Cosmology and String Theory: Conclusion

The Universe Before the Big Bang

Cosmology and String Theory

Conclusion

The key message we hope to have transmitted to the reader in this book is that there are neither observational data nor incontrovertible theoretical arguments to support the belief that the Big Bang represents the beginning of the Universe, and that before the explosion there was “nothing”. On the contrary, there are solid motivations – based upon recent developments in theoretical physics – for thinking otherwise. There are also scientifically valid tools for tracing the history of the cosmos back to epochs preceding the Big Bang, providing ways of testing such investigations with a series of effective experimental observations that are already feasible, and with even better prospects in the near future. The kinematical and dynamical details of the primordial cosmological epochs preceding the Big Bang are still quite uncertain.

There are various models, many hypotheses, and a number of possibilities that have not yet been fully explored. The situation will certainly become clearer following the theoretical and experimental work to be carried out over the next few decades. It is already evident, however, that the Big Bang could lose its rather mystical role as the beginning of everything, to become a more modest be- ginning of the current phase of the Universe, i.e., of the Universe as we currently know it, made up of radiation, matter, atoms, galaxies, and human beings. Nevertheless, it would still represent a crucial step in the history of our Universe, without which life in the form we now experience it would probably be absent. With regard to the beginning of the Universe, it is amusing to consult the book that can probably be considered the first and most authoritative text on cosmology of the modern age: Genesis.

Actually, by carefully rereading the initial verses of the Holy Bible, we find a description of the birth and the first moments of our Uni- verse which seems much closer to the pre-Big-Bang scenario than to the standard Big Bang scenario. Indeed, there is no mention at all of an explosion, and no reference to any hot, dense, highly curved concentration of energy. What is described is rather an initial state that is completely quiet, deserted, dark, and lifeless, which just resembles the typical initial state of pre-Big-Bang models (described in poetic, but very appropriate, terms).

We can read, in fact:

First God made heaven and earth.

The earth was without form and void,

and darkness was upon the face of the deep;

and the Breath of God was moving over the face of the waters . . .

(Genesis, The Holy Bible)

Here, terms like “heaven” and “earth” could denote, respectively, space-time itself – i.e., the environment where the Universe is brought to life and subsequently evolves – and the various forms of energy and natural forces. The “darkness” and the “deep” give us the idea of something immensely large, empty, and cold, like empty space, void of any interaction. Indeed, without interactions, matter is dark, since it does not emit radiation (i.e., light). The whole scenario actually makes us think of the string perturbative vacuum, which is a free state (i.e., without interaction) with a flat space-time geometry.

We should also recall that the fundamental string coupling determining the strength of all forces is controlled by the dilaton, and in particular by the exponential function of the dilaton field (see Chap. 4). In order to have an arbitrarily small coupling (i.e., arbitrarily weak interactions) in the initial state, the initial value of the dilaton field must be arbitrarily large and negative: this huge negative “abyss” could correspond – with a little imagination – to the “deep” mentioned in the Genesis.

Furthermore, “without form” is approximately synonymous with incoherent, chaotic, or stochastic. But this fits quite well with the description of the initial conditions given at the end of Chap. 5, where we presented an analogy between such an initial state and an ocean whose waves collide chaotically, and occasion- ally trigger non-trivial physical processes. The breath of life over this “face of the waters” could just represent the quantum oscillations of the dilaton and the geometry, aimed at triggering the inflation mechanism that eventually brings the Universe to its standard configuration through the explosive stage of the Big Bang.

Of course, everybody knows that the Bible’s words cannot be taken literally. It is also well known that it would be misleading to give a subjective interpretation to those words, forcing their meaning to fit one’s opinion. Nevertheless, it is difficult to refrain from proposing a personal translation of the above verses in scientific terms, those verses which so poetically describe the origin of the Universe in a language appropriate to ancient times when Gene- sis was written. Using a modern, less metaphorical language, the translation could sound more or less like this:

First God made the fields and the sources.

The sources were incoherent in the vacuum,

and this dark matter was without interactions;

and the dilaton

was fluctuating over the string perturbative vacuum . . .

The next sentence: And God said: let there be light! seems to de- scribe the Big Bang, i.e., the production of radiation marking the beginning of the standard cosmological phase! Hence, according to this personal translation, Genesis describes a scenario for the creation that seems to correspond quite closely to the pre-Big-Bang scenario suggested by string cosmology. But, as everybody knows, one can read anything into the Bible, provided one looks carefully enough for it.

More seriously, it would be naive to ask from science an ex- planation for all the big question marks of the creation. Beyond a certain point, each of us should look into himself/herself for the answers to the fundamental questions pertaining to the existence of the Universe and our own existence. My personal and modest opinion (as far as it may count in this case) is that the Universe was born according to God’s will, with an act of creation having its ultimate and complete purpose in human beings. However, with regard to how the Universe evolves after its creation, following those laws that God himself wanted to instill into nature, I think it is fully appropriate to apply the methods of scientific investigation.

In this spirit, string theory applied to cosmology seems to tell us that the Big Bang is not to be identified with the time of the initial creation, in the same way that – if I can take the liberty of using an analogy from biology – childbirth must not be identified with the moment when a new life is created (which corresponds rather to the act of conception). Well before the Big Bang, the supernatural act of creation was followed by a long cosmological “pregnancy”, required to prepare the explosion leading the Universe to its cur- rent form (similarly to what happens after the conception of a new living creature during the time preceding delivery). Actually, we may think of the Universe before the Big Bang as being in a sort of embryonic state, during which the various physical properties (that will be made manifest later on, during the post-Big-Bang epochs) were gradually taking form.

This “prenatal” life of the Universe is fully accessible to present and future experimental investigation. The hypothesis of a self-dual Universe, the scenarios described by string cosmology and brane cosmology models, and so on, can be tested in various ways. We may recall, in particular, that the phase of pre-Big-Bang evolution may produce backgrounds of relic gravitational radiation much stronger than the ones predicted by standard cosmology at high frequencies, and hence more accessible to direct observation.

In addition, the typical production of electromagnetic seeds for the cosmic magnetic fields, and of axion seeds for the CMB anisotropy, could very soon lead to other possible (even if indirect) confirmations or disproofs. Current and near-future experiments are thus able to open a window on the earliest history of the Universe, on epochs much more remote than ever envisaged. This is the other important message that this book hopefully puts across.
It seems appropriate to conclude with a historical remark.

The current status of cosmology, characterized by various possible models for the primordial Universe, looks similar to the situation about half a century ago, when there were two contrasting cosmological scenarios. They were somehow complementary, and corresponded to two radically different visions of the cosmos: the steady-state Universe of Herman Bondi, Thomas Gold, Fred Hoyle, and Jayant Narlikar, characterized by a continuous creation of matter, and the evolutionary Universe, hot and explosive, born from the Big Bang, of Georges Lemaitre, George Gamow, Robert Dicke, and others. One of the crucial differences between these scenarios was, respectively, the absence and the presence of a cosmic back- ground of thermal radiation. It was just the direct observation of this background, discovered by Arno Penzias and Robert Wilson in 1965, that definitively confirmed one scenario and disproved the other.

The current situation is rather similar. There are standard inflationary models predicting a very low, practically undetectable background of cosmic gravitational radiation at high frequencies.

Other models, based upon string theory, predict a much higher background. Once again we expect the choice between the different scenarios to be made on the basis of experiment, hopefully in a not too distant future.

Whatever the answer, we do believe that the experimental study of the relic gravitational background will be as important for cosmology as the study of the electromagnetic microwave back- ground. Probably even more important, since the electromagnetic radiation contains photons which provide us with a snapshot of a Universe younger than the current one, but still subsequent to the Big Bang. The relic gravitons, on the other hand, originate from a much more remote past, and may have retained in their spectrum a permanent imprint of the pre-Big-Bang Universe.