The new theory, developed by physicists at Aalto University, describes gravity in a way that’s compatible with the Standard Model of particle physics, opening the door to an improved understanding of how the Universe began.
The Standard Model of particle physics describes electromagnetic, weak, and strong interactions, which are three of the four known fundamental forces of nature. The unification of the fourth interaction, gravity, with the Standard Model has been challenging due to incompatibilities of the underlying theories — general relativity and quantum field theory. While quantum field theory utilizes compact, finite-dimensional symmetries associated with the internal degrees of freedom of quantum fields, general relativity is based on noncompact, infinite-dimensional external space-time symmetries. Mikko Partanen & Jukka Tulkki aim at deriving the gauge theory of gravity using compact, finite-dimensional symmetries in a way that resembles the formulation of the fundamental interactions of the Standard Model. Image credit: DESY / Science Communication Lab.
“If this turns out to lead to a complete quantum field theory of gravity, then eventually it will give answers to the very difficult problems of understanding singularities in black holes and the Big Bang,” said Aalto University’s Dr. Mikko Partanen.
“A theory that coherently describes all fundamental forces of nature is often called the Theory of Everything.”
“Some fundamental questions of physics still remain unanswered. For example, the present theories do not yet explain why there is more matter than antimatter in the observable Universe.”
The key was finding a way to describe gravity in a suitable gauge theory — a kind of theory in which particles interact with each other through a field.
“The most familiar gauge field is the electromagnetic field,” said Aalto University’s Dr. Jukka Tulkki.
“When electrically charged particles interact with each other, they interact through the electromagnetic field, which is the pertinent gauge field.”
“So when we have particles which have energy, the interactions they have just because they have energy would happen through the gravitational field.”
A challenge long facing physicists is finding a gauge theory of gravity that is compatible with the gauge theories of the other three fundamental forces — the electromagnetic force, the weak nuclear force and the strong nuclear force.
The Standard Model of particle physics is a gauge theory which describes those three forces, and it has certain symmetries.
“The main idea is to have a gravity gauge theory with a symmetry that is similar to the Standard Model symmetries, instead of basing the theory on the very different kind of spacetime symmetry of general relativity,” Dr. Partanen said.
Without such a theory, physicists cannot reconcile our two most powerful theories, quantum field theory and general relativity.
Quantum theory describes the world of the very small — tiny particles interacting in probabilistic ways — while general relativity describes the chunkier world of familiar objects and their gravitational interaction.
They are descriptions of our Universe from different perspectives, and both theories have been confirmed to extraordinary precision — yet they are incompatible with each other.
Furthermore, because gravitational interactions are weak, more precision is needed to study true quantum gravity effects beyond general relativity, which is a classical theory.
“A quantum theory of gravity is needed to understand what kind of phenomena there are in cases where there’s a gravitational field and high energies,” Dr. Partanen said.
“Those are the conditions around black holes and in the very early Universe, just after the Big Bang — areas where existing theories in physics stop working.”
“Always fascinated with the very big questions of physics, we discovered a new symmetry-based approach to the theory of gravity and began to develop the idea.”
“The resulting work has great potential to unlock a whole new era of scientific understanding, in much the same way as understanding gravity paved the way to eventually creating GPS.”
Although the theory is promising, the duo point out that they have not yet completed its proof.
The theory uses a technical procedure known as renormalization, a mathematical way of dealing with infinities that show up in the calculations.
So far Dr. Partanen and Dr. Tulkki have shown that this works up to a certain point — for so-called ‘first order’ terms — but they need to make sure the infinities can be eliminated throughout the entire calculation.
“If renormalization doesn’t work for higher order terms, you’ll get infinite results,” Dr. Tulkki said.
“So it’s vital to show that this renormalization continues to work.”
“We still have to make a complete proof, but we believe it’s very likely we’ll succeed.”
“There are still challenges ahead, but with time and effort he expects they’ll be overcome,” Dr. Partanen said.
“I can’t say when, but I can say we’ll know much more about that in a few years.”
The team’s paper was published in the journal Reports on Progress in Physics.
_____
Mikko Partanen & Jukka Tulkki. 2025. Gravity generated by four one-dimensional unitary gauge symmetries and the Standard Model. Rep. Prog. Phys 88, 057802; doi: 10.1088/1361-6633/adc82e
