Quantum and Classical Perspectives on Dark Matter and Gravity From Axion Dark Matter Squeezing to Novel Tests of Gravity

by Vasileios Fragkos (Stockholm University)

Friday 21 Mar 2025, 13:00 → 16:00 Europe/Stockholm

FP41 (AlbaNova House 1)

Abstract
This thesis in theoretical physics explores fundamental aspects of gravity and quantum mechanics, focusing on how
quantum effects arise in gravitational and cosmological contexts. Specifically, we investigate how novel quantum effects
in axion dark matter models—a promising dark matter candidate with numerous ongoing experimental detection efforts—
manifest and critically examine the implications of gravity-induced entanglement proposals as potential experimental tests
of quantum gravity. Additionally, we propose new approaches to testing fundamental principles of gravity, particularly
in identifying and constraining possible discrepancies between how gravitational fields are sourced and probed, both in
the classical and quantum domains. By drawing on insights from quantum optics and cosmology while leveraging recent
experimental advancements, this thesis challenges existing paradigms and proposes new methods for exploring the interplay
between gravity and quantum mechanics.
In Paper I, we focus on squeezing of axion dark matter, a quantum effect that accompanies the standard description of
axions. The commonly used mean-field treatment obscures potential quantum signatures in the system. We show that under
standard assumptions, the quantum state of axions is highly squeezed. This suggests that the mean-field description of
axion dark matter is incomplete, paving the way for studies beyond this approximation. Moreover, this thesis explores the
decoherence of axion dark matter, assessing whether squeezing remains robust in the presence of decoherence—an essential
step toward experimental probing such an effect. We demonstrate that squeezing persists under generic environmental
interactions, indicating that quantum effects may be expected for dark matter. Our results stem from an interdisciplinary
approach bridging cosmology, quantum optics, quantum open systems, and cold atoms.
In Paper II, we explore the quantum aspects of gravity. For nearly a century, reconciling gravity with quantum mechanics
has been a challenge, hindered by the lack of experimental evidence. Recent advances in quantum control of large systems
have renewed interest in testing this through tabletop experiments. An influential proposal aims to test whether gravity
mediates entanglement between two spatially superposed mesoscopic masses, using a quantum-information-theoretic
argument based on LOCC (Local Operations and Classical Communication) to infer quantized gravitational mediators.
However, there has been a heated debate about what conclusions can be drawn from such an experiment. We critically assess
this proposal, its assumptions, and its implications for quantum gravity. We conclude that the LOCC argument is insufficient
to unambiguously infer quantum mediators unless locality is elevated to a fundamental principle of nature. We support this
claim by showing that well-known relativistic field theories, beyond their local formulations, admit equally viable nonlocal
ones. Thus, the entanglement-generating quantum channel can be local or non-local, even within relativity. We also
highlight that cosmic microwave background observations already provide evidence for the quantization of the Newtonian
potential. Nevertheless, the experiment probes a new, untested regime—how quantum delocalized source masses gravitate.
Paper III builds on the conclusions of Paper II, specifically examining how delocalized masses source gravity. We
investigate whether it is sensible, within established theories, to distinguish between sourcing and probing gravity. To
explore this, we first analyze classical Newtonian gravity, revisiting studies on violations of active and passive gravitational
mass. We introduce novel parameters capturing a broader range of possible violations, including quantum mechanical ones.
Using existing experimental setups, such as Cavendish and free-fall experiments, we propose new methods to constrain
these parameters. Our constraints could match or surpass those from prior experiments. This work opens a new avenue
for testing the fundamental principle of the equality of active and passive gravitational masses—an area that has received
little attention. Additionally, we extend beyond classical tests, proposing novel quantum experiments. This stems from the
intriguing possibility that, while no distinction exists between active and passive gravitational mass in classical physics,
such violations could emerge, through mass-energy equivalence, in the deep quantum gravity domain.