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Big Bang as the beginning of time: science or myth?

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Was there ever really a beginning of everything? Including time? One can hardly imagine a more existential question in the spirit of each human being. Philosophies and religions have approached this issue incessantly since man left traces of his own existence. We find the same questions being asked by the Greek philosophers just as well as by the religious leaders of the Middle Ages. To the question "What was God doing before creating the world?» St.Augustine replied «The question is meaningless because God, together with the world, also created time! ». And just over a century ago the French painter Paul Gauguin began the title of one of his famous pictures with the question: "D'où venons-nous?"

And what about science? After Hubble discovery of the red shift of light (i.e. towards low frequencies) coming from more distant galaxies, we know that the universe has been expanding for approximately 15 billion years. Galaxies are moving faster and faster away from each other the farther away they are from one another. Going back in time, and following the laws of physics (in particular those of Einstein's general theory of relativity) we arrive at a moment - the Big Bang - in which the entire universe as we can observe it today was concentrated in one single point. As a consequence, some physical measures such as density and temperature, would have been infinitely large at the moment of Big Bang. It is precisely the existence of these infinites that leads us to conclude that that moment (conventionally defined as t = 0) is insurmountable and that it makes no sense to speak of what was there before, just as it makes no sense to ask what lies north of the north pole. However, can we really trust the physical theories that lead us to this conclusion?

In fact, current physical theories have had a resounding success. The so-called standard model of elementary particles has been confirmed, so far without fail, by the many data collected at various particle accelerators located almost all over the world. On the other hand, also Einstein 's theory of general relativity obtained spectacular confirmations (bending of light, precession of Mercury perihelion, just to mention the most classic ones). There seems to be no reason to doubt the validity of these theories, at least within the framework in which the experiments have been carried out. As a result of all this, modern cosmology rests on a solid foundation at least until when it aims at describing the universe starting from a few billionths of a billionth of a second after the big bang. But what about the reliability of these theories if we want to go back in time even further, up to t = 0, up to the Big Bang itself?

Hardly disputable arguments tell us that even in the most optimistic case, between t = 0 and a short time (one Planck time, see below) after the big bang, the theory of general relativity alone can not describe the evolution of the Universe. This is due to the fact that the general relativity fails to take into account the other great achievement of twentieth century physics, quantum mechanics, which is known to dominate the microscopic phenomena, such as atomic or nuclear phenomena. In fact, without quantum mechanics, an atom would be a very unstable system: the electron, while turning around the proton in a hydrogen atom should radiate electromagnetic waves and thereby lose energy until it would «crash» into the nucleus ( the proton in this case). Quantum mechanics steps in, with the famous Heisenberg uncertainty principle, on the basis of which, in order to minimize the energy of the system, the electron must remain at a respectful distance from the proton, a distance known as the Bohr radius, which amounts to about 10-10 m, that is exactly the size of an atom.

Similarly, when considering the effects of quantum mechanics on gravity, a characteristic length scale is determined (or, dividing by the speed of light, a time scale) which is called the length (or time) of Planck, named after the famous physicist Max Planck who introduced this length at the beginning of the last century. Its value, 10-35 m (10-43 s) is much smaller than the Bohr radius, but in any case is not zero. There is a consensus that, beyond that length or time scale, it is no longer possible to trust the relativity of Einstein. The question is: what should we replace it with? Here comes a problem: while, as far as the non-gravitational interactions are concerned, we know how to make them compatible with quantum mechanics, any attempt to do the same thing with the gravitational force has failed miserably ... well with one exception.

Since the late sixties there is a new theory of elementary particles known as "string theory" in which all elementary particles correspond to different states of vibration of a single entity, a vibrating string (as if in a certain way, different particles corresponded to different musical notes). By applying the laws of quantum mechanics, this string, in analogy with the hydrogen atom, acquires an optimum length, which is said length of the string, an a priori new constant of Nature. In the sixties and early seventies version of the theory, this length was chosen to be the size of an atomic nucleus (10-15 m) while in the modern versions this becomes of the magnitude of the above-mentioned Planck length, in other words no less than twenty times smaller.

The small, but finite size of the quantum string means that some physical measures such as density and temperature, can not become infinitely large in contrast with the outcomes that would result from the general relativity. As every infinite thus disappears, the necessity for a beginning of time disappears as well, and we are brought back to the question which St. Augustine had so brilliantly avoided answering: what was there before? Currently, technical difficulties prevent us from answering this question accurately and safely, but two possible answers seem to emerge.

According to the first one, the universe started at a stage which can not be described in terms of the usual notions of space and time, a stage completely dominated by quantum mechanics. Space and time concepts would "emerge" at the end of this primordial stage.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   This is the choice that most closely matches the ideas (not based on strings) of Steven Hawking, who speaks of time becoming "Euclidean", in other words similar to a fourth spatial coordinate.

The second possibility, which seems even more appealing as it is based on certain symmetries of the string theory is that the universe had a "prehistory", which, in some ways, is the mirror image of its history. At this stage, called pre-Big Bang, the universe would suffer a kind of gravitational collapse in a still "classical" system (i.e. without any relevant quantum effects ) reaching the maximum values of density, temperature and curvature allowed by the theory of strings. At this point the universe would enter a quantum phase from which it would then emerge creating the universe and its history as we know it today. The Big Bang is thus replaced by a "Big Bounce". This could also explain the "initial" very finely tuned conditions that are used to explain the current universe and that, in the absence of an earlier stage, would need additional paradigms such as the one of cosmological inflation. In other variations of this scenario, the process of contraction and expansion repeats endlessly (cyclic universe).

This latter debate seems to go beyond the context of physics to get into that of metaphysics, as it would use concepts that are not experimentally verifiable. However, this is not the case: in fact, from the studies of the inflationary models we already know that the early universe evolution normally leaves observable traces up to this day. This happens due to a phenomenon known as 'freezing' of certain physical quantities during very long cosmological periods. Today, these quantities "thaw out" and, like a prehistoric animal buried in ice millions of years ago, they reveal what the universe was like at around (or even before) the big bang / bounce.

An example of such prehistoric traces is that of a stochastic background of gravitational waves with a typical frequency spectrum that could be revealed in today's operating interferometers (LIGO, VIRGO, ...) or one of their successors (LISA?). Another one, is the fact that the "pre-bang" phase could generate "seeds" for the galactic magnetic fields whose origin is otherwise very difficult to explain. Therefore the question: "Did time begin?» may one day be answered experimentally.

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