Perturbations in string cosmology from double field theory

Stephen Angus; Shinji Mukohyama

doi:10.1140/epjc/s10052-025-13841-7

2023 Impact factor 4.2

Particles and Fields

Open Access

Eur. Phys. J. C (2025) 85: 173
https://doi.org/10.1140/epjc/s10052-025-13841-7

Regular Article - Theoretical Physics

Perturbations in $O (D, D)$ $\textbf{O}(D,D)$ string cosmology from double field theory

Stephen Angus¹^a and Shinji Mukohyama²^,3

¹ Center for Quantum Spacetime, Sogang University, 35 Baekbeom-ro, Mapo-gu, 04107, Seoul, Republic of Korea
² Center for Gravitational Physics and Quantum Information, Yukawa Institute for Theoretical Physics, Kyoto University, 606-8502, Kyoto, Japan
³ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, 277-8583, Kashiwa, Chiba, Japan

^a sangus@sogang.ac.kr

Received: 21 November 2024
Accepted: 19 January 2025
Published online: 11 February 2025

Abstract

The low-energy limit of string theory contains additional gravitational degrees of freedom, a skew-symmetric tensor B-field and a scalar dilaton, that are not present in general relativity. Together with the metric, these three fields are naturally embedded in the $O (D, D)$ $\textbf{O}(D,D)$ -symmetric framework of double field theory. The $O (D, D)$ $\textbf{O}(D,D)$ symmetry uniquely prescribes the interactions between the extended gravitational sector and other matter, leading to novel features beyond conventional string cosmology. In this work we present the equations of motion for linear perturbations around $O (D, D)$ $\textbf{O}(D,D)$ string cosmological backgrounds in $D = 4$ $$D=4$$ under a scalar-vector-tensor decomposition. We obtain analytic solutions in the superhorizon limit for scalar perturbations around various homogeneous and isotropic background solutions, including some candidate models for bouncing cosmologies. The generalized energy–momentum tensor includes source terms for the B-field and dilaton, and we show how the resulting generalized conservation laws modify the conditions for conservation of curvature perturbations.

© The Author(s) 2025

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