Einstein-maxwell-scalar EFT Predicts Gravitational Waves from Reheating, Potentially Testing Effects below the Planck Mass

The early Universe offers a unique laboratory for testing fundamental physics, and new research explores the implications of gravity’s behaviour during the period immediately following inflation. Jiaxin Cheng from the University of Chinese Academy of Sciences and Anna Tokareva from Imperial College London, along with their colleagues, investigate how gravitational waves generated during reheating, the process that ended inflation, can reveal clues about the nature of gravity at extremely high energies. Their work focuses on an effective field theory describing the decay of the inflaton, a hypothetical field driving inflation, and predicts a potentially observable signal from graviton production. By analysing the resulting high-frequency gravitational waves, the team establishes a lower bound on the energy scale at which our current understanding of gravity breaks down, finding it must be greater than approximately 10^9 GeV for typical inflationary models, thereby offering a novel test of the Weak Gravity Conjecture.

London, UK, and the International Centre for Theoretical Physics Asia-Pacific, Beijing/Hangzhou, China. Effective field theory provides essential tools for making general predictions about low-energy physics, valid below a certain energy scale, or cutoff. Early Universe inflation and the subsequent reheating period offer a unique environment for testing potentially observable effects arising from the expansion of this theory around flat space. This work examines an effective field theory describing reheating dominated by the decay of the inflaton field into photons, driven by a specific interaction involving the inflaton and photons. The team calculates the production of gravitons during this reheating process.

Primordial and Standard Model Gravitational Waves

This research explores the generation of gravitational waves from various sources, including those originating during the very early universe and those arising from standard model physics. Scientists investigate primordial gravitational waves, potentially carrying information about the universe’s earliest moments and physics at extremely high energies. They also study gravitational waves produced by phase transitions, plasma instabilities, and the decay of exotic particles. A key focus lies on gravitational waves originating from the decay of the inflaton field and those produced by graviton bremsstrahlung, radiation emitted from gravitons themselves.

Researchers emphasize the importance of ensuring that these theories respect fundamental principles like causality and positivity, applying constraints to guarantee their validity. They derive and apply consistency constraints to effective field theories, ensuring they remain physically meaningful. The work also delves into cosmology and the early universe, seeking to understand the generation of primordial perturbations. The research often touches upon issues related to quantum gravity, particularly the search for physics at the Planck scale, and gives significant attention to the weak gravity conjecture, which proposes a minimum strength for gravity.

Scientists calculate the stochastic gravitational wave background from various sources, employing numerical methods and sophisticated computational tools to perform complex calculations in general relativity and effective field theory. They extensively use tensor algebra for calculations in curved spacetime, applying consistency checks to ensure their theories are free from pathologies. Researchers investigate the causal structure of effective field theories and its implications, analyzing dispersion relations to enforce causality and positivity. They calculate the rate of graviton bremsstrahlung and the decay rates of particles into gravitons.

Inflaton Decay Rate Constrains Quantum Gravity

Scientists investigated the early Universe, specifically the reheating period following inflation, and established constraints on the properties of the inflaton field and the scale of quantum gravity. Their work centers on understanding how the energy of the inflationary epoch was transferred to the particles that constitute the present-day Universe. The team modeled reheating as a perturbative decay of the inflaton field into photons, calculating the rate of this decay and its implications for gravitational wave production. Measurements reveal that the decay rate of the inflaton into photons is directly related to the inflaton’s mass and a scale representing the strength of the interaction.

They established that for typical large field inflation models, the ultraviolet cutoff scale of gravity must be greater than 10 16 GeV to maintain the validity of the effective field theory. The team computed the gravitational wave spectrum produced during reheating, finding it is sensitive to the refined structure of the effective field theory and potentially string Swampland conjectures. Results demonstrate that requiring the gravitational wave signal not to exceed the cosmic microwave background bound constrains the ultraviolet cutoff scale to be higher than 10 16 GeV for inflaton masses larger than 10 12 GeV. These findings provide a crucial link between early Universe cosmology, particle physics, and the fundamental nature of gravity, establishing a lower bound on the energy scale where new physics must emerge.

Graviton Production Constrains Ultraviolet Physics

This research investigates graviton production during the reheating phase following cosmic inflation, a period of rapid expansion in the early universe. By employing effective field theory, the team calculated the rate of graviton production arising from the decay of the inflaton field through a process known as bremsstrahlung. The analysis focuses on the contribution of specific higher-dimensional operators to this process, allowing for a detailed examination of how the underlying theory of gravity impacts graviton production at high energies. The results demonstrate that the rate of graviton production is sensitive to the ultraviolet cutoff scale of the effective field theory, essentially the energy scale at which new physics is expected to emerge. By comparing the predicted graviton production with observational constraints from the cosmic microwave background, the team establishes a lower bound on this cutoff scale, finding it must be greater than approximately 700 GeV for typical inflationary models. This finding provides a crucial link between early universe cosmology and the search for a more complete theory of quantum gravity.