The Big Bang: when the Universe is not banging!

Is it because it concerns the origins of things? Anyway, the Big Bang is a scientific subject which everyone seems to have heard of. However, it appears that general ideas about it are not really clear.

As it will be question of the Big Bang in the subjects I will address soon in this website, here is an opportunity to inaugurate scientific popularisation articles in my blog: I propose to present the basics of the Big Bang, using simple experiments that everyone can do at home.

The term Big Bang was introduced in 1950 by the British astronomer and cosmologist Fred Hoyle during a BBC radio show entitled The Nature of Things. By the way, he used the term in an ironic sense since, at least at that time, he did not consider that this theory was founded. The expression has nevertheless prospered in such an extent that it is commonly used now.

However, it is misleading as it leaves us believe that the Big Bang is an explosive temporal event, that is to say localized, both in time and in space. What it is not …

The Big Bang theory is established within the framework of the general relativity, which main founder¹A. Einstein, 1916. Die Grundlage der allgemeinen Relativitätstheorie, Annalen der Physik, 49, pp. 769–822. is Albert Einstein. If this article raises sufficiently enthusiastic comments, I will present in detail this theory. For now on, I will try to give enough information to understand what follows.

In 1905, introducing the special relativity theory²A. Einstein, 1905. Zur Elektrodynamik bewegter Körper, Annalen der Physik, 17, pp. 891 – 921., Albert Einstein observed that the concept of time is relative. That is to say that each one (each observer) has its own proper time. If the proper time does not vary for this given observer, it is not necessarily synchronized with other observers. The more a given individual moves quickly relatively to an outside observer, the more his own time seems to expand from the point of view of the outside observer: while for the outside observer one second passes, less than a second passes for the moving individual. From the point of view of the individual in motion, however, its own proper time does not change: if you need 2 hours, 27 minutes and 43 seconds to read my complicated prose, no matter how fast you run, this time will not change! Anyway, you cannot really perceive a shift between proper times at speeds we are used to. Actually, astronauts hardly experience it as compared with people who are staying on the Earth. In general, the shift is a matter of stellar scales.

In order not to get lost, I will not explain the reasons for this shift in this post. If you want more details, again please let me know in the comments, I would do an article on the subject. In the meantime, I must ask you to accept these relativistic effects.

If motion has an effect on the synchronization of proper times, this means that a spatial phenomenon (a movement) has an effect on a temporal phenomenon. This leads to consider the space and time as two inseparable notions influencing each other, so we introduced the concept of space-time continuum: instead of a three-dimensional space and a one-dimensional time, we consider the unified concept of a four-dimensional space-time. According to the general relativity theory, the space-time continuum is under the influence of gravity, which deforms it – meaning that time is also subject to this deformation, you have followed me. The space-time continuum is often represented with a beautiful 3D drawing, but I find most striking to use a little experiment that can easily be replicated at home. It start with holding a piece of tissue (choose a not too fragile one), which will represent the space-time:

By placing a weighty object on it – choose it with enough weight to see the distortion well enough, but not too much so that you do not drop the piece of tissue nor tear it apart – you will distort your representation of the space-time. The more important the mass of the object, the greater the distortion – you can try with various objects, same remark as at the beginning of this paragraph:

It is a similar phenomenon that distorts the space-time continuum.

By introducing a new object – select it less weighty than the first one, and relatively round, the demonstration will be clearer –, it will then follow the deformation of the piece of tissue – thus, the deformation of the space-time – and approach the first object: things happen just as if a force (the gravitation) had attracted the second object to the first one. Incidentally, the second object distorts the piece of tissue as well – that is to say, the space-time, you have followed me! This is the way the general relativity describes gravitation:

In 1922³A. Friedmann, 1922. Über die Krümmung des Raumes, Zeitschrift für Physik, 10 (1), pp. 377–386., Alexander Friedmann, getting interested in the equations of general relativity, proposed as a solution to the fact that the Universe did not collapse under the effect of gravitation that it would not be static, as it was commonly thought at the time, but expanding. Such an expansion requires an initial singularity, that is to say what is now called the Big Bang. In 1927, Georges Lemaître gave a general description of this expansion⁴G. Lemaître, 1927. Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques, Annales de la société scientifique de Bruxelles, A47, pp. 49–59.. In 1929, Edwin Hubble shows that galaxies seem to move away from each other⁵E. Hubble, 1929. A relation between distance and radial velocity among extra-galactic nebulae, Proceedings of the National Academy of Sciences of the United States of America, 15 (3), pp. 168–173.. According to general relativity, which is otherwise strongly confirmed, it is attesting to the expansion of the Universe.

The expansion of the Universe is actually the deformation of the Universe itself. To understand it, lets do some more simple experiments. This time, we will represent the Universe with a balloon. The expansion of the Universe corresponds to inflate the balloon:

Note that this analogy has a real flaw: the balloon inflates in our space, therefore this phenomenon is called extrinsic. Instead, the expansion of the Universe is intrinsic, that is to say, it does not take place in an environment that contains the Universe, precisely because the container is none other than the Universe – I thank Jean-Pierre Luminet to have raised this issue in my presentation.

If we draw dots representing galaxies on the balloon, when inflating it (that is to say as a result of the expansion of the Universe), while the space between the dots (that is to say between galaxies) increases, galaxies seem to move away from one another:

Thus, the expansion of the Universe increases the distance between galaxies. Consequently, a less flawed vision of the expansion of the Universe would be the following:

In contrast, the solar system is subject to the gravitational field of the Sun, which keeps it in a coherent whole. As indicated above, this gravitational field caused by the Sun deforms space-time. This deformation exceeds the one caused by the expansion, so that the latter has no effect on the scale of the solar system. The expansion affects the relative position of the galaxies and clusters of galaxies between them, but does not affect the configuration of the galaxies themselves.

Since the Universe is not static but expanding, it has a history. The idea is then to use equations to determine the state of the Universe in the past, somehow passing the film backwards. If we turn the film of an expanding universe backwards, it seems to contract:

When the film is completely rewound, we obtain an extremely dense universe, which is contracted to a point and which temperature is infinite: in mathematical terms, it is called a singularity, and this is what Fred Hoyle called “the Big Bang”. As for the concept of time, at this singularity, it does not really make sense.

Has the origin of the Universe been definitely identified? Not exactly, as nowadays it can no longer be told about the Big Bang the way it used to be in 1950.

Gravitation is not the only fundamental force in the Universe. In fact, there are four of them: gravity, as said, but also electromagnetism, the strong nuclear interaction and the weak nuclear interaction.

General relativity is a theory of gravitation, it does not take into account the other three interactions. Unlike gravitation, the action of these three interactions is local. As a consequence, the model of expansion of the Universe based only on general relativity is established only for a universe whose expansion was sufficient, so that only gravitation has an appreciable effect on the scale of this expansion. Before that, the Universe is in what is called the Planck epoch (named after the German physicist Max Planck). The Universe has leaved this epoch about 13.82 billion years ago. To describe it, one must take into account simultaneously the four fundamental interactions. The initial singularity – obtained using only the equations of general relativity, thus taking into account no fundamental interaction but gravitation –, would take place in the Planck epoch.

This is where the shoe pinches, as electromagnetism and the weak and strong interactions are the field of quantum mechanics, which is incompatible with general relativity. So to be able to describe the Universe at the Planck epoch, we need a unifying theory for the four fundamental interactions.

At the time I write this, there are three serious candidates to become the unifying theory: string theories – note the plural form, as there are several variants of this theory – the loop quantum gravity, and non-commutative geometry. At present, none is fully completed and we are not able to determine which one is the most appropriate to describe the Universe, or even if one of them is appropriate …

However, these theories proposes elements for describing the Planck epoch and invites us to reconsider the model of an initial singularity. For example, in the framework of loop quantum gravity, it disappears in favour of the passage from a shrinking universe to an expanding one, as Martin Bojowald described in his book Once Before Time: A Whole Story of the Universe⁶M. Bojowald, 2010. Once Before Time: A Whole Story of the Universe, Knopf.. At present, the balance of evidence challenges the idea of an initial singularity, which would be the instant that created the Universe.

Western culture conveys the idea that the Universe has an origin. With the Big Bang, some believed we have obtained the evidence of that origin, such as Pope Pius XII. However, strictly speaking, at present we can not say anything definitive about the Universe during Planck epoch or before – not even if there may be a before. In fact, it is not certain that the notion of the origin of the Universe simply has a sense, nor that there is a unique universe.

These last remarks, which will perhaps deeply perplex you, conclude what I wanted to introduce to you about the Big Bang theory. I hope you found it interesting and it helped you to have clearer ideas about it. If that does not make you an expert on the subject, at least you should be able to detect when some people try to make you believe the Moon is made of cheese when the subject comes to the Big Bang.