Not-quite-primordial black holes seeded by cosmic string loops

Bryce Cyr

First listed 2025-07-23 | Last updated 2025-07-23

Abstract

Cosmic strings appear in many well-motivated extensions to the standard model of particle physics. If they exist, an abundant population of compact objects known as cosmic string loops permeate the Universe at all times, providing a secondary source of density perturbations that are large amplitude and non-gaussian in nature. In general, these loops are not stationary in the rest frame of the dark matter, thus their relative velocities will typically seed both spherical and filamentary overdensities in the matter era. Building upon previous work, we provide an improved framework to compute the complete halo mass function for these string seeded overdensities, valid for any loop velocity distribution. Using this mass function, we also compute the subset of halos capable of undergoing a direct collapse, forming a population of black holes with initial mass $10^{4-5} \, M_{\odot}$ at high redshifts. Interestingly, for reasonable values of the string parameters, one can reproduce the abundance of ``Little Red Dots" as inferred by JWST.

Short digest

This work builds an improved, unified framework for the halo mass function seeded by cosmic string loops, treating arbitrary loop-velocity distributions and capturing both spherical and filamentary growth. From this mass function, it isolates halos that meet direct-collapse conditions at high redshift, producing black-hole seeds of ~10^4–10^5 Msun. For reasonable string parameters, the predicted seed abundance reproduces the observed number density of JWST’s Little Red Dots, offering a non-Gaussian small-scale route to early SMBH growth. Because loop formation lacks large acoustic waves, the scenario avoids CMB spectral-distortion bounds, and a velocity–tension phase diagram (including fragmentation into “beads”) clarifies where this channel dominates.

Key figures to inspect

• Fig. 1: Use the three-phase schematic of stationary accretion (Regions I/II vs III) to see when the point-mass approximation is trustworthy and how the growth mode changes with time.
• Fig. 2: Read the contours of the redshift where the point-mass limit holds versus loop-formation redshift and Gμ; compare to the loop-decay track to pinpoint regimes where stationary accretion fails and moving/filamentary treatment is required.
• Fig. 3: Compare the growth histories across increasing loop velocities to watch the transition from spherical accretion to a single long filament, and finally to filament fragmentation into ‘beads’; note which channel actually governs the final halo mass relevant for direct collapse.
• Fig. 4: Examine the (Gμ, v) phase map showing stationary → filamentary → bead-fragmentation regimes; combined with the assumed velocity distribution (Eq. 22), it highlights that most loops fragment and identifies the parameter space that maximizes heavy-seed and LRD yields.