Quantum Gravity Models Demonstrate Emergent Inflation and Dynamical Dark Energy Transitions

The accelerating expansion of the universe remains one of the most profound mysteries in modern cosmology, and now Luca Marchetti from the Kavli Institute for the Physics and Mathematics of the Universe and Okinawa Institute of Science and Technology Graduate University, along with Tom Ladstätter from Ludwig-Maximilians-Universität München, and Daniele Oriti from Universidad Complutense de Madrid, present a novel mechanism linking this acceleration to the fundamental structure of quantum gravity. Their work demonstrates how models based on Group Field Theory naturally generate both an early period of rapid expansion, effectively inflation, and a late-time acceleration driven by a form of dynamical dark energy. This research is significant because it offers a potential solution to longstanding problems in cosmology, suggesting that the universe’s expansion isn’t driven by an unknown energy component, but emerges directly from the underlying quantum nature of spacetime itself, and avoids the need for special conditions to end the inflationary period.

The work investigates the fundamental principles of GFT, its mathematical foundations, and its application to understanding the early universe. A central theme involves using GFT condensates to model cosmology and the emergence of spacetime. Researchers are actively investigating phase transitions within GFT, employing techniques like Landau-Ginzburg analysis and renormalization group methods to understand how spacetime geometry arises. The research also delves into quantum cosmology, exploring emergent universe scenarios and investigating how quantum gravity might influence inflationary cosmology and the formation of large-scale structure.

Researchers are connecting these theoretical developments to observational cosmology, examining potential links to the Hubble Tension, the σ8 Tension, the Cosmic Microwave Background, and the large-scale structure of the universe. Key researchers driving this field include D. Oriti, L. Marchetti, S. Researchers developed a method to derive cosmological dynamics directly from the fundamental principles of this theory, bypassing traditional assumptions about spacetime. They calculated the effective action governing the dynamics of this condensate, determining the expansion rate of the universe. This achieves a natural emergence of Friedmann dynamics, the standard equations describing the expansion of the universe, from the underlying quantum gravity framework. Importantly, this emergent inflation avoids the “graceful exit problem” often encountered in traditional inflation models, transitioning smoothly into a non-accelerating phase consistent with classical expectations.

Researchers further refined this approach by investigating scalar cosmological perturbations, the seeds of structure formation. This involved calculating the effective dynamics of scalar fields arising from quantum fluctuations, demonstrating their consistency with observational constraints. The team also explored relational observables, quantities defined relative to the internal degrees of freedom of the quantum gravity system, to provide a background-independent description of cosmological dynamics, deepening understanding of the quantum origins of cosmic structure and the emergence of classical spacetime.

Group Field Theories Explain Cosmic Expansion and Inflation

Recent work demonstrates a connection between quantum gravity and the observed expansion of the universe, revealing a mechanism for both cosmic acceleration and early-universe inflation within a framework of Group Field Theories. Researchers have explored interacting mean-field models of these theories and found that a broad class of them consistently predict the emergence of a dynamical dark energy component, governing late-time cosmological evolution. Stability analyses confirm that these models exhibit a de Sitter (dS) attractor, indicating a persistent, non-accelerating phase consistent with classical expectations. Detailed investigations reveal that the effective equation-of-state parameter, characterizing the dark energy, evolves predictably across this class of models.

Importantly, the research demonstrates that models exhibiting repulsive dS regimes can successfully realize slow-roll inflation, where the inflaton itself emerges from the underlying quantum gravity dynamics. This suggests a unified framework where the same physical principles drive both the early, rapid expansion of the universe and its current, accelerating expansion. Furthermore, the study addresses existing tensions in cosmological measurements, specifically the H0 tension, a discrepancy between local and CMB determinations of the present-day expansion rate. The results align with recent data from the DESI experiment, which disfavors a pure cosmological constant and instead points toward a dynamical dark energy component with potentially “phantom” characteristics. The team discovered that, depending on interaction parameters, the model evolves towards either a late-time dark energy phase exhibiting phantom-like behaviour, or an initial period of rapid expansion consistent with inflation. Importantly, this emergent inflation avoids the typical “graceful exit” problem often encountered in inflationary cosmology and naturally transitions into a phase consistent with classical expectations for a universe dominated by matter. The researchers further investigated the dynamics following inflation, finding evidence for a transition to a classical phase governed by a scalar field.

Through both analytical approximations and preliminary numerical simulations, they showed that averaged quantities, such as the rate of change of density, remain roughly constant and satisfy expected relationships, suggesting the emergence of this classical behaviour. While acknowledging limitations in the numerical analysis due to computational constraints and the complexity of the system, the results provide qualitative support for the theoretical predictions. Future work will require more comprehensive numerical analysis and exploration of a wider range of parameters to definitively establish the robustness of these findings and fully characterise the emergent cosmological behaviour.