Latest century has witnessed of the rise of a new vision of the universe, thanks to technical and theoretical development of the various astronomy disciplines.
Thanks to the work that Edwin P. Hubble made since 1929, it was discovered that all extragalactic objects (ie galaxies) were moving awayfrom us (recession), in whatever direction they were observed: the larger was the distance of a galaxy from Milky Way, the greater was its recessional speed. The expansion effect was directly proportional to the distance of the galaxies, by means the so called the Hubble constant (H).
The discovery of the recession, which occurred by observing the red shift of spectral lines of galaxies, generated a series of new questions about the geometry of the Universe and its evolution. If the extragalactic objects had a motion away from each other, it was logical to assume that the universe has had an origin in both space and time.
To confirm this hypothesis , in the forties of last century, George Gamow and co-workers Ralph A. Alpher and Robert R. Herman, in order to explain the origin of matter in the Universe, developed a new theory of the atomic nucleosynthesis. Their calculations took the start from less than a second after the birth of the Universe, immediately after the immense explosion (Big Bang). At that time, the Universe was a “soup” extremely hot and dense of protons, neutrons, electrons and other elementary particles.
The Big Bang was not only the Act of birth of all matter and radiation observed, but the same space-time origin! The recessional velocity of galaxies, discovered by Edwin Hubble some years before, was the direct consequence of the expansion motion generated by the Big Bang.
To justify the observed abundances of the elements and the typical size of large-scale structures in the Universe, such as clusters of galaxies, must have existed a time in which the radiation (photons) and the matter had been heavily “interacting” in a physical condition defined thermal coupling.
In the first instants after the Big Bang, the Universe temperature was so high as to prevent the formation of stable atomic nuclei. The expansion of the Universe, however, had led to a gradual lowering of temperature down to the point where the energy of photons was not such as to prevent the formation of stable nuclei.
The gradual cooling of the Universe, up to a temperature of less than 4000 degrees Celsius above the absolute zero (−273.15 °C) , had marked the transition from an era “dominated by the radiation”, where most energy was in the form of radiation, to an era “dominated by matter”. In the matter era, most of the energy was (as today), trapped in the mass. At that point the “stories” of radiation and matter had taken two distinct ways: in other words, the radiation and matter were decoupled.
The Universe became transparent to the radiation, so the photons had begun to travel indisturbed through increasing distances. The aggregation processes of matter, due to gravitational collapse, no longer hampered by the “viscous” effect of the radiation, had slowly lead to the formation of the first structures.
How can one be certain that this “vision” was not due to an erroneous interpretation?
In 1948 Ralph Alpher and Robert Herman, on the basis of their theory, postulated the existence of a background radiation, due to the effect of the matter-radiation decoupling . According to their calculations, such rarefied and cooled radiation (to cause of the Universe expansion) , must have a temperature around 5 kelvins and, very important, had to be observable!
Now we have to skip in 1964, when Arno Penzias and Robert Wilson, on behalf of the “Bell Telephone Laboratory”, in order to measure the intensity of radio waves from the milky way, used a horn antenna, with a diameter of about 6 meters.
This kind of measure resulted very difficult, because the signal was very similar to what we hear by a radio in the presence of temporal discharges. It was therefore necessary to know every noise source, such as the interference due to electronics, the atmosphere emissions and the noises coming from the same antenna. Despite having been identified all possible sources of noise, the measures still showed a not justifiable ”excess of noise”. This was independent from the direction towards which the antenna was pointed, or from diurnal and seasonal cycles. Any further measure, adopted, was not sufficient to eliminate such “excess”, which corresponded to an antenna equivalent temperature ranges between 2.5 and 4.5 degrees above absolute zero.
At the same time, an experimental physicist at Princeton, Robert H. Dickie, on tha basis of Gamow theory, was organizing in collaboration with P.G., D.T. Wilkinson Rolle and later P.J.E. Peebles, an observational campaign in search of “fossil radiation” which had to be observable, if one considers valid the Big Bang theory. The two research teams began to cooperate, realizing with increasing clarity that “excess noise” observed by Penzias and Wilson was not only of extragalactic origin, but was almost certainly the remnant of the “hot phase” of the Universe: the “fossil” radiation expected. The “excess” of noise, measured by Penzias and Wilson, known today as the Cosmic Microwave Background Radiation, is one of the most important discoveries of the last century, which was awarded Nobel Prize in 1978 to the researchers of the “Bell Telephone Laboratory”.
This discovery not only upheld the ideas of Gamow and his team: what you were looking at was the oldest and most distant radiation that today we were able to detect.
Because the cosmic microwave background (Cosmic Microwave Background Radiation) is so important for Cosmology?
The discovery of the CMBR has opened a new series of questions about the geometry and the formation of large-scale structures in the universe. In 1990, the NASA satellite COBE (COsmic Background Explorer) has confirmed that the cosmic microwave background radiation has the same intensity behaviour provided by existing theories (incredibly in agreement with a “black body” behaviour). Furthermore, even more important discovery was the evidence if tiny inhomogeneities in the radiation temperature. These inhomogeneities was responsible for the formation of primordial cluster and super clusters of galaxies [see article "with eyes on the universe-the light ancient"]. By means of COBE observations, it was possible to calculate the value of the radiation temperature of 2.73 degrees above absolute zero.
The possibility of even more precise measurements of fluctuations in the CMBR will evaluate the cosmological parameters, necessary to understand the true nature of the universe in which we live. For this reason, today the CMB is subject of continuous studies, carried out with the aid of powerful telescopes, high altitude balloons and satellites.
The study of the cosmic microwave background is probably the key that will allow us to understand whether the Big Bang might, realistically, represents the mechanism that led to the current Universe or whether there will be the need to think back to new and revolutionary theories. ”