Cosmic Background Radiation and the Big Bang Theory

Background radiation and its isotropy are fundamental to our understanding of the universe. What background radiation is can be easily explained. When the theory that the universe was expanding was accepted in the 1940s, it became clear to the scientific community that if the universe was expanding, its beginning probably had to start from an initial singularity, later called the Big Bang. Explaining the universal origin through a Big Bang, while maintaining conservation of matter and energy, meant saying that at a certain moment all matter and radiation currently existing in space would have found itself in a state of incandescence of extremely restricted dimensions. Here physicists were presented with an apparent paradox. As was known, since the speed of light is constant, looking at ever greater distances one should see images reflecting an ever greater past. Therefore, observing far enough into space, and therefore far enough back in time, one should see the wall of light and radiation testifying to the beginning of the Big Bang. In any direction one looked, this wall of light had to appear - an experimental result that could be confirmed as false every night by any observer. Actually, it didn't take long to understand that this wall of light was, in effect, present. It simply was no longer composed of light in the visible spectrum but of invisible light in the microwave field. What had happened was that the expansion of the universe had dilated the wavelength of the light signal, shifting it from the visible spectrum to the invisible microwave spectrum. This wall of light coming from every direction of the sky and which would report the image of the initial states of the universe was fully confirmed in the microwave spectrum and took the name of Cosmic Microwave Background (CMB). The cross and delight of this cosmic background radiation, which would depict the universe in its initial stages, is its incredible isotropy. The cosmic background radiation appears almost perfectly identical in whatever direction it is observed. This isotropy of the cosmic background radiation, combined with the Copernican principle (whereby we are not at a privileged point in the universe), confirmed the cosmological principle relating to the isotropy and homogeneity of the universe on a large scale. From a mathematical point of view, the Cosmological principle has a direct implication on the equations governing the evolution of the cosmic scale factor. In particular, the hypothesis of having a homogeneous and isotropic universe translates into the possibility of using a very favorable metric for this type of calculation, which is the Friedmann-Robertson-Walker (FRW) metric and which is the basis of most of the expanding universe models we have seen. But what would happen if the Universe were not as isotropic and homogeneous as it seems? After all, observations account for enormous stellar walls formed by clusters of galaxies billions of light years away from us like the Sloan Great Wall. What would happen, for example, if we were at the center of a particularly empty zone of the Universe? From an intuitive point of view, what would happen is that galaxies distant from us would be attracted by more massive zones of the distant universe, thus being accelerated outward. If our galaxy were at the center of a Great Void, the universe would seem to expand radially as if moved by a dark and anti-gravitational force, while in reality the galaxies would only be attracted by the gravitational force of other more distant clusters. But how can we determine if our galaxy is actually at the center of this cosmic Great Void? The main answer in this sense derives precisely from the Cosmic Background Radiation. The cosmic background radiation should be a snapshot of the universe in its first moments of life. In particular, it should photograph the inhomogeneities and density variations present in the first moments of the universe. In this perspective, in 2001, a joint mission of NASA and Princeton University sent a satellite, called WMAP, specifically dedicated to tracking this anisotropy in cosmic radiation. These inhomogeneities are actually present in the image of cosmic radiation and modern cosmologists attribute to them the local inhomogeneity that currently manifests itself with the formation of various galaxies and stellar clusters. Therefore, if our galaxy is at the center of a huge empty bubble, this Great Void, in some way it must be able to be identified by the anisotropy of cosmic radiation. Some researchers hypothesize that this great void could be associated with a fairly well-known anisotropy of cosmic radiation.