In the beginning there was — well, maybe there wasn’t a beginning. Perhaps our universe has always existed – and a new theory of quantum gravity shows how that might work.
“There are so many things in reality that most people associate with science fiction or even fantasy,” says Bruno Bento, a physicist who studies the nature of time at the University of Liverpool in Britain.
In his work, he used a new theory of quantum gravity called causal set theory, in which space and time are broken down into discrete space-time fragments. According to this theory, there is a fundamental unit of space-time at some level.
Bento and his co-authors used this method of causal sets to study the beginning of the universe. They found that it is possible that the universe had no beginning – that it has always existed in the infinite past and only recently evolved into what we call the Big Bang.
Quantum gravity is probably the most annoying problem facing modern physics. We have two extremely effective theories of the universe: quantum physics and general relativity. Quantum physics has successfully described three of the four fundamental forces of nature (electromagnetism, weak forces, and strong forces) down to the microscopic scale. General relativity, on the other hand, is the most powerful and complete description of gravity ever created.
But for all its merits, the general theory of relativity is incomplete. In at least two specific places in the universe, the mathematics of general relativity simply breaks down without giving reliable results: at the centers of black holes and at the beginning of the universe. These areas are called “singularities,” that is, places in space-time where our current laws of physics collapse, and they are mathematical warning signs that general relativity theory is tripping over itself. In both singularities, gravity becomes incredibly strong on very small length scales.
Therefore, to unravel the mysteries of the singularities, physicists need a microscopic description of strong gravity, also called quantum theory of gravity. There are many contenders for this, including string theory and loop quantum gravity.
But there is another approach that completely rewrites our understanding of space and time.
Causal Set Theory
In all modern theories of physics, space and time are continuous. They form a smooth fabric that underlies all reality. In such continuous space-time, two points can be as close together in space as possible, and two events can occur as close together in time as possible.
But another approach, called causal set theory, presents space-time as a series of discrete fragments, or “atoms” of space-time. This theory places strict limits on how close events can be in space and time, since they cannot be closer than the size of an “atom.
For example, if you look at your screen and read this article, everything seems smooth and continuous. But if you looked at the same screen through a magnifying glass, you would see pixels that divide space, and you would find that it is impossible to bring two images on the screen closer than one pixel.
This physical theory excited Bento. “I was thrilled to find this theory that not only tries to be as fundamental as possible – being an approach to quantum gravity and actually redefining the very notion of space-time – but also gives a central role to time and what the passage of time means physically, how physical your past is and whether or not the future already exists,” Bento told 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 said. “So, in causal set theory, a causal set will grow one ‘atom’ at a time and get bigger and bigger.”
The causal set approach neatly removes the problem of the Big Bang singularity because, in the theory, singularities can’t exist. It’s impossible for matter to compress down to infinitely tiny points — they can get no smaller than the size of a space-time atom.
So without a Big Bang singularity, what does the beginning of our universe look like? That’s where Bento and his collaborator, Stav Zalel, a graduate student at Imperial College London, picked up the thread, exploring what causal set theory has to say about the initial moments of the universe. Their work appears in a paper published Sept. 24 to the preprint database arXiv. (The paper has yet to be published in a peer-reviewed scientific journal.)
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.”
Their work implies that the universe may have had no beginning — that it has simply always existed. What we perceive as the Big Bang may have been just a particular moment in the evolution of this always-existing causal set, not a true beginning.
There’s still a lot of work to be done, however. It’s not clear yet if this no-beginning causal approach can allow for physical theories that we can work with to describe the complex evolution of the universe during the Big Bang.
“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 said. “So at least mathematically, this can be done.”
In other words, it’s … a beginning.