That general relativity and quantum theory get along so badly has profound consequences for the world of cosmology. The first perfectly describes the movement of planets, stars and galaxies and has led us to the discovery of the expansion of the universe (in fact, it was first deduced theoretically and then the experimental evidence was found); it is impossible to understand the large-scale structure of the universe without it. The second is the most comprehensive and fundamental theory we have developed: it allows us to understand how nature works at its lowest level. And here’s the problem: when the universe began, about 14 billion years ago, the densities of matter and energy were extremely high and the entire universe was less than the size of a hydrogen atom, so quantum considerations are essential to understand what happened. The fabric of space-time, which for relativity is nothing more than a continuous trampoline, undergoes important fluctuations as a result of the fact that we have descended to the quantum level . It is these fluctuations that we find ourselves unable to explain, much less understand; without a quantum theory of gravity we can’t say what blew up. And all because gravity resists being quantized.
Physicists are hopeful that a quantum gravity will remove the primordial singularity to which general relativity hopelessly leads . As happens in the center of black holes, Einstein’s theory predicts that at the initial moment all the mass and energy of the universe was concentrated in a mathematical point without dimensions. That means that all physical quantities take on an infinite value, and when that happens in a theory it is a sign that something is wrong. This is where the well-known string theory and the other competing theory come into play, the quantum loop theory , which appeared in 1986 at the hands of the Hindu physicist Abhay Ashtekar: according to it, space-time is made up of ‘loops’ and loops of infinitesimal dimensions; that is, that space-time itself is discrete, it is “atomized.”
In recent years, different proposals have emerged that, with their feet set on the two previous theories, seek to explain what happened at the time of the Big Bang. It is not surprising, because everyone wants to be the one who solves the problem. One of them that currently has quite a lot of prestige is of purely European origin and has the esoteric name ‘Lorentzian quantum gravity network’, but is better known as Causal Dynamic Triangulation . Formulated in 2006 by physicists Renate Loll, Jan Ambjørn and Jerzy Jurkiewicz, it emerges as the middle ground in the fight for the quantization of gravity.
All those working on it know that drastic decisions will surely have to be made regarding general relativity or quantum theory , but they disagree on which direction to take them. For string physicists, quantum theory is essential, and if general relativity has to be mutilated in order to fit it in the end… nobody’s hand should shake. At the other extreme are those who think that general relativity is sacrosanct and must be respected at all costs: if any kind of surgery is to be applied, it is to the foundations of quantum mechanics.
An example of this confrontation is what the string theorist Leonard Susskind baptized as the Black Hole War : Stephen Hawking, a deep relativist, defended that information disappears inside black holes; Susskind, convinced quantum, affirmed that this violated one of the fundamental laws of the universe, the conservation of information. The fight -scientific, not personal- was settled with the triumph of Susskind. At least for the moment.
The interesting thing about the cosmology that follows from loop quantum gravitation is that it claims to solve the problem of what happened before the Big Bang: it is usually assumed that the Big Bang marked the beginning of everything, including time. In the quantum loop theory our universe arises from what has been called a Big Bounce : a universe before ours collapsed but did not become a singularity, but a little earlier, when the energy density reaches a critical value , quantum effects make a repulsive force appear. As we can suppose, this produced a rebound from which the universe in which we live today was formed.
This is not a new idea: the cosmologists Robert Dicke and James Peebles already played with it in the 1960s, but it was only towards the end of the 1980s and the beginning of the 1990s that this idea was taken more seriously, which implies the idea of multiple rebounds, following an infinite cycle of collapses and expansions. The problem is that to assume this it is necessary to impose strong modifications to the theory, since in general relativity there is nothing that says that the universe would bounce if it became a singularity. Too many news.
The main problem with this mathematical game is that there is no way to test experimentally whether this view of the origin of the universe has any ring of reality . And the worst thing is that no one knows what the hell to look for.