Modified Gravity with Curvature-Matter Couplings Enables Gravitationally Induced Particle Creation and Energy Exchange

The persistent mysteries of dark energy and dark matter have prompted physicists to explore alternatives to standard cosmological models, and a compelling approach involves modifying our understanding of gravity itself. Francisco S. N. Lobo, from the Instituto de Astrofísica e Ciências do Espaço and the Universidade de Lisboa, Tiberiu Harko of Babeș-Bolyai University, and Miguel A. S. Pinto, also from the Universidade de Lisboa, investigate theories where gravity interacts directly with matter, potentially explaining the observed accelerated expansion of the universe without invoking dark energy. Their work demonstrates that this interaction leads to the creation of particles, generating a negative pressure that drives the late-time acceleration, and importantly, upholds the fundamental generalized second law of thermodynamics. By calculating the rate of particle creation and its impact on entropy, the team provides a novel framework for understanding cosmic expansion and offers constraints on the dynamics of matter creation in a time-varying gravitational field.

Modified Gravity and Cosmology Alternatives

This extensive collection of research focuses on exploring alternatives to the standard model of cosmology, investigating theories beyond traditional General Relativity. A central theme is the investigation of modified gravity, where the relationship between matter and geometry is more complex than previously understood, potentially offering explanations for phenomena like dark matter and dark energy. Researchers are particularly interested in theories involving non-minimal coupling between matter and the curvature of spacetime, suggesting a dynamic interplay between these fundamental aspects of the universe. This work also delves into the realm of non-equilibrium physics, recognizing that the early universe was far from a state of thermal equilibrium, and that understanding particle distributions requires sophisticated tools from kinetic theory.

The research explores exotic solutions to Einstein’s equations, such as wormholes, and how these might be supported by modified gravity theories. Investigations into neutron star physics within these modified gravity frameworks aim to test the theories against observations of compact objects. The research points towards potential avenues for testing these theories with observational data, including the cosmic microwave background, large-scale structure, and gravitational waves.

Particle Creation in Expanding Spacetimes via Thermodynamics

Scientists developed a sophisticated methodology to investigate particle creation in expanding universes, moving beyond traditional dark energy and dark matter explanations. The study centers on modified gravity theories where matter and geometry interact, leading to the emergence of forces and non-conservation of energy-momentum. Researchers adopted a thermodynamic interpretation, viewing this interaction as an exchange of energy between matter and the geometry of spacetime, allowing them to explore particle creation processes. To model this, the team employed the Boltzmann equation, a powerful tool from statistical physics, to describe the evolution of particle distributions.

Crucially, the research pioneered a modification of the Boltzmann equation to incorporate gravitational particle production. The team introduced a non-collisional source term, representing gravitationally induced matter creation, proportional to the Christoffel symbols and inversely proportional to the expansion rate. This innovative approach allows scientists to model particle creation directly within the framework of the Boltzmann equation, offering a new pathway to understanding the evolution of the universe.

Matter-Curvature Coupling Alters Fluid Dynamics

This work presents a detailed investigation into modified gravity theories where gravity interacts directly with matter, offering an alternative to concepts like dark energy and dark matter. The core idea is that matter and the geometry of spacetime aren’t separate entities, but exchange energy and momentum, leading to observable effects. Researchers demonstrate this interaction causes a deviation from standard expectations of how objects move, specifically an extra force acting on fluid elements. This force arises from the non-conservation of energy and momentum, a direct consequence of the coupling between matter and curvature.

The team reformulated the theory using scalar fields, effectively recasting the complex gravitational interaction as a more manageable system of interacting scalar fields and matter. Crucially, the analysis reveals that the divergence of the energy-momentum tensor is modified due to this coupling, indicating a continuous exchange of energy between matter and the scalar field. Furthermore, the study applies the principles of irreversible thermodynamics to understand particle creation within this framework, demonstrating that this interaction can drive a late-time acceleration of the universe.

Gravity, Thermodynamics, and Accelerated Expansion

This research demonstrates that modified theories of gravity, incorporating a direct interaction between matter and the curvature of spacetime, offer a compelling explanation for the observed accelerated expansion of the universe without invoking dark energy. By examining a scalar-tensor formulation of these theories, scientists derived the rate at which particles are created by the gravitational field, alongside the associated creation pressure and entropy production. The results show that this irreversible particle creation generates a negative creation pressure, naturally driving a late-time, de Sitter-type expansion of the universe. Importantly, this work establishes a link between gravitational processes and thermodynamics, demonstrating that the total entropy of the universe, considering both the cosmological horizon and enclosed matter, increases monotonically and reaches a stable state during the de Sitter phase. This thermodynamic consistency reinforces the viability of the model and suggests that the accelerating expansion of the universe may not require the existence of dark energy, but instead arises from the fundamental interaction between gravity and matter.