Universe might be full of singularities, and they could make up dark matter (Patreon)

[This is a transcript with links to references.]

Ok, I have seen a lot of weird ideas for what dark matter could be, but this one surprised even me. A team of researchers proposes that the universe might be filled with singularities  and those could make up what we call dark matter.  I had a look at the paper.

This new paper is about what’s called a naked singularity.  They’re called naked not because they’re missing panties -- that’s knickers for the British audience -- but because they’re missing a horizon like black holes have.  A horizon is the boundary of a region from within which no light can escape. It’s why we can’t see what’s inside a black hole. For a black hole it’s like part of space become entirely inaccessible.  Inside the black hole horizon, there is singularity  that’s a place where matter is crushed by its own gravity until it’s infinitely dense. A naked singularity, on the other hand, has a plain view on that singularity, there is no horizon hiding it.

Black holes and naked singularities are both solutions of Einstein’s theory of General Relativity. Yes, it’s Albert again.

We know that black holes exist, but we don’t know whether naked singularities exist.  There’s a conjecture called the cosmic censorship hypothesis  that has it that all singularities in our universe are hidden behind a horizon. But it’s never fully mathematically been proved and it’s somewhat controversial whether it’s correct and fulfilled in nature.  This is because there are computer simulations that strongly suggest naked singularities can indeed form.

 Of course a computer doesn’t actually ever spit out infinity, you just calculate until the number is so large it might as well be infinity. And that’s just as well because most physicists don’t think that singularities are real.  They think that, well, most of the time they probably think there’s a bug in their code.  But if it’s not the code, when a singularity appears in the mathematics of a theory, not the simulation they think it means that the theory breaks down and needs to be replaced by something better.

When it comes to infinite energy densities created by gravity, they think what happens is that when the densities get very high, and the curvature of space very large, then we need to take into account the quantum properties of space and time. For this we would need a theory of quantum gravity, which we don’t have.

But that doesn’t make naked singularities any less interesting, it makes them more interesting.  It’s because they would us you a plain view onto a region with strong quantum gravitational effects. There’s no other place where we can find that.  This is why I was very interested in naked singularities ten years ago or so because I was wondering whether in case they’re there and make gravitational lensing if that could tell us something about quantum gravity.  But then someone did the maths and it turns out that they would be very very difficult to tell them apart from black holes, so that idea never went anywhere.

In any case that brings me to the new paper.  These guys now say that naked singularities, a lot of them, could form in the early universe. This idea isn’t totally crazy.  You see in the early universe all matter is in the form of a hot plasma.  The plasma isn’t entirely smooth, it’s more like my attempt to make hollandaise sauce, in some places the density is a bit higher, in some places it’s a bit lower. And the places with higher density will go on to contract under the pull of gravity.

Normally we say that they go on the form stars and later galaxies. But if the fluctuations are large enough, they can form what is called “primordial black holes”.  And the authors of the new paper now say, well they could also form “primordial naked singularities”.  

 The black holes that we observe out there in the cosmos are formed from the collapse of stars or from the accumulation of matter onto those stars. They are extremely massive because there are no other astrophysical processes in the late universe to create them. But primordial black holes could have been created at any mass. In particular they could be fairly small and now be distributed throughout the universe.

This makes primordial black holes possible candidates for dark matter,  if that exists, which it may not. However, this idea has meanwhile been mostly discarded because it’s in conflict with observations.  The reason is that we know how much mass there must be in dark matter in total. It must add up about 4 times as much as the normal mass. That’s a lot.  And that’s the average over the entire universe. Within galaxies like the milky way you need 10 or 20 times as much mass in dark matter than normal matter. If that’s all-black holes, where are they?

Depending on the mass of the black holes they would lead to different observable consequences. They would do gravitational lensing which we haven’t seen.  They would distort the cosmic microwave background which we haven’t seen.  They could go through starts and collapse  them which we haven’t seen, the very small ones would evaporate  and leave behind flashes which we haven’t seen. They could still make a contribution to dark matter which is why some people are still working on it, but it looks like they don’t make up the bulk.

 In the new paper now they say that maybe what’s happening is that the early universe didn’t create primordial black holes but primordial naked singularities.  And while these would look similar for what gravitational lensing is concerned, they might not do much to collapse stars and so make more viable dark matter candidates.

This paper seems to be the first one about the idea and they didn’t look at a lot of details, but I assume they will soon follow up with another paper.

Is this possible. Yes.  I am pretty sure they will be able to work out the maths so that it’s compatible with observations. The problem I see coming is this.  While naked singularities form under somewhat different conditions as black holes, overall the process is kinda similar. I suspect it will be very difficult to come up with an explanation for what happened in this plasma in the early universe that creates a lot of primordial naked singularities but not also a lot of primordial black holes. And since we’ve pretty much ruled out primordial black holes, that will rule out the naked singularities along with that.

Then again, that’s really just a guess, and I’ll have an eye on how this idea develops, so stay tuned.