Ultra-Light Dark Matter (2005.03254v2)

Abstract: Ultra-light dark matter (ULDM) is a class of dark matter models (DM) where DM is composed by bosons with masses ranging from $10^{-24}\, \mathrm{eV} < m < \mathrm{eV}$. These models have been receiving a lot of attention in the past few years given their interesting property of forming a Bose-Einstein condensate (BEC) or a superfluid on galactic scales. BEC and superfluidity are one of the most striking quantum mechanical phenomena manifest on macroscopic scales, and upon condensation, the particles behave as a single coherent state, described by the wavefunction of the condensate. The idea is that condensation takes place inside galaxies while outside DM behaves like a normal cold particle DM. This wave nature of DM on galactic scales that arise upon condensation can address some of the curiosities of the behaviour of DM on small scales while maintaining the successes of LCDM on large scales. There are many models in the literature that describe a DM component that condenses in galaxies. In this review, we are going to describe those models and classify them according to the different ways they achieve condensation. For that, we review the phenomena of BEC and superfluidity, and apply this knowledge to the DM in order to explain their construction and phenomenology. We describe the small scale challenges these models aim to solve and how ULDM alleviates them. These models present a rich phenomenology that is manifest in different astrophysical consequences. We review here the astrophysical and cosmological tests used to constrain those models, together with new and future observations that promise to test these models in different regimes. We finalize by showing some predictions that are a consequence of the wave nature of this component, like vortices and interference, that could represent a smoking gun in the search of these rich and interesting alternative class of DM. (Abridged)

• The paper presents an extensive review of ULDM models addressing small-scale structure formation by categorizing them into FDM, SIFDM, and DM superfluid frameworks.
• It employs observational tests such as galaxy rotation curves and gravitational lensing to constrain key parameters including the ~10⁻²² eV mass scale.
• The analysis underscores the potential of ULDM to bridge quantum mechanics with cosmological behavior, offering a viable alternative to traditional CDM models.

An Overview of "Ultra-Light Dark Matter"

The paper by Elisa G. M. Ferreira offers an extensive review of ultra-light dark matter (ULDM) models, focusing on the potential of these models to address the shortcomings of the cold dark matter (CDM) paradigm on small scales while maintaining its successes on large scales. The work comprehensively categorizes ULDM models into three main classes based on their distinct phenomenologies on small scales: Fuzzy Dark Matter (FDM), Self-Interacting Fuzzy Dark Matter (SIFDM), and Dark Matter (DM) as a Superfluid. Each category is analyzed for its dynamics, structure formation in galactic halos, and cosmological consequences, offering insightful perspectives into the applicability and challenges of ULDM models in modern cosmology.

Classification and Dynamics of ULDM Models

The paper begins by classifying ULDM models into three distinct categories:

1. Fuzzy Dark Matter (FDM): This class involves a scalar field subjected solely to gravity, forming a Bose-Einstein condensate (BEC) on galactic scales. The wave-like nature of FDM is intended to address issues such as the cusp-core problem by predicting cored density profiles within halos due to quantum pressure effects.
2. Self-Interacting Fuzzy Dark Matter (SIFDM): This model includes weak self-interactions among the dark matter particles, leading to the formation of a superfluid in galaxies. The self-interaction introduces additional phenomenology, such as modified structure formation dynamics and potential resolution to small-scale challenges.
3. DM Superfluid: This innovative model suggests that DM forms a superfluid within galaxies, leading to emergent phenomena like MONDian behavior. By utilizing effective field theory (EFT) of superfluids, the model reproduces the MOND phenomenon on small scales, while still acting as CDM on large scales.

Implications for Structure Formation and Cosmology

The detailed exploration of these models shows that ULDM can offer alternative explanations for the distribution of dark matter on small scales, especially in addressing the "cusp-core" problem, the paucity of satellite galaxies (known as the "missing satellites problem"), and the "too big to fail" problem. The quantum pressure inherent to ULDM can naturally lead to cored density profiles in galaxies, in contrast to the cuspy profiles predicted by CDM with NFW halos.

Numerical Results and Observational Constraints

The synthesis of astrophysical tests within the paper highlights the constraints these models must satisfy to remain viable:

• For the FDM model, observational data on galaxy rotation curves and gravitational lensing are key tests. The paper notes, however, that the mass range where interesting small-scale effects are predicted ($\sim 10^{-22} \, \mathrm{eV}$) is becoming increasingly constrained by current observations.
• The SIFDM model, despite its potential to address small-scale discrepancies with additional self-interaction, remains weakly constrained due to the complexity added by self-interactions.
• For the DM Superfluid framework, the ability to match the predicted MOND-like dynamics with observed galactic rotation curves is critical for its validation.

Theoretical and Practical Implications

Theoretical implications of ULDM suggest a paradigm shift in how dark matter might be understood, given the influence of quantum mechanics on macroscopic astrophysical phenomena. Practically, as observational techniques advance, particularly CMB, LSS, and future surveys, these novel dark matter models face significant predictive challenges that could either consolidate or refute their standing as credible alternatives to CDM.

Future Developments and Speculations

As for future developments, ULDM offers a flexible platform for integrating quantum phenomena with astrophysical observations. The nuanced predictions of coherence and interference patterns, formation of vortices, and the potentially vast phenomenology that emerges in combination with baryonic processes, provide numerous avenues for both theoretical and observational advancements.

In conclusion, the paper by Ferreira underlines the fascinating potential of ULDM models as promising candidates for solving long-standing puzzles in galaxy formation and the nature of dark matter. It highlights the delicate balance between preserving large-scale cosmological successes while innovating on small scales. Continued development and testing of these models, underpinned by rigorous theoretical and observational scrutiny, remain crucial as the field evolves toward a deeper understanding of dark matter.