Black Hole Chemistry Reveals Phase Transitions and Broadens Gravitational Physics

Black holes, once considered simple gravitational sinks, now reveal surprisingly complex behaviour reminiscent of everyday chemistry, a field of study that began to emerge fifteen years ago. Robert B. Mann from the University of Waterloo, along with colleagues, initiated this shift by incorporating the concept of pressure into black hole thermodynamics, effectively expanding the possibilities for phase transitions and critical phenomena. This work establishes that black holes can exhibit behaviours analogous to Van der Waals fluids, undergoing transitions with features like triple points and reentrant phases, previously thought exclusive to conventional matter. The resulting field, known as Black Hole Chemistry, not only deepens our understanding of these enigmatic objects but also introduces novel perspectives on gravity, cosmology, and the fundamental nature of spacetime, suggesting that black holes may still hold many secrets yet to be uncovered.

This discovery launched the field of black hole thermodynamics, proposing that black holes possess temperature and entropy despite lacking conventional matter. Early investigations established fundamental laws governing black holes, drawing parallels with classical thermodynamics but adapting them to curved spacetime and event horizons. A crucial development was the identification of Bekenstein-Hawking entropy, linking entropy to the black hole’s event horizon area, and the associated Hawking temperature, arising from quantum effects near the event horizon.

Over the last 15 years, the field expanded into black hole chemistry, extending the analogy to include chemical potentials and reactions. This progression views black holes not simply as thermodynamic entities, but as systems capable of undergoing chemical transformations, with the cosmological constant acting as a thermodynamic pressure. This approach stems from a desire to understand the microscopic origin of black hole entropy and to explore connections between gravity, thermodynamics, and information theory. Furthermore, the chemical perspective offers new insights into black hole stability and phase transitions, potentially revealing links to condensed matter physics and string theory. Concepts of temperature, entropy, work, and phase changes, initially introduced into gravitational physics and applied to black holes, were later extended to cosmological horizons and other settings.

This includes exploring relationships between mass and thermodynamic quantities, and the first law of black hole mechanics. Investigations extend general relativity by incorporating higher-order curvature terms, and researchers are actively investigating the existence of phase transitions and critical points in black hole systems. Stability analysis of black hole solutions in various gravity theories is a key focus, and researchers are extending the understanding of black hole thermodynamics by treating pressure and volume as thermodynamic variables. This research paints a picture of active investigation into black hole physics, with a strong emphasis on extending the standard framework of general relativity and exploring the thermodynamic and chemical properties of black holes in more complex scenarios. Recent work demonstrates that black holes can exhibit a rich variety of phase transitions, analogous to those observed in laboratory fluids, including behaviours like Van der Waals fluids and reentrant phase transitions. Importantly, research shows that the ‘molecules’ comprising black holes can experience both attractive and repulsive interactions, a distinction from simpler fluid models. Analysis using Ruppeiner curvature confirms that critical exponents and dimensionless constants for black holes match those of Van der Waals fluids, suggesting a universal underlying structure.

While the precise nature of the black hole’s microconstituents remains unknown, the existence of a molecular-like structure appears robust. Current research actively explores the chemistry of de Sitter black holes and related topics, indicating the field remains vibrant and continues to offer new insights into the fundamental nature of gravity and black holes. Future work will likely focus on identifying these constituents and further exploring connections between black hole chemistry and other areas of physics, such as condensed matter systems and cosmology.