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There are supermassive black holes. There is ultramassive black holes. How big can these strange objects be? Well, there could be something even bigger than ultramassive: prodigiously large black holes, according to the latest research.
Such hypothetical black holes – more than 100 billion times the mass of the Sun – have been explored in a new article which names 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 residing at the center of our own galaxy,” explained astronomer Bernard Carr of Queen Mary University of London.
“Although there is currently no evidence for the existence of SLABs, it is conceivable that they could exist and that they could also reside outside galaxies in intergalactic space, with consequences of d interesting observation.
Black holes only have a few fairly broad categories of mass. There are stellar mass black holes; they are black holes that are around the mass of a star, up to about 100 solar masses. The next category is intermediate mass black holes, and their size seems to depend on who you’re talking to. Some say 1000 solar masses, some say 100,000 and others 1 million; whatever the upper limit, these seem 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 *, at 4 million solar masses, and the most photogenic SMBH in the Universe, M87 *, at 6.5 billion solar masses.
The clearest black holes we have detected are ultra-massive, more than 10 billion (but less than 100 billion) solar masses. These include an absolute beast clocked at 40 billion solar masses at the center of a galaxy named Holmberg 15A.
“However, surprisingly, the idea of SLABs has been largely overlooked until now,” Carr said.
“We have offered options for the formation of these SLABs and hope that our work begins to motivate discussions within the community.”
The point is, scientists don’t really know how much black holes form and grow. One possibility is that they form in their host galaxy, then grow larger and larger absorbing large numbers of stars, gas and dust, and colliding with other black holes as the galaxies merge.
This model has an upper limit of about 50 billion solar masses – this is the limit at which the prodigious mass of the object would require an accretion disk so massive that it would fragment under its own gravity. But there is also a significant problem: Supermassive black holes have been found in the beginning of the Universe at masses too high to have increased by this relatively slow process since the Big Bang.
Another possibility is what are called primordial black holes, first proposed in 1966. The theory is that the varying density of the early Universe could have produced pockets so dense that they collapsed into black holes. . These would not be subject to the size constraints of black holes in collapsed stars, and could be extremely small or, well, unbelievably large.
The very little ones, if they had ever existed, would probably have evaporated from the Hawking radiation by now. But the bigger ones could have survived.
So, based on the primordial black hole model, the team calculated exactly the size of these black holes, between 100 billion and 1 quintillion (or 18 zeros) of solar mass.
The purpose of the article, the researchers said, was to consider the effect of these black holes on the space around them. We may not be able to see SLABs directly – black holes that do not catch material are invisible, as light cannot escape their gravity – but massive invisible objects can still be detected depending on how whose space around them behaves.
Gravity, for example, bends space-time, so that light passing through these regions also follows a curved path; it’s called a gravitational lens, and the effect could be used to detect SLABs in intergalactic space, the team said.
The huge objects would also have implications for the detection of dark matter, the invisible mass that injects far more gravity into the Universe than there should be – depending on what we can actually detect directly.
A hypothetical dark matter candidate, Massive Weak Interaction Particles (WIMP), would accumulate in the region around a SLAB due to the immense gravity, at concentrations such that they would collide and annihilate, creating a halo of gamma radiation.
And primordial black holes are themselves a candidate for dark matter.
“The SLABs themselves couldn’t provide the dark matter,” Carr said. “But if they do exist, it would have important implications for the Early Universe and make it plausible that lighter primordial black holes could do so.”
In addition, we could not resist calculating the size of a black hole with a solar mass of 1 quintillion. The event horizon would end more than 620,000 light years away. Uh. Prodigious.
The team’s research has been published in the Monthly notices from the Royal Astronomical Society.
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