Non-Relativistic Formation of Scalar Clumps as a Candidate for Dark Matter

Abstract

We propose a new mechanism for the formation of dark matter clumps in the radiation era. We assume that a light scalar field is decoupled from matter and oscillates harmonically around its vacuum expectation value. We include self-interactions and consider the nonrelativistic regime. The scalar dynamics are described by a fluid approach where the fluid pressure depends on both quantum and self-interaction effects. When the squared speed of sound of the scalar fluid becomes negative, an instability arises and the fluctuations of the scalar energy-density field start growing. They eventually become nonlinear and clumps form. Subsequently, the clumps aggregate and reach a universal regime. Afterwards, they play the role of cold dark matter. We apply this mechanism first to a model with a negative quartic term stabilised by a positive self-interaction of order six, and then to axion monodromy, where a subdominant cosine potential corrects a mass term. In the first case, the squared speed of sound becomes negative when the quartic term dominates, leading to a tachyonic instability. For axion monodromy, the instability starts very slowly after the squared speed of sound first becomes negative and then oscillates around zero. Initially the density perturbations perform acoustic oscillations due to the quantum pressure. Eventually, they start growing exponentially due to a parametric resonance. In both scenarios, the scalar-field clumps span a wide range of scales and masses, running from the size of atoms to that of galactic molecular clouds, and from $10^{-3}\mathrm{gram}$ to thousands of solar masses. Because of finite-size effects, both from the source and the lens, these dark matter clumps are far beyond the reach of microlensing observations.

40 pages, 19 figures, published version

References in corpus (16)

• Ultralight scalars as cosmological dark matter
• Too big to fail? The puzzling darkness of massive Milky Way subhaloes
• Cosmic Structure as the Quantum Interference of a Coherent Dark Wave
• Limits on the Macho Content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds
• A search for ultra-light axions using precision cosmological data
• The Powers of Monodromy
• Dilute and dense axion stars
• Inflaton Fragmentation and Oscillon Formation in Three Dimensions
• Axion as a Cold Dark Matter candidate
• Dynamical and Gravitational Instability of Oscillating-Field Dark Energy and Dark Matter
• Classical Decay Rates of Oscillons
• Merger of Dark Matter Axion Clumps and Resonant Photon Emission
• Collisional interactions between self-interacting non-relativistic boson stars: effective potential analysis and numerical simulations
• Perturbations of ultralight vector field dark matter
• Efficient self-resonance instability from axions
• K-essence scalar dark matter solitons around supermassive black holes

Cited by in corpus (10)

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• Beyond Schrödinger-Poisson: Nonrelativistic Effective Field Theory for Scalar Dark Matter
• Gravitational waves from binary black holes in a self-interacting scalar dark matter cloud
• Enhanced Early Galaxy Formation in JWST from Axion Dark Matter?
• Detecting dark matter oscillations with gravitational waveforms
• The Invisible Dilaton
• Cosmological models based on a complex scalar field with a power-law potential associated with a polytropic equation of state
• Dark Spin-2 Field Solitons as a Source of Electromagnetic Radiation
• Formation of solitons and their transitions in scalar-field dark matter models with a non-polynomial self-interaction potential