That’s the title of a new study by two physicists from the University of Liverpool and Imperial College London. While scientists have been trying for decades to unravel the mysteries of the birth of the Universe, it may be that the Universe does not actually have a beginning. A certain approach to quantum gravity, called causal set theory, indeed suggests that our universe may have always existed.
According to this theory, space-time is fundamentally discrete (on the contrary, in all current theories of physics, space and time are continuous) and the events of space-time are linked by a partial order; this partial order has the physical significance of the causal relations of the events of space-time. So at some level there would be a fundamental unity of space-time.
Bruno Valeixo Bento, from the Department of Mathematical Sciences at the University of Liverpool and Stav Zalel, from the Blackett Laboratory at Imperial College, set out to use this causal approach to explore the beginning of the Universe. This is how they discovered that it is possible that our universe never had a beginning. It would therefore have always existed in the infinite past and would only have evolved “recently” towards what we call the Big Bang.
Unify quantum physics and general relativity
The Universe is based on two major theories: quantum physics and general relativity. The first allows to effectively describe three of the four fundamental forces of nature (the electromagnetic interaction, the weak interaction and the strong nuclear interaction). The theory of general relativity, on the other hand, is the most powerful and complete description of gravity ever established; it models gravity as a curvature of space-time, the radius of which is proportional to the energy density.
However, general relativity has certain shortcomings. As it turns out, this theory does not produce consistent results for at least two particular places, namely the center of black holes and the beginning of the Universe. These regions, called singularities, are points in space-time where the current laws of physics break down. Within these two singularities, gravity especially becomes incredibly strong at very small scales of length.
To get around the problem, and thus attempt to unify quantum physics and general relativity, physicists have sought to establish a theory of quantum gravity; this was to help explain phenomena involving large quantities of matter or energy over small spatial dimensions. From this effort were born several approaches, such as supergravity theory, string theory or loop quantum gravity.
Current theories of physics assume that space and time are continuous. In such space-time, two points can be as close to each other in space as possible, just as two events can occur as close to each other as possible. But causal set theory describes space-time as a series of discrete elements, units or “atoms” of space-time. This theory therefore imposes strict limits on the proximity of events in space and time, because they cannot logically be any closer than the size of one of these “atoms”.
A theory that eliminates the problem of the Big Bang singularity
This theory, which questions the notion of space-time itself, aroused great interest in Bruno Bento. ” I was delighted to find this theory, which not only tries to be as basic as possible, but also gives time a central role and what the passing time physically means. “Bento told Live Science.
The theory of causal sets has indeed important implications for the nature of time. As the physicist explains, it is based on the principle that the passage of time is something truly physical and that it is not some kind of illusion generated by our mind. In this theory, a causal set grows one unit at a time, becoming larger and larger. This approach thus completely eliminates the problem of the Big Bang singularity, because it suggests that it is impossible for matter to compress to infinitely small points; they cannot be smaller than the size of a space-time unit.
Without this initial singularity, represented by a point of infinite density containing all the energy of the Universe, what then does the beginning of the Universe look like? In an article available in preprint, Bento and Zalel attempted to determine whether a debut must necessarily exist or not. Their thinking suggests that the Universe may not have had a beginning, that it has simply always existed. ” In the original formulation and dynamics of the causal set, classically speaking, a causal set develops from nothing to end up in the Universe we see today. In our work, on the contrary, there would be no Big Bang as a beginning, because the causal set would be infinite until the past, and therefore there is always something before ”Says Bento. What we call the Big Bang would therefore only be a particular moment in this development.
It remains for the researchers to determine if this causal approach without beginning can make it possible to develop theories making it possible to describe the complex evolution of the Universe during the Big Bang.