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  • Open Access

Modular fluctuations in cosmology

Lars Aalsma1,2,* and Sang-Eon Bak2,†

  • *Contact author: laalsma@asu.edu
  • †Contact author: sbak2@asu.edu
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Phys. Rev. D 112, 026017 – Published 15 July, 2025

DOI: https://doi.org/10.1103/cpyr-w532

Abstract

Verlinde and Zurek (VZ) have proposed that quantum gravity fluctuations in causal diamonds lead to observable effects. In particular, they argued that in quantum gravity causal diamonds have an uncertainty in their size that scales as δL∼ℓpL, i.e. fluctuations are enhanced from the Planck scale. In this work, we explore the origin of the VZ scaling by studying scalar perturbations in inflationary cosmology. We reinterpret these cosmological perturbations as fluctuations of the modular Hamiltonian associated with a causal diamond, establishing a connection between modular fluctuations and observable primordial perturbations. Independent of assumptions about the modular Hamiltonian or the background spacetime, our results show that—whenever an (effective) spherically symmetric perturbation is quantized—it can lead to the VZ scaling. However, whether such a mode exists in the vacuum of any theory of quantum gravity remains an open question.

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Phys. Rev. D 112, 026017– Published 15 July, 2025
  • Received 8 April 2025
  • Accepted 19 June 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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