Revisiting constraints on asteroid-mass primordial black holes as dark matter candidates (1906.05950v2)

Abstract: As the only dark matter candidate that does not invoke a new particle that survives to the present day, primordial black holes (PBHs) have drawn increasing attention recently. Up to now, various observations have strongly constrained most of the mass range for PBHs, leaving only small windows where PBHs could make up a substantial fraction of the dark matter. Here we revisit the PBH constraints for the asteroid-mass window, i.e., the mass range $3.5\times 10^{-17}M_\odot < m_{\mathrm{PBH}} < 4\times 10^{-12}M_\odot$. We revisit 3 categories of constraints. (1) For optical microlensing, we analyze the finite source size and diffractive effects and discuss the scaling relations between the event rate, $m_{\mathrm{PBH}}$ and the event duration. We argue that it will be difficult to push the existing optical microlensing constraints to much lower m$_{\mathrm{PBH}}$. (2) For dynamical capture of PBHs in stars, we derive a general result on the capture rate based on phase space arguments. We argue that survival of stars does not constrain PBHs, but that disruption of stars by captured PBHs should occur and that the asteroid-mass PBH hypothesis could be constrained if we can work out the observational signature of this process. (3) For destruction of white dwarfs by PBHs that pass through the white dwarf without getting gravitationally captured, but which produce a shock that ignites carbon fusion, we perform a 1+1D hydrodynamic simulation to explore the post-shock temperature and relevant timescales, and again we find this constraint to be ineffective. In summary, we find that the asteroid-mass window remains open for PBHs to account for all the dark matter.

• The paper updates constraints on asteroid-mass PBHs as dark matter by reassessing observational windows in a previously unexplored mass range.
• It refines methodologies in optical microlensing and dynamical capture by stars to address challenges in probing low-mass black holes.
• Hydrodynamic simulations of white dwarf interactions show that PBH-induced explosions are rare, supporting the potential of PBHs as dark matter candidates.

Constraints on Asteroid-Mass Primordial Black Holes as Dark Matter

The paper explores the potential of primordial black holes (PBHs) in the asteroid-mass range as viable dark matter candidates. The examination covers several observational and theoretical methods to constrain these PBHs, set within the mass window of $$3.5\times 10^{-17}M_\odot < < 4\times 10^{-12}M_\odot$$. This mass range escapes current observational constraints, rendering it an open window for PBHs to account for dark matter.

The research revisits three primary constraints:

1. Optical Microlensing: The investigation into optical microlensing accounts for finite source size and diffraction effects, which have historically restricted the capability to probe low-mass PBHs. The derived scaling relationships demonstrate how intricacies, such as event rate dependence on mass and duration, make further exploration into lower masses challenging. Consequently, advancing current optical microlensing constraints to lower masses with present technology proves formidable.
2. Dynamical Capture by Stars: The paper develops a comprehensive framework to calculate the PBH capture rate in stars based on phase space dynamics. While previous assumptions held that survival of stars amidst PBH disruption could provide constraints, the paper argues otherwise. Stars capturing PBHs, particularly low-mass ones, could go unobservable as these disruptions are rare. However, if PBHs were substantial contributors to dark matter, significant stellar disruptions would become noticeable over time. This presents a prospective pathway to impose constraints, albeit dependent on discerning signatures of such disruptions.
3. White Dwarfs and PBH-Induced Explosions: The authors employ hydrodynamic simulations to assess the potential for PBH-induced detonations in white dwarfs. The analysis reveals that under typical conditions, significant stellar disruptions due to PBHs are unlikely. White dwarfs have previously been considered susceptible to ignition through PBH passages, yet a combination of nuclear reaction times and subsequent instabilities make this eventuality rare. As a result, existing white dwarfs in observed quantities cannot exclude the possibility of PBHs in this mass range being dark matter.

The outlined findings redefine the constraints on asteroid-mass PBHs. With contemporary data and technology, this range remains a potential domain for PBHs to constitute the entirety of dark matter. The implications of the research are profound, urging the pursuit of newly formulated methodologies or observational techniques capable of resolving the asteroid-mass PBH dark matter hypothesis.

The paper posits that while established observational approaches cannot currently probe sufficiently low masses, future technological and methodological advancements could either substantiate this potential or necessitate further theoretical refinement. Recognizing the limitations of present-day techniques, this paper advocates new explorative avenues that could test the PBH dark matter hypothesis, particularly focusing on direct observational signatures or novel astrophysical constraints.