A new theoretical framework suggests that the emergence of galaxies and other complex structures in the universe may be explained by quantum information theory, potentially resolving a longstanding paradox in cosmology. Ginestra Bianconi of Queen Mary University of London has demonstrated how entropy density can decrease locally while the total entropy of the universe continues to increase, according to her gravity from entropy (GfE) theory.
The second law of thermodynamics states that the entropy of a system must increase over time. It is the only fundamental law of physics that captures the direction of time we experience. Entropy is a measure of disorder: lower entropy means more structure, while higher entropy means less structure. The law explains why gases expand, why diffusion occurs from high to low concentration, and why coffee goes cold.
Cosmologists believe the universe itself follows this law. The early universe was in a state of low entropy and has evolved toward states of high entropy. Yet during this evolution, the universe has produced incredibly complex structures such as galaxies, which are low in entropy but have formed in a high-entropy environment. Their creation seemingly violates the second law, a contradiction that has motivated Bianconi's gravitational framework.
In a recent paper titled «Thermodynamics of the gravity from entropy theory», Bianconi built on her previous work on the GfE theory and applied it to the entire universe. The theory treats the metric of space-time—the mathematical object that describes how space-time is curved in Einstein's general relativity—as a quantum operator. This quantization leads to two notions of the metric: one that is the «true» metric and one induced via mass and energy. The distinguishability between them is measured by a quantity from quantum information theory called quantum relative entropy (QRE).
It is this interplay between the two geometries, measured through QRE, that defines the dynamics of space-time. In other words, the QRE leads to equations of motion that describe gravity. Bianconi found that in the low curvature limit, classical general relativity is reproduced exactly. Outside that limit, however, the equations must be modified by a mathematical object called the G-field, which generates a dynamical dark-energy term. In standard general relativity, dark energy is driven by a static cosmological constant, so a dynamical term could potentially be testable.
Bianconi then applied the GfE theory to a Friedmann–Robertson–Walker (FRW) metric universe, a model that approximates the universe as isotropic, homogeneous, and expanding. As the universe expands, its volume increases along with its total entropy. Because the same amount of entropy is contained in a larger volume, the local entropy per unit volume decreases over time. This decrease in local entropy density in a universe of growing total entropy provides a mechanism for low-entropy structures such as galaxies and even life forms to emerge.
The question remained whether this mechanism violates the second law of thermodynamics. Bianconi addressed this for a non-empty FRW universe by integrating entropy density over a space-time region. Because the volume increases faster than the entropy density decays, the total entropy increases with time for both radiation- and matter-dominated universes. Thus, while entropy density decreases locally, the total entropy of the universe still rises, preserving the second law.
Bianconi emphasizes that the GfE theory is still in its infancy and requires experimental verification. Nevertheless, the work highlights the intrinsically thermodynamic nature of the theory and opens new avenues for both classical and quantum gravity. By framing gravity in terms of quantum information and entropy, the theory may offer an alternative route toward quantizing gravity.
