There are supermassive black holes. There are ultrahuge black holes. How big can these strange objects get? Well, there could be something even bigger than ultramassive: astonishing large black holes, according to the latest research.
Such hypothetical black holes – larger than 100 billion times the mass of the sun – have been explored in a new paper calling them SLABs, an acronym that stands for “Stupendously LArge Black holeS”.
“We already know that black holes exist over a wide range of masses, with a supermassive black hole of 4 million solar masses at the center of our own galaxy,” explains astronomer Bernard Carr of Queen Mary University London.
“Although there is currently no evidence for the existence of SLABs, it is conceivable that they could exist and be located outside galaxies in intergalactic space, with interesting observational implications.”
Black holes only have a few somewhat broad mass categories. There are black holes of enormous mass; those are black holes that are around the mass of a star, up to about 100 times that of the Sun. The next category is medium mass black holes, and how big they get seems to depend on who you’re talking to. Some say 1,000 solar masses, some say 100,000, and others say 1 million; whatever the upper limit, these seem to be quite rare.
Supermassive black holes (SMBHs) are much, much larger, on the order of millions to billions of solar masses. These include the SMBH at the heart of the Milky Way, Sagittarius A *, with 4 million solar masses, and the most photogenic SMBH in the universe, M87 *, with 6.5 billion solar masses.
The thickest black holes we’ve detected are ultramassive, more than 10 billion (but less than 100 billion) solar masses. These include an absolute beast clocking 40 billion solar masses at the center of a galaxy called Holmberg 15A.
“But surprisingly, the idea of SLABs has been largely neglected so far,” Carr said.
“We have proposed options for how these SLABs could be formed, and hope that our work will motivate discussions within the community.”
The thing is, scientists aren’t quite sure how really big black holes form and grow. One possibility is that they form in their host galaxy and then get bigger and bigger by gobbling up a lot of stars and gas and dust, and colliding with other black holes when galaxies join.
This model has an upper limit of about 50 billion solar masses – that’s the limit at which the object’s miraculous mass requires an accretion disk so large that it would fragment under its own gravity. But there is also a big problem: supermassive black holes were found in the early Universe with a mass too high to have grown over time since the Big Bang through this relatively slow process.
Another possibility is something called primal black holes, first proposed in 1966. It is theorized that the varying density of the early Universe could have produced cavities so dense that they collapsed into black holes. These would not be subject to the black hole size limitations of collapsed stars, and could be extremely small or, well, astonishingly large.
The extremely small ones, if they ever existed, would likely have evaporated by Hawking radiation by now. But the much, much bigger ones could have survived.
So, based on the primordial black hole model, the team calculated exactly how awesome large these black holes could be, between 100 billion and 1 quintillion (that’s 18 zeros) solar masses.
The purpose of the paper, the researchers said, was to reflect on the effect of such black holes on the space around them. We may not be able to see SLABs directly – black holes that are not accretion material are invisible, because light cannot escape their gravity – but huge invisible objects can still be detected based on the way the space around them behaves.
For example, gravity curves space-time, so that the light traveling through those areas also follows a curved path; this is called a gravitational lens, and the effect could be used to detect SLABs in intergalactic space, the team said.
The huge objects would also affect the detection of dark matter, the invisible mass that injects much more gravity into the universe than there should be – based on what we can actually detect directly.
A hypothetical candidate for dark matter, weakly interacting massive particles (WIMPs), would accumulate in the area around an SLAB due to immense gravity, in concentrations such that they would collide and destroy each other, creating a halo of gamma rays. .
And primal black holes are themselves candidates for dark matter.
“SLABs couldn’t provide the dark matter themselves,” Carr said. “But if they do exist, that would have important implications for the early Universe and make it plausible that lighter primordial black holes could do that.”
We also couldn’t help but calculate the size of a 1 trillion solar mass black hole. The event horizon would be more than 620,000 light years across. Uh. Astonishing.
The team’s research is published in the Monthly Communications from the Royal Astronomical Society.