Theoretical astrophysics
To model any astrophysical object (whether it's one you see in the sky or one you step on), a set of equations is required to describe the fundamental interactions: gravity, electromagnetic, weak, and strong forces (and potentially a fifth force if scalar fields are considered). Modifying these fundamental equations, particularly those governing gravity, can lead to observable effects on astrophysical bodies. Einstein’s General Theory of Relativity, while revolutionary, fails to fully explain certain phenomena, such as dark matter, the accelerated expansion of the Universe, and other discrepancies at cosmological scales.
To address these challenges, scientists develop Modified Gravity or Extended Theories of Gravity, including scalar-tensor and metric-affine models. These theories not only aim to bridge the gaps in General Relativity but also affect the Newtonian limit, influencing the structure and evolution of stars, white dwarfs, and planets, such as Jupiter and other substellar objects. Such modifications provide tools to test gravity theories and constrain dark matter models through astrophysical observations. Furthermore, certain observational techniques, such as the lithium depletion method for dating stars, depend sensitively on the underlying gravitational framework, underscoring the interplay between fundamental physics and stellar evolution. Of particular interest are neutron stars—extremely dense remnants of collapsed stars, with masses up to about two times that of the Sun but radii comparable to a small city (~15 km). By applying a suitable description of their extraordinary and not yet fully understood interior properties, Modified Gravity theories can alter their observable characteristics. These changes provide potential tests for our gravitational models, especially for supporting the claim that Modified Gravity is necessary to explain certain phenomena.
Planetary Seismology as a Test of Fundamental Physics
The internal structures of planets like Earth and Mars vary slightly in extended gravity models, making them ideal for testing these theories. Using seismic data to refine gravity models reveals how planetary interiors respond to modified physics. Future neutrino tomography is expected to further enhance constraints, particularly on Earth’s core, providing a deeper understanding of fundamental interactions at play within planetary interiors.
Planets in our Solar System, such as Jupiter and Saturn, as well as exoplanets and their evolutions, may exhibit behaviors diverging from Newtonian physics. Studying their seismic and structural features under extended gravity theories allows researchers to probe the limits of our understanding of gravitation and its implications for planetary evolution and formation.
Big Bang Nucleosynthesis beyond GR
The abundance of light elements formed shortly after the universe’s creation is sensitive to the cosmological models provided by alternative gravity theories. These theoretical predictions are compared with observed ratios of light elements like lithium to hydrogen. I have shown that such element abundances differ in gravity theories, challenging common approaches to solving the “cosmological lithium problem.”
Thermodynamics and Gravity
The consistency of equations in Modified Gravity (MG) has been called into question based on findings from previous research. For instance, the chemical potential’s relationship with gravity suggests that changes in gravitational field descriptions may have substantial impacts. MG has also been shown to affect the geodesic deviation equation on stellar surfaces, introducing adjustments resembling Hooke’s law within the polytropic equation of state. Additionally, microscopic properties like opacity undergo changes, leading to the emergence of effective quantities. Corrections to thermodynamic laws, stability criteria for relativistic stars as well as white dwarf ones. Moreover, I have demonstrated that properties of Fermi gases are also altered.
Theoretical models of thermonuclear processes within stellar interiors experience significant alterations under MG, impacting energy generation rates (see those papers: [1, 2, 3, 4]). Some gravity models suggest a dependence of particle interactions on local energy-momentum distributions. Moreover, quantities such as specific heat capacities, Debye temperatures, and crystallization process in white dwarfs are influenced by the gravitational model.
Chemical reaction rates are also affected by MG, as are equations of state in curved spacetime for degenerate stars, where metric components influence chemical potentials and temperature profiles (soo, e.g. [1, 2, 3, 4]). Finally, thermodynamic properties and equations of state are further altered by the presence of (pseudo-)scalar fields, including axions.
