[Paper Review] A Firewall Argument from the Swampland

Himanshu Chaudhary, Chethan Krishnan|arXiv (Cornell University)|Mar 11, 2020

Cosmology and Gravitation Theories1 citations

TL;DR

This paper investigates whether super-Planckian scalar field ranges can emerge during gravitational collapse, using spherically symmetric, massless scalar field models inspired by Choptuik's work. It finds that while such large field movements occur generically in supercritical collapses, they are always hidden behind apparent horizons, suggesting a firewall-like mechanism that protects Effective Field Theory from breakdown by confining large field excursions within horizons.

ABSTRACT

A classical solution where the (scalar) field value moves by an ${\cal O}(1)$ range in Planck units is believed to signal the breakdown of Effective Field Theory (EFT). One heuristic argument for this is that such a field will have enough energy to be inside its own Schwarzschild radius, and will result in collapse. In this paper, we consider an inverse problem: what kind of field ranges arise during the gravitational collapse of a classical field? Despite the fact that collapse has been studied for almost a hundred years, most of the discussion is phrased in terms of fluid stress tensors, and not fields. An exception is the scalar collapse made famous by Choptuik. We re-consider Choptuik-like systems, but with the emphasis now on the evolution of the scalar. We give strong evidence that generic spherically symmetric collapse of a massless scalar field leads to super-Planckian field movement. But we also note that in every such supercritical collapse scenario, the large field range is hidden behind an apparent horizon. We also discuss how the familiar perfect fluid models for collapse like Oppenheimer-Snyder and Vaidya should be viewed in light of our results.

Motivation & Objective

• To investigate whether super-Planckian scalar field ranges can arise during gravitational collapse, challenging the assumption that such ranges always signal EFT breakdown.
• To re-analyze classical collapse scenarios—particularly Choptuik-like systems—by focusing on scalar field evolution rather than fluid approximations.
• To clarify the role of apparent horizons in shielding large field ranges from external observers, thereby preserving EFT validity in observable physics.
• To reinterpret standard fluid-based collapse models like Oppenheimer-Snyder and Vaidya in light of field-theoretic collapse dynamics.

Proposed method

• Analyzes spherically symmetric, massless scalar field collapse using numerical relativity techniques, focusing on the scalar field's evolution rather than fluid stress-energy tensors.
• Applies the framework of Choptuik's critical collapse to scalar fields, identifying the threshold between dispersion and black hole formation.
• Tracks the scalar field's range in Planck units during collapse, particularly in supercritical regimes where horizon formation occurs.
• Identifies the formation of apparent horizons as a key feature that isolates regions of super-Planckian field movement from external observers.
• Compares field-based collapse dynamics with standard fluid models (e.g., Oppenheimer-Snyder, Vaidya) to assess their consistency with field-theoretic behavior.
• Uses geometric and dynamical criteria to determine when field ranges exceed O(1) in Planck units and whether such ranges are causally accessible.

Experimental results

Research questions

• RQ1 Can super-Planckian scalar field ranges emerge during gravitational collapse of a classical scalar field?
• RQ2 Under what conditions does the scalar field exceed O(1) in Planck units during collapse, and is this behavior generic?
• RQ3 Are such large field ranges observable, or are they hidden behind apparent horizons in supercritical collapse?
• RQ4 How do standard fluid-based collapse models (e.g., Oppenheimer-Snyder) compare to field-theoretic collapse in terms of field range evolution?
• RQ5 What is the role of apparent horizons in shielding large field excursions from external observers, and how does this relate to the swampland program?

Key findings

• Generic spherically symmetric collapse of a massless scalar field leads to super-Planckian field movement in the supercritical regime.
• In every case of super-Planckian field range, the large field excursion is hidden behind an apparent horizon, making it causally isolated from external observers.
• The formation of an apparent horizon coincides with the onset of large field movement, suggesting a natural firewall-like mechanism.
• Standard fluid models like Oppenheimer-Snyder and Vaidya, when reinterpreted in field-theoretic terms, fail to capture the full dynamics of scalar field evolution during collapse.
• The results support the idea that the swampland program's prohibition on super-Planckian field ranges may be evaded if such ranges are always hidden behind horizons.
• The paper provides a field-theoretic justification for why EFT may remain valid despite apparent field ranges that would otherwise signal its breakdown.