Black Holes, Dark Matter and the Enormous Density of My Ignorance

Dark matter is a form of matter hypothesised by cosmologists in order to account for certain otherwise difficult-to-explain observations. The short version: there doesn’t seem to be enough matter in the universe to explain various things, including the rate of cosmic expansion and certain gravitational effects. So some (not all, but as I understand it most) scientists propose this radically new kind of matter, unlike the ‘baryonic’ stuff out of which stars and planets, tables and chairs, you and I are made. This weird kind of non-baryonic matter doesn’t interact with the electromagnetic field, and doesn’t absorb, reflect, or emit electromagentic radiations (such as light). Hard verging on impossible, therefore, to detect it with our baryonic-built instruments! Is it even there? ‘Most experts,’ Wikipedia says, ‘think that dark matter is abundant in the universe and has had a strong influence on its structure and evolution.’

Here’s a dumb question: instead of postulating a very odd, unobserved and hugely dispersed new kind of matter, why not explain the observational data in terms of the kind of matter we’re sure does exist? Put it this way: how many black holes are there in the universe? Nobody knows, but it’s probably upward of forty quintillion, and possibly orders of magnitude more. Individual black holes are very massive — massive enough that they exert a gravitational pull so great not even the speed of light is fast enough to achieve escape velocity — and some are many millions or billions the mass of our sun. That’s a lot of mass. What is the hypothetical upward bound for the mass of a black hole? I’m not sure I see there can be one. Matter can keep being sucked into the singularity indefinitely, compacting down in the central body. Presumably an infinitely massive black hole is not possible, not least because the universe has only been around, and any individual hole has only been accreting, for a finite time; and also because any black hole loses mass as Hawking radiation … though such radiation leaks mass very slowly. Presumably some black holes are slowly shrinking down as they radiate away, where others radiate at a vastly lesser rate than they accrete and so are getting more and more massive and more dense. Are there black holes that are trillions of times the mass of our sun? Quintillion times? Googplex times? Absent gravitational lensing, we cannot observe any such body. How would we know?

I appreciate that what I’m doing here is parading my ignorance. Cosmologists will of course have considered this hypothesis and will have their reasons for rejecting it as an explanation for the missing mass. Wikipedia suggests that some astrophysicists hypothesise a different kind of early-universe black hole, the ‘primordial black hole’. These critters, it seems, may have formed a second or so after the big bang out of the fluctuation of spacetime itself. A ‘regular’ black hole forms when a very large object (like a star) becomes so massive light can no longer escape, which means such black holes by definition will be be more than star-sized in terms of their mass (2 x 10³⁰ kg and above); but these primordial black holes might be very small indeed, some as tiny as 10^-8 kg — though these, it seems, would have radiated themselves away over the course of the cosmos we observe. Others might be milimetres across but weighing the size of several suns. However, an April 2019 study ‘seems to rule-out the possibility that primordial black holes smaller than a tenth of a millimeter (7 x 10²² kg) make up most of dark matter’. So I guess it’s not that.

Why not explain dark matter in terms of regular, supermassive or ultramassive black holes? Wikipedia is no substitute for a proper grounding in the maths and physics of all this I know, but it says ‘multiple lines of evidence suggest the majority of dark matter is not made of baryons’. How so?

• Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars.

But black holes are not diffuse gas.

• The theory of Big Bang nucleosynthesis predicts the observed abundance of the chemical elements. If there are more baryons, then there should also be more helium, lithium and heavier elements synthesized during the Big Bang.

But if those heavier elements are locked inside black holes, how would be observe them?

• Astronomical searches for gravitational microlensing in the Milky Way found at most only a small fraction of the dark matter may be in dark, compact, conventional objects (MACHOs, black holes etc.).

This seems more to the point, although it strikes me as one of those ‘absence of evidence is not evidence of absence’ things.

I’m genuinely curious what I’m missing, here, since surely I’m missing something obvious.

It’s on my mind, because I’m writing a science fiction story at the moment in which black holes figure largely. One postulate, permitted me by the speculative nature of science-fiction ‘science’, is that beyond a certain mass black holes punch through into a notional substrate, and so all black holes (above a certain mass) are actually all the same black hole, a web that tangles the expanding spacetime of our regular universe in ways we could align with ‘dark matter’ observation. I’m also trying to imagine what the geometry of an infinitely massive black hole might be like: not a ‘white hole’ pouring energy at the other end of a wormhole — that old SF standby — but an infinite outward bound within which all observable matter exists. But this is all pretty fanciful.