Year: 2025
Type: article
Abstract: The quantum nature of gravity remains an open question in fundamental physics, lacking experimental verification. Gravitational waves (GWs) provide a potential avenue for detecting gravitons, the hypothetical quantum carriers of gravity. However, by analogy with quantum optics, distinguishing gravitons from classical GWs requires the preservation of quantum coherence, which may be lost due to interactions with the cosmic environment causing decoherence. We investigate whether GWs retain their quantum state by deriving the reduced density matrix and evaluating decoherence, using an environmental model where a scalar field is conformally coupled to gravity. Our results show that quantum decoherence of GWs is stronger at lower frequencies and higher reheating temperatures. We identify a model-independent amplitude threshold below which decoherence is negligible, providing a fundamental limit for directly probing the quantum nature of gravity. In the standard cosmological scenario, the low-energy density of the universe at the end of inflation leads to complete decoherence at the classical amplitude level of inflationary GWs. However, for higher-energy densities, decoherence is negligible within a frequency window in the range $100\text{ }\text{ }\mathrm{Hz}\ensuremath{-}{10}^{8}\text{ }\text{ }\mathrm{Hz}$, which depends on the reheating temperature. In a kinetic-dominated scenario, the dependence on reheating temperature weakens, allowing GWs to maintain quantum coherence above ${10}^{7}\text{ }\text{ }\mathrm{Hz}$.
Authors: Hiroki Takeda, Takahiro Tanaka
Institutions: Kyoto University, Yukawa Institute for Theoretical Physics
Language: English
Subfield: Statistical and Nonlinear Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Awards: G1146995298, Testing theories of gravity on cosmological distance scales by searching for polarization modes in the gravitational wave propagation, Long wavelength dynamics and the issue of Infrared divergence during inflation, G7243721012, Searching for new scalar particles with gravitational waves.
OpenAlex ID: W4410775488