Stochastic scalar-tensor inflation and beyond

summary

Video file (mp4)

The gist

The gist: This work provides a generalisation of stochastic inflation to scalar-tensor theories going beyond General Relativity by applying a gauge-agnostic coarse-graining procedure to the linear

In short

This work generalizes stochastic inflation to complex scalar-tensor theories beyond General Relativity. The method uses a gauge-agnostic coarse-graining procedure applied to the linear equations of motion from the Effective Field Theory of Dark Energy (EFToDE) to find stochastic sources for nonlinear models. This provides a unified framework applicable across various theories and scales.

Key concepts

Stochastic Inflation Paradigm
This approach addresses limitations in quantum field theory during inflation by using a stochastic description. It treats inhomogeneities arising from quantum vacuum fluctuations as classical, super-Hubble systems described by stochastic partial differential equations (SPDEs). This allows for the study of nonlinear dynamics in the infrared.
Effective Field Theory of Dark Energy (EFToDE)
The EFToDE acts as a unified framework to describe UV modes and compute sources simultaneously. It provides the most general action for a single scalar degree of freedom coupled to gravity, using time-dependent mass coefficients that encode self and metric interactions.
Gauge-Agnostic Coarse-Graining
This is the core methodology where linear equations from the EFT are coarse-grained to find stochastic sources. It maps different nonlinear theories onto their specific sets of stochastic equations by identifying corresponding coefficients within the EFToDE framework, regardless of the theory's specific gauge.

Terminology used across episodes

This episode discusses

The paper

Stochastic scalar-tensor inflation and beyond · Read on arXiv

Centre for Theoretical Cosmology · Department of Applied Mathematics and Theoretical Physics, University of Cambridge

During cosmological inflation, inhomogeneities arising from quantum vacuum fluctuations are stretched to become super-Hubble and effectively classical. As many scenarios of the origin involve nonlinearities or a breakdown of perturbativity in the infrared, the limitations of quantum field theory can be addressed using a stochastic description of the dynamics, the so-called stochastic inflation paradigm. However, the stochastic formalism was only recently formulated consistently within full General Relativity and has not yet been extended to more general theories of the early universe, which is the subject of this work. In order to find the stochastic sources for a wide class of fully nonlinear scalar-tensor theories, we apply our gauge-agnostic coarse-graining procedure to the linear equations of the associated effective field theory. Each theory can then be mapped to its own set of stochastic equations of motion by identifying the corresponding coefficients in the former. We illustrate this with a few concrete and, in most cases, unprecedented examples, including Gauss-Bonnet, generalized Brans-Dicke, Horndeski, and braiding theories. Finally, we discuss other natural extensions to provide a phenomenologically complete stochastic framework. For example, we showcase the coarse-graining of multifield inflation in full General Relativity and argue for the generality of our procedure and thus its potential applications beyond the realm of inflation.

DOI: 10.1088/1361-6382/aeb29c

Transcript

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: We're looking at the paper: "Stochastic scalar-tensor inflation and beyond".

Jocelyn: The gist:

Vera: First, who's behind it and why it matters.

Paper summary: Vera: So to wrap up segment one, we’ve seen how they use this gauge-agnostic coarse-graining procedure to find sources for various theories. Now let's talk about what this whole paper is trying to say and why it matters cosmologically.

Jocelyn: The thesis is that the stochastic formalism, which was only recently formulated consistently within full General Relativity, can be extended to more general theories of the early universe.

Subrahmanyan: They are addressing scenarios where nonlinearities or a breakdown of perturbativity in the infrared occur in inflation, and they are using stochastic dynamics to provide a nonperturbative model sourced by quantum vacuum fluctuations’ noise.

Vera: So why is that important? Because standard quantum field theory has limitations there, but this stochastic description makes it possible to study those scenarios semiclassically.

Jocelyn: It matters because the stochastic inflation paradigm offers a way to provide a nonperturbative model of the dynamics when perturbativity breaks down, which is something you can’t always do with standard approaches in full spacetime.

Subrahmanyan: This work expands that formalism beyond just standard models by showing how it applies to scalar-tensor theories and other complex gravity theories.

Vera: So it means we can now look at a wider range of cosmological scenarios, not just the simplest ones, using this stochastic tool.

Conclusion: Jocelyn: So thinking about the title "Stochastic scalar-tensor inflation and beyond" and the authors Yoann Launay and his team, what’s the big picture takeaway here?

Subrahmanyan: The implication is that this method fundamentally has no obstacle to generalize to any theory as long as they admit a classical set of scales and a perturbative one, whether it is classical or quantum.

Vera: So essentially, the method isn't stuck on GR; it’s robust enough to handle different kinds of gravity theories.

Jocelyn: It means this technique could be used for studying late-time dark energy phases where modes start superHubble and end up subHubble.

Subrahmanyan: That opens up avenues for studying the phenomenological implications of these models in a much broader context than just the very beginning of inflation.

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