Gravity disappears at the Planck temperature

The cosmic background radiation

The cosmic background radiation is a remnant from the time of the Big Bang. With the COBE satellite, scientists have measured them for the first time and improved the models for the structure of the universe on the basis of the data obtained.

The theories of electromagnetic radiation and its propagation through space made great strides around 1900. Scientists also discussed the structure of the universe at this time. Einstein's theories of relativity had a major impact in both areas. It had gradually become clear that the universe must be extremely large. Since gravity is omnipresent and effective everywhere, one speculated about the stability of the universe. Gravity would pull everything together, so one wondered how the universe could apparently be so stable and so big.

The relatively simple basic equations that describe such a large structure therefore required a factor that counteracts gravity. For this purpose Einstein introduced the factor \ (\ Lambda \) (pronounced lambda), but later regretted it. Today we know that this so-called cosmological constant is actually necessary. Theoretically, the Dutch astronomer Willem de Sitter and the Russian scientist Alexander Alexandrowitsch Friedmann showed in 1922 that the universe must either expand or collapse if the pressure and density of matter in it are very small. This statement did not fit into the picture of the universe at that time and received little attention.

The origin of the big bang model

In the mid-1920s, work by American astronomer Edwin Hubble suggested that the universe was expanding. He had measured the so-called "escape speed" of galaxies: the further away a galaxy is from us, the faster it seems to be moving away from us. In the years that followed, the “Big Bang model” gradually established itself for the universe - in 1947 the British astronomer and mathematician Fred Hoyle defined the term Big Bang. Today the universe is big and cool. Since it is observed to expand, it must have been very compact a long time ago and therefore very hot.

Scientists assume that the energy density of the Big Bang - the beginning of space and time - was so great that initially there were not even atomic nuclei. The building blocks for it were still moving around freely. After a short period of time (according to our current knowledge within a few minutes) the lightest atomic nuclei were formed in the expanding and cooling universe, mainly hydrogen and helium, but also very small amounts of boron, beryllium and lithium. In a very hot gas, all atoms are ionized: the electrons normally bound to an atom are detached and buzz through space, as are the ions that are created in this way.

As the atom cools down further, the atomic nuclei can gradually capture free electrons. In the initially still dense and hot gas of the young universe, however, the existing intense radiation quickly releases the electrons. Due to this constant interplay - capture of an electron, release by a photon - the radiation field is both spatially homogeneous and homogeneous in terms of wavelengths. This radiation field always has the same spectral shape, but the radiation maximum lies at a wavelength that is correlated with the temperature: the cooler the material, the longer the wavelengths the radiation maximum. At that time only theorists thought about these properties of radiation in connection with the properties of the early universe.

Discovery of the cosmic background radiation

Sky view in the infrared

Researchers at Bell Laboratories in New Jersey (USA) examined the sky in the radio range in the early 1960s. In doing so, they encountered annoying background noise for which there seemed to be no known cause. This unexplained, weak radiation came from all directions and had a radiation temperature of around three degrees Kelvin, which corresponds to minus 270 degrees Celcius. Cosmologists immediately knew how to interpret the phenomenon: The cooled-down radiation field of the Big Bang had been discovered. In 1978 researchers Arno Penzias and Robert Wilson of Bell Laboratories were awarded the Nobel Prize in Physics for their discovery of the microwave radiation background.

Shortly after the Cosmic Microwave Background (CMB) - as cosmic background radiation is called in English - was detected in 1965, scientists have already carried out various measurements on it. The homogeneity across the sky, it turned out, is not really homogeneous. The cold dust that exists between the stars of the Milky Way and that predominantly radiates at far-infrared wavelengths, for example, also contributes a little to the wavelength range of the CMB signal. However, this effect can only be observed in areas of the sky where there is a lot of dust, for example in the direction of the galactic disk. There is also radiant dust in interplanetary space. Both contributions to the far-infrared background can be seen well in the sky survey carried out in the DIRBE experiment.

On earth, noise effects would have a strong influence on the measurement of cosmic background radiation, and the transmittance of the earth's atmosphere in the microwave range also fluctuates. As a result, radiation can best be measured from space. In the mid-1980s, NASA created the first specially designed satellite: the Cosmic Background Explorer, in short COBE. The start took place in 1989. The scientific management of the project was in the hands of John Mather, George Smoot and David Wilkinson. The three scientists supervised the project and managed the creation of the results (the first version was published in 1991). Wilkinson died in 2002, and John Mather and George Smoot received the 2006 Nobel Prize in Physics for their work on and with the COBE satellite.

COBE 1-3

The result of the COBE mission is a map of the sky in which tiny inhomogeneities can be seen in the CMB. This result was achieved after laborious data processing. Why was this so difficult?

  • The measurements, which recorded the sky in strips, had to be absolutely calibrated and the intensities at every point in the sky had to be determined from the multiple measurements. In the first approach, this resulted in a homogeneously luminous sky.
  • From other measurements it was known that there is a so-called "dipole effect" in the CMB. The Milky Way is moving towards the neighboring Andromeda Galaxy, while the Sun moves in almost the same direction on its orbit around the galactic center. If we now look in the direction in which we are moving, we measure a somewhat lighter and hotter background radiation than from the direction from which we are moving away. This dipole effect in the CMB is caused by the Doppler effect and has to be subtracted from the overall picture.
  • The contribution of the dust radiation (see recording of the DIRBE experiment above) must be removed from the measurement data. So-called synchrotron radiation and thermal radio radiation also make smaller contributions to the measured background radiation, which must be corrected. Since none of these radiation contributions are spatially evenly distributed, they create a certain uncertainty in the end result.
  • If you remove all disturbances from the measurement data, you get a sky map in which tiny fluctuations in intensity can be seen. With the help of these fluctuations, scientists can study the original state of the universe.

What does the CMB reveal?

COBE 4

When the universe grew to a sufficient size around 380,000 years after the Big Bang and the primordial gas had already cooled down considerably, the free electrons could be captured by the ions and form neutral atoms. Immediately afterwards the radiation hardly interacted with the matter and was able to spread freely in the universe - physicists say that radiation and matter “decoupled” from each other. This only occurs when the gas has fallen below a certain temperature and density limit. It is estimated that the transition temperature at that time was around 3000 Kelvin (2730 degrees Celsius).

At that point in time, the intensity spectrum of the background radiation almost matched that of a so-called black body with a temperature of 3000 Kelvin. This means an idealized body which absorbs all radiation that hits it and which itself has a characteristic spectrum that only depends on its temperature. Because of the expansion of the universe, the wavelength of the background radiation has increased over time, so that we now observe it in the microwave range. Their spectrum now corresponds to a black body with a temperature of 2.728 Kelvin, or –270.42 degrees Celsius.

However, the radiation field was not completely homogeneous at the time of decoupling. It is assumed that small fluctuations in the density of matter (and thus also in the local temperature) formed during the expansion of the universe. In the somewhat cooler areas, the free electrons were captured earlier and the radiation decoupled from the matter a little earlier than in the somewhat hotter areas. As a result, small spatial inhomogeneities arose in the radiation field - inhomogeneities that are visible today as fluctuations in the CMB. Thus, the pattern of these fluctuations detected by COBE tells researchers something about the conditions in the early universe.

However, the structures in the background radiation, as can be seen on the sky maps recorded by COBE, must not be equated with real density structures. Since there have been such structural fluctuations everywhere in space and we can see the radiation in all directions over great distances, these fluctuations are, as it were, added up over the entire line of sight. So you can see the effect of a sum of fluctuations.

What the fluctuations show us can only be determined with the help of model calculations. Such models show, for example, that fluctuations can only arise if there is a sufficiently high density in the universe. Scientists see this as another argument for the existence of so-called dark matter. The analysis of the properties of the cosmic background radiation has thus contributed significantly to the development of models for the structure of the universe.