Key Takeaways:
- A recent theory of quantum gravity called causal set theory suggests the universe might have always existed and the Big Bang was just a point in its evolution.
- Causal set theory divides space and time into discrete chunks, unlike the smooth fabric we imagine in current physics theories.
- This theory eliminates singularities, points where physics breaks down, because matter can only get as small as a space-time “atom”.
- The universe might be infinite to the past according to causal set theory, meaning there never was a beginning.
- More work is needed to see if this theory can explain the complexities of the Big Bang but mathematically it’s a possibility.
There once was — well, perhaps there never was a beginning. Perhaps the universe has always existed; the reason for this possibility is revealed by a recent theory of quantum gravity.
“Reality has so many things that most people would associate with sci-fi or even fantasy,” said Bruno Bento, a physicist who studies the nature of time at the University of Liverpool in the U.K.
He used a novel theory of quantum gravity in his work called causal set theory, which divides space and time into discrete space-time sections. This theory suggests that space-time has a fundamental unit at some level.
Bento and his collaborators studied the universe’s beginning using this causal-set method. It is possible, they discovered, that the universe never began, existing indefinitely in the past and only recently evolving into what is known as the Big Bang.
A quantum of gravity
Perhaps the most annoying issue in modern physics is quantum gravity. Quantum physics and general relativity are our two incredibly powerful theories of the universe. Three of the four fundamental forces of nature—electromagnetism, the weak force, and the strong force—have been successfully defined at the microscopic level by quantum physics. General relativity, on the other hand, is the most powerful and complete description of gravity ever devised.
Despite all of its advantages, general relativity is not perfect. In at least two specific places in the universe, the math of general relativity simply breaks down, failing to produce reliable results: in the centers of black holes and at the beginning of the universe. These regions, also known as “singularities,” are places in space-time where the laws of physics that control us today break down. They are mathematical indications that the general theory of relativity is about to collapse. At very small length scales, gravity becomes extraordinarily strong within both of these singularities.
Therefore, physicists require a microscopic description of strong gravity, also known as a quantum theory of gravity, in order to unravel the mysteries surrounding the singularities. There are many contenders, such as loop quantum gravity and string theory.
And there’s another approach that completely rewrites our understanding of space and time.
Causal set theory
Time and space are continuously defined in all of the current physics theories. They form a smooth fabric that underlies all of reality. In such a continuous space-time, two points can be as close to each other in space as possible, and two events can occur as close in time to each other as possible.
An alternative method known as causal set theory, on the other hand, interprets space-time as a collection of discrete parts, or “atoms.” According to this theory, events are only allowed to occur as close together in space and time as the size of a “atom.”
If you’re reading this on your screen, for example, everything appears to be flowing smoothly. It is impossible to get two images on your screen closer than a single pixel, though, if you were to look at the same screen through a magnifying glass and see the pixels that divide up the space.
Bento was excited by this physics theory. “I was thrilled to find this theory, which not only tries to go as fundamental as possible — being an approach to quantum gravity and actually rethinking the notion of space-time itself — but which also gives a central role to time and what it physically means for time to pass, how physical your past really is and whether the future exists already or not,” Bento said on Live Science.
Beginning of time
Causal set theory has important implications for the nature of time.
“A huge part of the causal set philosophy is that the passage of time is something physical, that it should not be attributed to some emergent sort of illusion or to something that happens inside our brains that makes us think time passes; this passing is, in itself, a manifestation of the physical theory,” Bento stated. “So, in causal set theory, a causal set will grow one ‘atom’ at a time and get bigger and bigger.”
Since singularities are impossible in the theory, the causal set approach neatly solves the Big Bang singularity issue. Matter can never compress to an infinitely small point; it can only get as small as an atom in space-time.
What then would the origin of our universe look like in the absence of a Big Bang singularity? That’s where Bento and his collaborator, Imperial College London graduate student Stav Zalel, took up the thread, investigating the implications of causal set theory for the early stages of the universe. Their work can be found in a paper uploaded to the preprint database arXiv. (The article has not yet been released in a scientific journal subject to peer review.)
The paper examined “whether a beginning must exist in the causal set approach,” Bento said. “In the original causal set formulation and dynamics, classically speaking, a causal set grows from nothing into the universe we see today. In our work instead, there would be no Big Bang as a beginning, as the causal set would be infinite to the past, and so there’s always something before.”
According to their research, the universe might not have had a beginning at all and has just always existed. Perhaps rather than being a true beginning, what we understand as the Big Bang was simply a specific point in the evolution of this everlasting causal set.
There’s still a lot of work to be done, however. It is not yet clear whether physical theories that can be used to explain the complex evolution of the universe during the Big Bang can be accommodated by this no-beginning causal approach.
“One can still ask whether this [causal set approach] can be interpreted in a ‘reasonable’ way, or what such dynamics physically means in a broader sense, but we showed that a framework is indeed possible,” Bento stated. “So at least mathematically, this can be done.”
In other words, it’s … a beginning.
