Gravity could be a manifestation of thermodynamics, the science of heat and energy.
This is the idea explored by researchers at Imperial College London, who propose that it acts as a heat engine, transforming heat into work. A path that could solve major cosmological puzzles.
Einstein's theory of general relativity describes gravity as a curvature of spacetime. But it struggles to explain why the vacuum energy is so small compared to quantum predictions: the famous cosmological constant problem. This discrepancy has led physicists to search for alternatives. The thermodynamic approach could offer a new path, linking gravity to heat and entropy processes already observed in black holes.
It all started in 1995, when physicist Ted Jacobson reversed the logic of Hawking and Bekenstein. Instead of deducing a temperature from gravity, he used thermodynamics to derive Einstein's equation. A brilliant idea but one that remained unexplored for decades. Joao Magueijo, lead author of the new study, remembered it while on vacation on a remote Greek island. He realized that previous attempts tried to fit existing theories into Jacobson's framework, rather than letting thermodynamics guide the way.
With his PhD student Ray Isichei, Magueijo modeled gravity as an Otto cycle, the principle behind gasoline engines. In an engine, heat is not the only energy source: there is also work and chemical reactions. So they added this "something else" to the thermodynamic process, without assuming the outcome. This open approach allowed them to derive a new theory of gravity, where matter and energy can be created or destroyed.
The thermodynamic cycle proposed in the paper. Einstein's gravity corresponds to the degenerate case where only heat flow steps are present. By adding work production steps, new theories of gravity emerge, including those where matter-energy conservation is modified.
Credit: Isichei and Magueijo / Physical Review Letters. This result almost ended up being discarded, because it seemed absurd. But the researchers realized that this property could explain the accelerated expansion of the universe without invoking dark energy or a cosmological constant. Normally, ordinary matter slows down expansion. But if its conservation is modified, a continuous creation of matter could instead accelerate expansion. A bold idea that could solve a decades-old problem.
Of course, this theory remains speculative. The authors plan to test it against cosmological observations, such as supernovae or the cosmic microwave background. Cosmology has become a precision science, and any new idea must pass the test of data. If confirmed, our understanding of gravity and the universe could be profoundly shaken.