Nick Huggett: Cosmological Aspects of Quantum Gravity”

The speaker begins by setting the stage for a discussion on quantum gravity, referencing a previous talk given at the Philosophy of Science Association. They mention the challenge of expanding a 20-minute talk to fit a 45-50 minute format and acknowledge the contributions of Chris and other institutions. The speaker also provides a bit.ly address for downloading the presentation slides and clarifies the URL difference between the online version and the one being presented. The talk aims to explore the philosophical implications of the emergence of classical space-time from quantum systems, a project known as 'Beyond Space-Time,' which is currently funded by a Templeton grant. The speaker introduces the concept of emergence and the philosophical questions it raises, particularly in the context of cosmology.

This paragraph delves into the possibility that classical space-time emerges from more fundamental quantum systems, a concept emphasized by Daniela in her talk. The speaker uses the analogy of phase transitions in matter, such as the change from solid to liquid, to illustrate the potential for a transition from a non-geometric state to a geometric one in the universe. They reference a paper by Rudy that catalogs various scenarios in which space-time could emerge from a non-spatio-temporal state. Philosophical implications include shedding light on the nature of non-spatio-temporal degrees of freedom and how they form space-time. The speaker also raises the issue of empirical incoherence for quantum gravity inquiries, suggesting that there may be regions of the universe that are fundamentally unobservable.

The speaker transitions to discussing string cosmology, referencing work by Gasparini and Venustiano, and providing a brief overview of string theory for those unfamiliar with the subject. They describe the basic elements of string theory, such as strings propagating in time and their ability to split and join. The focus then shifts to the worldsheet, a two-dimensional space-time on which strings live, and the addition of a scalar field that interacts with the geometry of the string. The speaker discusses the implications of these concepts for the nature of the universe, including the potential for an always-expanding universe with decelerating expansion, and the philosophical and physical questions that arise from these models.

In this section, the speaker explores the role of dilatation fields in string theory, which can affect the gravitational interaction strength. They discuss how the dilatation field, when varied, can change the string coupling and potentially lead to different gravitational behaviors. The speaker also touches on the renormalization group analysis and beta functions, which are crucial for understanding the quantum properties of strings. The summary of this paragraph highlights the complex interplay between the dilatation field, the background fields, and the fundamental nature of space-time in string theory.

The speaker delves into the quantum mechanical nature of string theory, discussing the concept of coherent states built from string modes and the transition from strings to quantum fields. They emphasize the importance of understanding the background fields in string theory not as independent objects but as classical approximations of string excitations. The paragraph also touches on the challenges of perturbation theory and the need to consider higher-order effects and different topologies in the worldsheet, which are crucial for a comprehensive understanding of string dynamics.

This paragraph focuses on finding solutions to the equations derived from string cosmology models, specifically in four dimensions with spatially homogeneous, isotropic, and flat space-time. The speaker introduces the Hubble parameter and discusses the symmetries of the equations, including time reversal and scale inversion. They present solutions for both positive and negative times, suggesting an expanding universe with a curvature singularity at time equals zero. The speaker also mentions the possibility of an inflating universe for negative times and the philosophical implications of these findings.

The speaker contemplates the quantum nature of the Big Bang, suggesting that the classical singularity might be resolved through quantum stringy effects. They discuss three main possibilities for what could happen at the Big Bang: curvature corrections, string coupling corrections, or a combination of both that might involve M-theory. The speaker emphasizes the difficulty of understanding what happens at the Big Bang due to the breakdown of perturbative methods and the need for a full quantum theory of gravity.

In this paragraph, the speaker explores the concept of a transition from a non-spatio-temporal phase to a spatio-temporal phase, which is a central theme in the discussion of quantum gravity and cosmology. They discuss the philosophical implications of such a transition, including the challenges of understanding what it means for time to emerge or dissolve. The speaker suggests that thinking about the Big Bang in reverse might provide a more comfortable perspective on the emergence of time and space, highlighting the need for further exploration of these complex ideas.

The speaker concludes with a discussion of the philosophical implications of the models and theories presented, particularly regarding the concept of empirical incoherence. They raise questions about the nature of observation and inference in the context of quantum gravity, where some regions of the universe may be fundamentally unobservable. The speaker also touches on the challenges of understanding the past and the role of time in our comprehension of the universe, suggesting that further philosophical inquiry is needed to address these complex issues.

In the final paragraph, the speaker reflects on the concept of transition in the context of quantum gravity, especially when considering the emergence of space-time from a non-spatio-temporal state. They break down the notion of transition into several components, discussing the challenges of applying temporal concepts to a state that is non-temporal. The speaker suggests that the dynamics of such a transition may not require a literal before and after in time but rather a process in which time itself is dissolving. This reflection invites further philosophical and scientific exploration into the nature of time and space in quantum gravity theories.