Beyond the Standard Model Explanations of GW190521

The LIGO-Virgo Collaboration has recently announced the detection of a heavy binary black hole merger, with component masses that are difficult to account for in standard stellar structure theory. In this Letter, we propose several explanations based on models of new physics, including new light particle losses, modified gravity, large extra dimensions, and a small magnetic moment of the neutrino. Each of these affect the physics of the pair instability differently, leading to novel mechanisms for forming black holes inside the mass gap.

Figure 1

Beyond the standard model explanations of GW190521—the lower edge of the black hole mass gap $M_{\mathrm{BHMG}}$ in various theories of new physics, as well as the upper edge in the case of modified gravity. The ticks indicate values of model parameters shown on the left. In cyan, we show the GW190521 primary black hole mass $m_1=85_{-14}^{+21}{M}_{\odot }$ and, in pink, the secondary mass $m_2=66_{-18}^{+17}{M}_{\odot }$. The dashed line gives the standard model prediction for the lower edge of the black hole mass gap $M_{\mathrm{BHMG}}^{(\mathrm{SM})}$ from stellar simulations including pulsations; see text for details. Some of the model parameters are constrained by or in tension with other probes. In most models explored here, additional model freedom (such as that given by the particle mass) allows one to reconcile these disparate probes, although this is not possible in the case of the $\nu$ magnetic dipole moment, for example; see text for details.