“It is no good getting furious if you get stuck. What I do is keep thinking about the problem but work on something else. Sometimes it is years before I see the way forward. In the case of information loss and black holes, it was 29 years.” -Stephen Hawking
When we look out at the Universe, you know what it is that we see.
Light! Light from stars, galaxies, clusters, etc., covering all the different wavelengths you can measure.
But there weren’t always stars, galaxies, and clusters in the Universe. When the Universe was first “born,” in fact, the densest places were only a few thousandths of a percent denser than average. It took gravity tens of millions of years in the densest locations to form even the very first stars.
But our earliest pictures of the Universe come from when it was much younger than that: when it was just 380,000 years old! That would be the Cosmic Microwave Background, where the “cold” spots (shown in blue) and the “hot” spots (shown in red) represent the “thousandths-of-a-percent” difference from one location to another.
This image is more than just a pretty picture, it provides some of the best data of the early Universe that we’ve ever collected.
One of the fun things we can do (well, fun if you’re a theoretical cosmologist) is to look at the size of these temperature differences on different-sized scales. Are the temperature fluctuations bigger on large scales? On small scales? Let’s take a look…
Now, if you’re like most people, this just looks like a bunch of points and wiggles. Which is fine; I don’t expect you to have spent a decade (or more) of your life studying this as an undergraduate, graduate student and scientist. But you can learn a whole lot about the Universe from doing this. For example, you might want to know how much of your Universe is made up of dark matter?
And, as you can see, if you adjust the amount of dark matter in your Universe, the shape of your wiggles adjusts, too! We can do similar things for the amount of normal (atomic) matter, dark energy, neutrinos, etc.
But we can also learn whether we get bigger, smaller, or similar-sized fluctuations on smaller, non-cosmic scales. And what we find is that fluctuations are scale-invariant, which means that these fluctuations are the same few thousandths-of-a-percent on all scales.
If the fluctuations were much, much larger on very tiny scales, we could have a Universe filled with tiny black holes, ranging in scale from the size of Jupiter down to sub-atomic sizes.
Now, our observations disfavor this idea, but they aren’t sufficient to rule it out. But I was nonetheless surprised to come across this National Geographic article:
Their major idea is that there are these tiny, minuscule black holes that are physically smaller than a single atom, but are about the mass of a thousand cars.
Now, those of you familiar with microlensing searches know that black holes from the early Universe that are more massive than our Moon or so are ruled out as existing in great numbers, but we can’t definitively rule out less massive ones.
Unless they’re less massive than a large mountain: about a billion tonnes. They are all gone by now, because black holes evaporate!
Even though we are talking about gravity here, we can’t just turn off quantum mechanics! The vacuum itself — populated by particle-antiparticle pairs — causes black holes to slowly radiate their mass away through the process of Hawking radiation.
Perhaps paradoxically, the smaller and lighter your black hole is, the faster it evaporates! For a black hole of the mass that these authors are proposing, the evaporation time is about two minutes!
You can’t just pretend that since Hawking radiation hasn’t been directly observed (it’s not like we have a convenient black hole to study in this type of detail) that the laws of physics suddenly don’t apply. In fact, the experimental analog of black holes that was built showed the analogous effects to Hawking radiation!
As far as we can tell, all the evidence points away from the existence of primordial black holes. While they’re a fun theoretical toy, there’s no reason to expect they actually exist. Furthermore, if they did exist, they would still be subject to the laws of quantum mechanics, and thus would evaporate. You can’t selectively turn off quantum physics when it’s inconvenient for you, as I’ve detailed before. If you want to say something sensible about what physically exists, you need to follow the laws of physics.
So, in summary, if you did turn off quantum mechanics (which you can’t), and you allowed 100% of the dark matter to be these thousand-tonne black holes (for which there’s no reason or mechanism), then you’d have “miniature black holes” everywhere. Or, at least, one or two of them passing through Earth per day. At least the authors get one thing right: even in this worst case scenario, they’re still totally harmless.