New theoretical research offers a potential way to address one of modern physics’ most difficult puzzles: How did the universe give rise to galaxies, stars, planets, and life while still obeying the second law of thermodynamics?
The research was led by mathematician Professor Ginestra Bianconi from Queen Mary University of London. Her research examines whether a theory known as gravity from entropy can help explain how the complexity of the universe emerges even as the total entropy of the universe increases.
cosmic entropy puzzle
Einstein famously said that “the second law of thermodynamics occupies a unique place among the laws of nature,” reflecting his view that it is one of the safest and most fundamental principles in physics.
The second law states that the total entropy of an isolated system generally increases with time. Entropy is often described as a measure of disorder, but more accurately reflects how energy and information are distributed within a system.
This principle poses a major challenge to cosmology. Scientists generally believe that the early universe began in a state of low entropy and steadily moved toward high entropy. At the same time, matter has been organized into increasingly complex structures, from galaxies and stars to planets and living things.
Explaining how this increasing complexity and continuous entropy growth can coexist remains an open question.
A new perspective from entropy to gravity
In a paper published in Physical Review DBianconi investigates the puzzle using Gravity from Entropy (GfE), a proposed approach to quantum gravity.
This theory uses ideas from statistical mechanics to explain gravity as emerging from microscopic properties of space-time geometry. Rather than treating gravity only as a fundamental force or the curvature of space-time, GfE connects it to information and entropy at the quantum level.
Bianconi’s analysis reveals potentially important differences. The total entropy of the universe increases over time, but the amount of entropy per unit volume decreases as the universe expands.
This behavior could provide a new way to understand how organized structures develop locally without violating the second law of thermodynamics.
Black holes combine gravity and heat
The idea that gravity and thermodynamics are deeply connected dates back to the pioneering work of Jacob Bekenstein and Stephen Hawking in the 1970s.
Their findings showed that black holes have entropy and can emit thermal radiation. These discoveries transformed the scientific understanding of black holes and pointed to deeper connections between space-time, information, gravity, and heat.
Gravity from entropy builds on this broader relationship.
According to GfE, gravity arises from the information tension between the actual space-time metric and another metric generated by the matter field and the curvature of space-time. A metric is a mathematical construct used to describe distance and geometry in space and time.
This interpretation is expressed through the GfE Lagrangian defined by the quantum geometric relative entropy (QGRE) between two metrics.
Possible connection with dark energy
Under conditions of low energy and weak spacetime curvature, the equation of gravity from entropy reproduces general relativity. However, under more extreme conditions, the predictions start to differ.
Beyond this weak limit, the GfE equation produces a varying dark energy contribution. Because the term evolves dynamically, it may generate predictions that researchers may eventually test through cosmological observations.
This study investigates these thermodynamic effects in Friedman-Robertson-Walker cosmological spacetime. Friedman-Robertson-Walker cosmological spacetime is a commonly used mathematical model to describe a universe that is uniformly expanding on large scales.
This result shows that the local geometric components of spacetime obey the first law of thermodynamics. In this explanation, the emerging dark energy contribution acts as internal energy, and the quantum geometric relative entropy (QGRE) represents the local entropy per unit volume.
Quantities corresponding to effective temperature and effective pressure also arise naturally from theory. Taken together, these results suggest that the quantum state underlying gravity from entropy may have intrinsic thermal properties.
Expansion spreads entropy throughout space
This study also highlights the importance of local volume elements determined by physical spatiotemporal metrics.
As the universe expands, its volume also increases. Within the GfE framework, increasing volume increases the total entropy, even though the local QGRE within each unit of volume gradually decreases.
In other words, while the entire universe can contain more entropy, entropy becomes more widely distributed across expanding space. This unusual thermodynamic pattern may help elucidate how local structural regions and complexity arise.
Gravity and spacetime may be thermodynamic
This discovery supports the possibility that gravity and spacetime have both an informational and thermodynamic basis.
Such an interpretation could provide new ways to investigate the relationships between gravity, quantum theory, dark energy, the evolution of the universe, and the emergence of complex structures.
This proposal is still at an early theoretical stage. But researchers say this could contribute to efforts to link general relativity, thermodynamics, quantum mechanics and cosmology within a broader framework.
“This study reveals how entropic gravity theory can tackle the difficult problem of reconciling the second principles of thermodynamics with the emergence of cosmic complexity. These results could open new avenues for studying the long-standing problem of reconciling the basis of cosmological irreversibility, the emergence of complex structures, and ultimately life with fundamental gravitational mechanics,” said Professor Bianconi.
