Today is the fourth Thursday in November, which by tradition and law and fundamental cosmic force is the day for the American holiday of Thanksgiving. I’ve written quit a bit before about this holiday, and I don’t think I have anything new to add, so instead let me just link to previously written but still heartfelt words, and then we can move on to the Universe.

Dark matter is still a mystery. We know it exists. For one thing, the way stars orbit in galaxies doesn’t make sense unless a lot — like 80%, typically — matter in the galaxy is dark; that is, emits no light. Galaxies also orbit each other in galaxy clusters, and again they move too quickly to be constrained by the matter we see; the gravity just isn’t enough to hold them in place. They should fly apart, but they don’t, implying a lot of the matter there is dark. We have lots of other reasons to know it exists, too.

But what is it? The current best guess is a subatomic particle called an axion, but it’s possible it could be made of — seriously — teeny tiny black holes, called primordial black holes, or PBHs.

The smallest black holes we know can exist have about three times the mass of the Sun, and are created when stars explode as supernovae and their cores collapse to form black holes. But it’s possible even smaller ones might exist, created in the moments after the Big Bang. Huge densities and pressures might have been able to compress matter enough to form black holes of all mass ranges, including ones that could be as lightweight as a typical asteroid! They’d be very small, atomic sized, but possibly created in vast numbers at that time.

This is all highly theoretical, but if they exist, they’d be extremely difficult to detect, yet could account for dark matter. So, how could we find them?

A new paper outlines one way: use the solar system as a PBH detector. Really! [link to journal paper]

The idea is actually simple to explain. Black holes, even ones that small, have gravity (for example, an asteroid-mass BH has the same gravity an asteroid would). If one were to pass through the solar system, it would very slightly affect the positions of the planets and other known objects. For example, one passing even a few hundred million kilometers from Mars could shift it in its orbit by about a meter! That effect would take many weeks to transpire, but in principle could be detected. We have a lot of probes orbiting Mars, and so we know the planet’s position with better than one meter accuracy.

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