Cosmic microwave background

Cosmic microwave background. If our radio telescope were able to tune to frequencies close to 280 GHz, we would observe that the intensity of the signal decreases on both sides in a particular way and surprisingly equivalent to the signal that we would measure at the output of a small hole made in the perfectly absorbent walls. from a hollow object (a black body) at about 2.73 degrees above absolute temperature zero. Technically this signal is usually called the Cosmic Microwave Background (it is common to use the English acronym CMB, for Cosmic Microwave Background).


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  • 1 Features
  • 2 History
  • 3 Static and expansion models of the universe
  • 4 Evolution of the universe
  • 5 Observations of the microwave background
  • 6 The future of the cosmic microwave background
  • 7 Bibliography
  • 8 External links
  • 9 Source


Only the Big Bang model gives us a simple answer to the existence of this microwave background. If the universe is expanding, it could have been smaller, denser, and hotter in the past. At some point the temperature was so high that not even atoms could exist as such, with the electrons being detached from the nuclei.

Under these conditions, electrons interact with light particles ( photons ) in a very efficient way. In other words, the light was in close contact with matter, both reaching a perfect thermal equilibrium. But the expansion of the universe cooled the environment until reached about 3000K the electrons began to rapidly combine with the nuclei forming atoms

At that moment, light began to travel freely, finding fewer and fewer electrons in its path. That light is still between us (about 400 photons per cubic meter), but the expansion of the universe has had the effect of drastically decreasing the frequency until it becomes microwave.


CMB was first detected by two technicians from Bell Labs, Arno Penzias and Bob Wilson in 1965 (see a detailed history of predicting and detecting this signal).

Shortly after the discovery of background radiation, in 1967 Sachs and Wolf (1967, ApJ, 147, 73) suggested that the first clusters of matter that would eventually form the great galactic structures that we see today may have produced fluctuations in intensity of the background radiation in different regions of the sky. This would be basically because the photons that have reached us from regions of higher density of matter have to scale the greater barrier of gravitational potential and lose energy (see Martin White & Wayne Hu 1996 for a pedagogical derivation of the effect).

But don’t be disappointed by the reader. The measured fluctuation range is equivalent to about 30 microKelvin in temperature. This can be converted into gravitational potential by the result obtained by Sachs and Wolf and the result in a potential difference equivalent to a height difference of 2 ua ( astronomical units ), considering a constant gravity acceleration equal to that in Earth’s surface: the valleys of potential in the early universe were actually quite appreciable.

Static and expanding models of the universe

The Models of the Universe According to the generally accepted theory of the Big Bang , the Universe originated between 10,000 and 20,000 million years ago and has been expanding ever since. The future of the Universe is uncertain: the expansion could be limited (closed Universe), the Universe contracting on itself, or it could be infinite (Open Universe), in which case the Universe will continue to expand forever. In the limit case between these two possibilities (flat Universe), the expansion will not stop either.

In 1917 Albert Einstein proposed a model of the Universe based on his new theory of general relativity. He considered time as a fourth dimension and showed that gravitation was equivalent to a curvature of the resulting fourth-dimensional space-time. His theory indicated that the Universe was not static, but should either expand or contract. The expansion of the Universe had not yet been discovered, so Einstein raised the existence of a repulsive force between the galaxiesit compensated for the gravitational pull of attraction. This led him to introduce a “cosmological constant” into his equations; the result was a static universe. However, he missed the opportunity to predict the expansion of the Universe, what Einstein would describe as “the biggest mistake of my life”.

Evolution of the universe

One of the unsolved problems in the expanding Universe model is whether the Universe is open or closed (that is, whether it will expand indefinitely or contract again). One attempt to solve this problem is to determine if the mean density of matter in the Universe is greater than the critical value in Friedmann’s model.. The mass of a galaxy can be measured by observing the motion of its stars; multiplying the mass of each galaxy by the number of galaxies it is seen that the density is only 5 to 10% of the critical value. The mass of a galaxy cluster can be determined in an analogous way, by measuring the movement of the galaxies it contains. Multiplying this mass by the number of galaxy clusters gives a much higher density, which is close to the critical limit that would indicate that the Universe is closed. The difference between these two methods suggests the presence of invisible matter, the so-called dark matter , within each cluster but outside the visible galaxies. Until the phenomenon of hidden mass is understood, this method of determining the fate of the Universe will be unconvincing.

Many of the usual works in theoretical cosmology focus on developing a better understanding of the processes that must have led to the Big Bang. Inflationary theory, formulated in the 1980s , solves major difficulties in Gamow’s original approach by incorporating recent advances in elementary particle physics. These theories have also led to speculation as bold as the possibility of an infinity of universes produced according to the inflationary model. However, most cosmologists are more concerned with locating the whereabouts of dark matter, while a minority, led by Swede Hannes Alfvén, winner of the Nobel Prize in Physics, maintain the idea that not only gravity but also plasma phenomena have the key to understanding the structure and evolution of the Universe.

Microwave background observations

After the CMB discovery, hundreds of cosmic microwave background experiments have been performed to measure and characterize the nature of radiation. The most famous experiment is probably the satellite COBE of NASA that orbited between 1989 – 1996 , which detected and quantified the large scale anisotropies at the limit of its detection capabilities. Inspired by the initial results of COBE , an extremely isotropic and homogeneous background, a series of balloon and soil-based experiments quantified CMB anisotropies at small angular scales over the next decade.

The main objective of these experiments was to measure the first acoustic peak, for which COBE did not have sufficient resolution, on an angular scale. These measures could exclude cosmic strings as the main theory of cosmic structure formation and suggest that cosmic inflation is the proper theory. During the 1980s , the first peak was measured with increasing sensitivity, and in 2000 , the BOOMERanG experiment reported that higher energy fluctuations occurred at scales of approximately one degree. Along with other cosmological data, these results imply that the geometry of the Universe is flat. Various interferometersThey provided highly accurate fluctuation measurements over the next three years, including the Very Small Array , Degree Angular Scale Interferometer (DASI), and the Cosmic Background Imager (or CBI). The first detection of the DASI was the polarization of the CMB while the CBI obtained

The future of the cosmic microwave background

Given that as the Universe expands, the redshift suffered by the cosmic background radiation increases, a very distant moment will come, assuming an open Universe , in which it will be completely undetectable, ending up being “covered” by the one caused by starlight the light emitted by the stars and this in turn as the Cosmos continues to expand will suffer the same effect and will be replaced by that of other processes that occur in the distant future.


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