THE BIG BANG AND THE UNIVERSE

The Big Bang theory is currently the most reliable scientific theory about the origins of the cosmos. It postulates that our universe started approximately 13.8 billion years ago from an extremely hot and dense state and, since then, has expanded basically continuously. The Big Bang theory monopolised the attention of the scientific community over the course of last century, when a series of experimental observations gradually undermined the models of a stationary universe, much in vogue until at least halfway through the 20th century. According to such models, the cosmos was considered to be immutable in space and time.

The two decisive proofs in favour of the Big Bang theory were the observation of the expansion of the universe, thanks to the measurements gathered by the astronomer Edwin Hubble in the 1920s, and the discovery of cosmic background radiation. The latter is a kind of “echo” of the Big Bang that still permeates the whole universe, accidentally observed for the first time in 1965 by the radio astronomers Arno Penzias and Robert Wilson. In between, it is important to also mention the work of the physicist George Gamow, who, in the immediate post-war period, provided solid theoretical bases to the Big Bang theory and predicted background radiation. More recent cosmological observations made by various space probes (including the Planck satellite of the European Space Agency) made it possible to map in an extremely precise way the distribution of cosmic background radiation. Small disparities (small fluctuations in temperature and density) in this radiation are the sign of fluctuations from which, it is thought, the structures of the current universe originated. The Big Bang theory is still being tested by more and more detailed theoretical and observational research, which investigates the many still open questions on the origins of the universe.

Globular Star Cluster NGC 6355 from Hubble (© ESA/Hubble & NASA, E. Noyola, R. Cohen)

Representation of the evolution of the universe (© INFN)

The evolution of the universe and its destiny

From the extremely hot and dense initial state, immediately after the Big Bang, the universe went through a series of transformations that led to the gradual formation of particles, atoms, planets, stars, galaxies, and increasingly complex structures. The various phases that characterised the evolution of the universe were reconstructed with great precision according to the standard cosmological model. This model is the theoretical framework (founded on the hypothesis of the Big Bang) that most effectively and precisely describes the history of the cosmos to date. However, there remain many open problems, on which current theoretical and experimental research is concentrated in the field of cosmology. To mention just a few of them, the nature of dark matter, whose role was crucial for the formation of the early galaxies, and dark energy, associated with the observation of an accelerated expansion of the universe.

Another phenomenon that is the subject of debate is so-called cosmic inflation, a very rapid phase of expansion that is supposed to have occurred in the first moments of life of the cosmos, multiplying by an enormous quantity the scale factor of the universe. This is a widely accepted paradigm in the scientific community, especially because it would justify the current homogeneity of the universe on a large scale. Still, definitive experimental proof of inflation is still lacking. The future and destiny of the universe are also open questions, which have always fascinated cosmologists. The most probable hypothesis is that the universe continues its expansion indefinitely (encouraged by the “thrust” of dark energy), reaching a kind of sad “thermal death” that would lead to the gradual extinguishing of all the stars. An alternative hypothesis is the so-called big crunch, according to which, at a certain point, gravitational attraction would prevail over expansion. This would lead the universe to collapse on itself, in a kind of opposite Big Bang (the latter hypothesis is, however, considered improbable, especially after the observation of the accelerated expansion of the universe).

Representation of the different theories on the future of the universe (© INFN)

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