The effect of quantum decoherence on inflationary gravitational waves

• Main Authors: de Kruijf, Jessie, Bartolo, Nicola
• Format: Journal Article
• Language: English
• Published: 05-08-2024
• Subjects: Physics - Cosmology and Nongalactic Astrophysics; Physics - General Relativity and Quantum Cosmology; Physics - High Energy Physics - Phenomenology; Physics - High Energy Physics - Theory; Physics - Quantum Physics
• DOI: 10.48550/arxiv.2408.02563

The theory of inflation provides a mechanism to explain the structures we
observe today in the Universe, starting from quantum-mechanically generated
fluctuations. However, this leaves the question of: how did the
quantum-to-classical transition, occur? During inflation, tensor perturbations
interact (at least gravitationally) with other fields, meaning that we need to
view these perturbations as an open system that interacts with an environment.
In this paper, the evolution of the system is described using a Lindblad
equation, which describes the quantum decoherence of the system. This is a
possible mechanism for explaining the quantum-to-classical transition. We show
that this quantum decoherence leads to a scale-dependent increase of the
gravitational wave power spectrum, depending on the strength and time
dependence of the interaction between the system and the environment. By using
current upper bounds on the gravitational wave power spectrum from inflation,
obtained from CMB and the LIGO-Virgo-KAGRA constraints, we find an upper bound
on the interaction strength. Furthermore, we compute the decoherence criterion,
which indicates the minimal interaction strength needed for a specific scale to
have successfully decohered by the end of inflation. Assuming that the CMB
modes have completely decohered, we indicate a lower bound on the interaction
strength. In addition, this decoherence criterion allows us to look at which
scales might not have fully decohered and could still show some relic quantum
signatures. Lastly, we use sensitivity forecasts to study how future
gravitational-wave detectors, such as LISA and ET, could constrain the
decoherence parameter space. Due to the scale-dependence of the power spectrum,
LISA could only have a very small impact. However, ET will be able to
significantly improve our current constraints for specific decoherence
scenarios.