Nordita Astrophysics Seminars

Non-classicality of scalar field dark matter

Name: Non-classicality of scalar field dark matter
Start: 2021-09-01T13:30:00+02:00
End: 2021-09-01T14:30:00+02:00
Location: https://stockholmuniversity.zoom.us/j/530682073

by Dr Michael Kopp (Nordita)

Wednesday 1 Sept 2021, 13:30 → 14:30 Europe/Stockholm

https://stockholmuniversity.zoom.us/j/530682073

Description

The axion-like particle (ALP) is a class of dark matter candidates where single particle states are highly occupied. The usual approach to model the non-relativistic ALP dynamics relevant to cosmic large scale structure formation is to treat it as a single classical field, in contrast to cold dark matter (CDM) which is treated as a collection of classical particles. Astrophysical “smoking gun” signatures of ALPs that set it apart from CDM are due to this difference.

Previous literature on possible effects beyond the classical field description of ALPs is sparse and controversial, with some having found that quantum effects are essential for an accurate description of large scale structure, while others concluded that quantum effects will arise only at times exceeding the age of the universe. These discrepancies are to some extent due to the application of different benchmarks for ``quantumness,'' or ``non-classicality.''

Here we introduce a quantum benchmark that is novel for ALPs, based on gravitationally induced self-squeezing. Self-squeezing, or Kerr-squeezing, is a process well known in the context of quantum optics and Bose-Einstein condensates.

We show using a simple yet conservative model based on the validity of the classical Schrödinger-Poisson equation (a type of Gross-Pitaevskii equation) that for a typical QCD axion, squeezing can become as large as r=36 on very short time scales of order few thousand years, with the onset of squeezing r=1 reached within 0.1 milliseconds. According to this benchmark any viable cosmological ALP, such the QCD axion or ultralight ``fuzzy dark matter’', is non-classical. This indicates that a deeper understanding of the quantum-classical transition of the ALP is required. All our conclusions were based on enforcing the validity of the Gross-Pitaevskii equation, so that exploring the observability of squeezing in future work is tantamount to scrutinizing the validity of the classical field description in ALP cosmology.

Preprint: https://arxiv.org/abs/2105.13451