There are supermassive black holes. There is ultramassive black holes. How big can these foreign objects grow? Well, there could be something even bigger than ultramassive: steaming large black holes, according to the latest research.
Such hypothetical black holes – greater than 100 billion times the mass of the sun – were investigated in a new article they call SLABs, an acronym for “Stupendously LArge Black holes”.
“We already know that black holes exist in a wide variety of masses, with a supermassive black hole of 4 million solar masses living in the center of our own galaxy,” explained astronomer Bernard Carr of Queen Mary University in London.
“Although there is currently no evidence for the existence of SLABs, it is possible to exist and may also live outside galaxies in intergalactic space, with interesting observational consequences.”
Black holes have only a few broad mass categories. There are black mass holes; these are black holes that are around the mass of a star, up to about 100 solar masses. The next category is black mass holes, and how big they hang depends on who you are talking to. Some say 1,000 solar masses, others 100,000 and others 1 million; regardless of the upper limit, it seems pretty rare.
Supermassive black holes (SMBHs) are much, much larger, in the order of millions to billions of solar masses. These include the SMBH in 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 nicest black holes we have detected are ultramassive, more than 10 billion (but less than 100 billion) solar masses. These include an absolute animal that clocks in 40 billion solar masses in the center of a galaxy called Holmberg 15A.
“Surprisingly, the idea of SLABs so far has been largely neglected,” Carr said.
“We have proposed options for the formation of these SLABs, and we hope to begin our work to motivate community discussions.”
The thing is, scientists do not quite know how big black holes really form and grow. One possibility is that they form in their host system, then get bigger and bigger by swallowing a lot of stars and gas and dust and colliding with other black holes when galaxies merge.
This model has an upper limit of about 50 billion solar masses – this is the limit at which the amazing mass of the object would require a growth disk, so massive that it would shatter under its own gravity. But there is also a big problem: supermassive black holes have been found in the early universe in masses that are too high to grow through this relatively slow process in the time since the big bang.
Another possibility is something called original black holes, which was first proposed in 1966. According to the theory, the different densities of the early universe may have made bags so dense that they collapsed into black holes. It would not be subject to the size restrictions of black holes of collapsing stars, and it could be very small or, well, incredibly large.
The extremely small ones, if they ever existed, would probably have evaporated now due to Hawking radiation. But the much, much larger could have survived.
So, based on the primeval black hole model, the team calculated exactly how incredibly large these black holes can be, between 100 billion and 1 quintillion (that is 18 zeros) solar masses.
According to the researchers, the purpose of the article was to consider the effect of such black holes on the space around them. We may not be able to see SLABs directly – black holes that do not collect material are invisible, as light cannot escape their gravity – but massive invisible objects can still be detected based on the way the space around them behaves.
Gravity, for example, bends space-time, causing the light moving through those regions to also follow a curved path; it is called a gravitational lens, and the effect can be used to detect SLABs in intergalactic space, the team said.
The large objects also have consequences for the detection of dark matter, the invisible mass that inflicts more gravity in the universe than there should be – based on what we can actually detect directly.
One hypothetical candidate for dark matter, poor interaction with solid particles (WIMPs), would accumulate in the area around a SLAB due to the tremendous gravity, in such concentrations that they would collide and destroy each other and create a gamma ray hall.
And the original black holes are also a candidate for a dark cause.
“SLABs themselves could not provide the dark matter,” Carr said. “But if it exists at all, it will have important implications for the early Universe and make it easier for lighter black holes to do so.”
We also could not resist the calculation of the size of a black hole of 1 millimeter solar mass. The event horizon will eventually end at more than 620,000 light years. Uh. Stupendous.
The team’s research is in the Monthly notices from the Royal Astronomical Society.