Theory of Dark Matter Superfluidity (1507.01019v2)

Abstract: We propose a novel theory of dark matter (DM) superfluidity that matches the successes of the LambdaCDM model on cosmological scales while simultaneously reproducing the MOdified Newtonian Dynamics (MOND) phenomenology on galactic scales. The DM and MOND components have a common origin, representing different phases of a single underlying substance. DM consists of axion-like particles with mass of order eV and strong self-interactions. The condensate has a polytropic equation of state P~rho^3 giving rise to a superfluid core within galaxies. Instead of behaving as individual collisionless particles, the DM superfluid is more aptly described as collective excitations. Superfluid phonons, in particular, are assumed to be governed by a MOND-like effective action and mediate a MONDian acceleration between baryonic matter particles. Our framework naturally distinguishes between galaxies (where MOND is successful) and galaxy clusters (where MOND is not): due to the higher velocity dispersion in clusters, and correspondingly higher temperature, the DM in clusters is either in a mixture of superfluid and normal phase, or fully in the normal phase. The rich and well-studied physics of superfluidity leads to a number of observational signatures: array of low-density vortices in galaxies, merger dynamics that depend on the infall velocity vs phonon sound speed; distinct mass peaks in bullet-like cluster mergers, corresponding to superfluid and normal components; interference patters in super-critical mergers. Remarkably, the superfluid phonon effective theory is strikingly similar to that of the unitary Fermi gas, which has attracted much excitement in the cold atom community in recent years. The critical temperature for DM superfluidity is of order mK, comparable to known cold atom Bose-Einstein condensates.

• The paper introduces a hypothesis where light axion-like particles form a superfluid in galaxies under conditions such as an eV mass, 0.1 cm²/g cross-section, and mK critical temperature.
• The paper derives a MOND-like force law through superfluid phonon excitations, naturally explaining the Baryonic Tully-Fisher Relation and galaxy rotation curves without altering gravity.
• The paper predicts distinct observational signatures, including vortex patterns and unique merger dynamics, highlighting differences in dark matter behavior between galaxies and clusters.

Overview of "Theory of Dark Matter Superfluidity"

The paper "Theory of Dark Matter Superfluidity" by Lasha Berezhiani and Justin Khoury presents a hypothesis to reconcile the successes of the $\Lambda$CDM model on cosmological scales with the MOND phenomenology observed on galactic scales. The authors propose that dark matter (DM), composed of light axion-like particles, can form a superfluid within galaxies. This superfluidity results in a polytropic equation of state, $P \sim \rho^3$, leading to unique galactic dynamics governed by collective excitations of the condensate, specifically superfluid phonons. These phonons mediate a MONDian acceleration between baryonic matter particles, exhibiting characteristics that are reminiscent of MOND without the need to explicitly modify gravitational laws.

Key Numerical Results and Theoretical Claims

1. Criteria for Superfluidity: The paper identifies the necessary conditions for DM to form a superfluid. The DM particle mass should be around the eV range, with a strong self-interaction cross-section of at least 0.1 cm$^2$/g. The critical temperature for superfluidity within galaxies is estimated to be of order mK, similar to cold atom gases.
2. Galactic Dynamics: Within galaxies, the DM behaves as a superfluid with phonons being the primary dynamic agents. The phonon effective action mirrors a MOND-like force law, linking the Baryonic Tully-Fisher Relation (BTFR) and galaxy rotation curves without necessitating CDM’s non-baryonic density profiles.
3. Cluster Dynamics: The model naturally differentiates between galaxies and galaxy clusters. In clusters, DM might exist in a mixed phase or a normal phase, depending on temperature and velocity dispersion, explaining why MOND is less successful in these contexts.
4. Observation and Implications: The authors predict unique observational signatures, such as vortex patterns in the DM superfluid, interference in cluster mergers, and differing dynamics in galaxy mergers based on the relative infall velocities compared to phonon sound speeds.

Implications and Future Perspectives

The theoretical framework presented has profound implications for our understanding of DM and the composition and dynamics of cosmic structures. By bridging the gap between $\Lambda$CDM and MOND, this model provides a unified perspective that accounts for both large-scale structure formation and the peculiarities of galaxy dynamics without modifying the laws of gravity.

However, a critical aspect of future work will involve the experimental validation of this hypothesis, particularly through astronomical observations and potential laboratory analogs provided by advances in cold-atom physics. Identifying exact cold atom systems that replicate the behaviors predicted in this paper could provide pivotal insights, potentially enabling laboratory simulations of galactic behavior.

Additionally, this theory prompts a reevaluation of cosmological simulations and DM detection experiments, given that DM in this model may reflect properties significantly different from those assumed in traditional CDM scenarios. Future research should aim to refine the theoretical constructs and reconcile them with observational data, while also exploring the multi-faceted implications of superfluid DM on cosmic evolution, structure formation, and fundamental physics.