Dark Black Holes in the Mass Gap

TL;DR

This paper explores the possibility that dark sector stars, composed of dark electrons and dark protons, could collapse into black holes within the observed mass gap, challenging standard stellar evolution models.

Contribution

It introduces a dark sector model with specific cooling processes that can produce black holes in the mass gap, supported by detailed numerical analysis.

Findings

Dark sector black holes can populate the mass gap under certain particle mass conditions.

Heavier dark sector particles cannot be significantly lighter than visible protons for mass gap black holes to form.

Future merger data can test the dark sector black hole hypothesis.

Abstract

In the standard picture of stellar evolution, pair-instability -- the energy loss in stellar cores due to electron-positron pair production -- is predicted to prevent the collapse of massive stars into black holes with mass in the range between approximately 50 and 130 solar masses -- a range known as the "{\em black hole mass gap}". LIGO detection of black hole binary mergers containing one or both black holes with masses in this {\em mass gap} thus challenges the standard picture, possibly pointing to an unexpected merger history, unanticipated or poorly understood astrophysical mechanisms, or new physics. Here, we entertain the possibility that a "dark sector" exists, consisting of dark electrons, dark protons, and electromagnetic-like interactions, but no nuclear forces. Dark stars would inevitably form given such dark sector constituents, possibly collapsing into black holes with masses within the mass gap. We study in detail the cooling processes necessary for successful stellar collapse in the dark sector and show that for suitable choices of the particle masses, we indeed predict populating the mass gap with dark sector black holes. In particular, we numerically find that the heavier of the two dark sector massive particles cannot be lighter than, approximately, the visible sector proton for the resulting dark sector black holes to have masses within the mass gap. We discuss constraints on this scenario and how to test it with future, larger black hole merger statistics.