Astronomy:Stupendously large black hole

Short description: Hypothetical type of extremely massive black hole exceeding 100 billion solar masses

A stupendously large black hole (also abbreviated as SLAB) is a hypothetical type of extremely large supermassive black hole, with a mass in excess of a hundred billion or even a trillion of times the mass of the Sun (M_☉). They are larger than ultramassive black holes (UMBHs) as observed in quasars and massive galaxies, which are currently the most massive confirmed black holes with between 5 billion M_☉ and up to approximately 50 billion M_☉, which represents the upper natural mass limit for at least nonspinning black holes that feature an accretion disk. As such, they could have been seeded by primordial black holes (PBHs), which have formed soon after the Big Bang in the early universe.

Some such candidates are the central black holes of IC 1101 and the Phoenix Cluster, which are as well both among the most massive black holes with 40–100 billion and at least 100 billion M_☉.

Description

Whilst there is not currently direct evidence for the existence of stupendously large black holes, it is conceivable that SLABs exist given the existence for ultramassive black holes such as TON 618 of up to nearly a hundred billion M_☉ in galactic nuclei, and as well as quasars in the early universe at high redshifts, albeit the idea of SLABs has largely been neglected with no published constraints on a SLAB population until early 2021.

Stupendously large black holes are classically defined as black holes with a mass on order of 100 billion M_☉ or even possibly beyond 1 trillion M_☉ depending on models. This is larger than the absolute upper limit for the most massive black holes. Such black holes may even have been seeded by primordial black holes, which has been long been stated that they could have formed in the early radiation-dominated Universe, and as primordial black holes did not form from a stellar gravitational collapse, they could have a wide range of initial masses, including very small (e.g around 10⁻⁸ kg, also known as the planck mass) and stupendously large ones. Some of these might also reside outside galaxies in intergalactic space and must have formed independently of galaxies with interesting observational consequences, and likely have primordial origin as well. However, it has been noted that primordial black holes are not usually expected to be as large as SLABs with some arguments stating they might not exceed 10⁵ M_☉, but in principle their mass could be anything up to the horizon mass at the time of matter-radiation equality, which is of the order of 10¹⁷ M_☉. A scenario in which the PBH mass function extends up to the SLAB range has been once suggested. Any of these primordial SLABs could have been formed independently of galaxies and located in intergalactic space. Dynamical arguments suggest that the maximum possible mass for a SLAB is 1×10¹⁸ M_☉. Not only a dozen times the binding mass of the Laniakea Supercluster, but also corresponds to a Schwarzschild diameter (thus if nonrotating black hole) of at least 620,000 light-years (190 kiloparsecs), as large as the largest modern galaxies in the universe.

Although ultramassive black holes within quasars and galactic nuclei cannot grow beyond around (5–27)×10¹⁰ M_☉ through the accretion disk and as well given the current age of the universe, some of these monster black holes in the universe are predicted to still continue to grow up to perhaps 10¹⁴ M_☉ during the collapse of superclusters of galaxies in the extremely far future of the universe. Even these would evaporate over a timescale of up to 10¹¹⁶ years via Hawking radiation. By comparison, a stupendously large black hole with a mass of 10¹¹ M_☉ will evaporate in around 3.66×10¹⁰⁷ years.

Formation

There is two possible classes of model regarding the origin of stupendously large black holes. Maybe the most natural scenario, stupendously large black holes may have been formed within galactic nuclei, despite reasons to believe that the mass function of black holes cannot extend beyond 10¹¹ M_☉ as mentioned above. As such, this value was taken to be the lower mass limit for a SLAB.

As mentioned above although it is more speculative and depends on the possible existence of primordial black holes, the SLABs may have been formed primordially within the first few minutes after the Big Bang, in spite of it does not usually envisage that PBHs could be necessarily in the SLAB range, to which several studies stated that PBHs cannot have initial mass higher than ∼10⁵ M_☉. Nonetheless, there are several scenarios of producing PBHs even larger, with very massive ones that will inevitably be able to grow enormously through merging and accretion, so one must distinguish between the initial and final PBH mass. It was also stated that primordial black holes could be good candidates for being the seeds of the supermassive black holes at the center of supergiant galaxies, so the distinction between the two classes of explanation is not clear-cut. In that case, this might raise an interesting connection to the suggestion that primordial black holes can be provide the dark matter although SLABs are themselves clearly unlikely to do this, as due too their size and mass too large to reside in galactic halos, while primordial black holes can provide the dark matter in a much lower mass. Even so, if SLABs does exist, this could have important implications for the early universe and would make it plausible that lighter primordial black holes could do this.

See also

• Direct collapse black hole
• Quasi-star
• Dark star (dark matter)

Further reading

• Chakraborty, Amlan; Chanda, Prolay K.; Pandey, Kanhaiya Lal; Das, Subinoy (2022). "Formation and Abundance of Late-forming Primordial Black Holes as Dark Matter". The Astrophysical Journal 932 (2): 119. doi:10.3847/1538-4357/ac6ddd. Bibcode: 2022ApJ...932..119C. 
• Carr, Bernard; Kühnel, Florian (2022). "Primordial black holes as dark matter candidates". SciPost Physics Lecture Notes. doi:10.21468/SciPostPhysLectNotes.48. 

External links

• September 2020, Paul Sutter 29 (29 September 2020). "Black holes so big we don't know how they form could be hiding in the universe" (in en). https://www.space.com/ginormous-black-holes-could-lurk-in-universe.html.