Fuzzy Dark Matter Paradigm and Ultralight Axions

Dear Colleagues,

Cosmology requires a new unified theoretical framework to describe thermal radiation on high and low redshifts as well as structure on large and small distance scales. Such a framework is likely rooted in certain paradigm changes in fundamental physics. Some of our present CDM Cosmological Standard Model’s parameter values are at tension when confronted with low-redshift vs. high-redshift data. For example, observations/CDM simulations of the Cosmic Microwave Background including intensity, polarisation, as well as weak gravitational lensing effects (and cluster identification through the thermal Sunyaev–Zeldovich effect) and of BAO signatures in matter correlations have extracted a Hubble constant which deviates by more than five sigma from the value extracted by various CDM fits to low-redshift luminosity-distance redshift data for standard candles, based on independent calibrations of the cosmic distance ladder. Also, high-redshift CDM modelled parameter extractions from data typically produce baryon densities, which are significantly larger than those coming from local censuses, and redshifts for the onset of re-ionisation of the intergalactic medium are higher than those detected directly by the Gunn–Peterson trough in quasar spectra. At the same time, CDM simulations of structure formation and observations clash in characterising the dark-matter distributions in the central regions of low-surface-brightness dwarf galaxies, disagree in typical numbers of satellite galaxies per central galaxy, galaxy morphologies, and the onset of galaxy formation.

A wide range of proposals to mitigate these parameter tensions is put forward in the literature. Here, we focus on the Fuzzy Dark Matter (FDM) paradigm which posits that ultralight yet highly non-relativistic scalar bosons acquire, mostly under the influence of self-gravity, collective quantum states (isolated or condensed granules obeying Schrödinger–Poisson (SP) quantum dynamics). These reproduce the successes of CDM on large scales and match observed structure on small scales much better than massive Cold Dark Matter particles. If visible matter and radiation is subjected to Yang–Mills theory not only in describing their interactions, like in the Standard Model of Particle Physics, but also underlying their very (nonperturbative) emergence, then these ultralight bosons can productively be attributed to axial anomalies (axions) and must occur in various species. Finally, by cosmic depercolations of axion granules (discontinuous), changes in Dark Matter and Dark Energy densities take place.

For this Special Issue, we invite the submission of reports on SP simulation results, ranging from subgalactic to large scales, and astrophysical/cosmological data analysis in the framework of the FDM approach to Dark Matter and Dark Energy to strengthen the evidence for or to rule out this paradigm shift for the Dark Sector.

We look forward to receiving your valuable contributions.

Best wishes,

Dr. Ralf Hofmann
Guest Editor

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• fuzzy dark matter
• ultralight axions
• cold dark matter
• Schrödinger–Poisson simulations