Seminar – Dr. Michael Kopp – Nordita

Non-classicality of scalar field dark matter

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 ALP’s dynamics relevant to cosmic large scale structure formation is via a single classical field, rather than classical particles, as is the case for cold dark matter (CDM).  Astrophysical “smoking gun” signatures of ALPs that set it apart from CDM are based on this single classical field description. 
Previous literature on possible effects beyond that description 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 extend 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. Since we have obtained the large squeezing by enforcing the Gross-Pitaevskii equation, exploring the fate of squeezing is tantamount to scrutinizing the validity of the Gross-Pitaevskii equation in ALP cosmology.