wavelengths. The spectrum of the cosmic background radiation resembles the radiation of a black body with a temperature of 2.728 Kelvin with high accuracy. This radiation, which we measure today coming from all directions in space, is, so to speak, the afterglow of the Big Bang. Competing models such as the idea of ​​an eternal universe (steady state theory) cannot explain the cosmic background radiation.

After Penzias and Wilson published their measurements in 1965, cosmology began to transform from a more philosophical discipline to an empirical science. With the help of measurement data, important parameters can now be derived with which the properties of the universe can be described. Deficiencies in the original Big Bang model were eliminated through extensions - such as the idea that the cosmos expanded extremely quickly in the first fractions of a second after the Big Bang (inflation phase). With modern measurement technology it has been possible in the last decades to detect tiny fluctuations in the cosmic background radiation, which were required by the theory. And since 2002 we have also known that the background radiation is polarized - this was also a prediction of the model. With the discovery of the B modes in this polarization by the BICEP2 experiment in March 2014, another indicator has now been added that proves the correctness of the big bang model. However, the interpretation of the BICEP2 results is still controversial - other experiments will have to confirm the measurement data.