Shadows of Giants: Constraints on Stupendously Large Black Holes from Negative Sources against the Cosmic Microwave Background

TL;DR

This paper proposes a novel constraint on stupendously large black holes (SLABs) by analyzing their shadows against the cosmic microwave background, ruling out certain mass ranges and abundance levels.

Contribution

It introduces a new method to constrain SLABs using their negative microwave signatures and discusses implications for their growth and accretion effects.

Findings

SLABs with mass > 10^{17} M_sun} are ruled out within the last scattering surface.

The abundance of SLABs is constrained to 0^{-5} for masses between 10^{15} and 10^{18} M_sun.

Shadow detection becomes easier at redshifts above 1.6 due to angular diameter distance effects.

Abstract

Stupendously large astrophysical black holes (SLABs) are hypothetical black holes with masses of more than a trillion Suns. Because observable consequences of their existence have only recently been seriously considered, there have been relatively few constraints on their abundance. This work motivates a simple yet powerful constraint on SLABs: their huge shadows are visible against the cosmic microwave background. SLABs could thus appear as negative sources in microwave data. In fact, the shadow of a SLAB with a fixed mass becomes easier to detect with increasing redshifts past $1.6$ where the angular diameter distance starts falling. The limits are powerful enough to rule out SLABs of mass $\gtrsim 10^{17}\ M_{\odot}$ within the last scattering surface, and imply $\Omega_{BH} \lesssim 10^{-5}$ for masses $10^{15}$--$10^{18}\ M_{\odot}$. I also discuss the effects of accretion and their implications for the limits: SLAB growth, positive accretion luminosity, and obscuring material.