Tension-Filled Black Holes

Most matter pushes against nearby matter, via a positive “pressure.” That’s why Earth holds its shape, instead of collapsing down to a point mass. An odd feature of general relativity (GR) is that the pressure in matter that tends to push it away from each other also tends to push the space it is in to shrink, not expand. 

Sometimes matter is in tension, such as in a cable holding a bridge above the ground. And here that same odd feature of GR says that tension pushes the space it is in to expand. But as most stuff near us is under pressure, not high tension, overall that stuff pushes space to shrink. Which is why the expansion of a universe filled with ordinary stuff tends to slow down and reverse to become a contraction.

We think we know of two big exceptions in the universe, where tension dominates, and so space is pushed to expand. One is the very dense hot matter that in the early universe caused an “inflation”. The tension there apparently caused the universe to expand exponentially over many orders of magnitude until it decayed into familiar matter. The other exception is the “dark energy” that is now causing our universe to expand exponentially, though at a vastly slower rate. 

We know little about either of these kinds of matter, but one of the most intriguing physics hypotheses I’ve seen in years says that these are both actually the same matter, except recently this stuff exists only inside black holes. As ordinary matter falls into a black hole, it is crushed and heated to the extreme levels previously seen in the early universe, and is then converted into a material that has strong tension, not pressure, like that early inflation matter. 

Even though black holes constitute only a tiny fraction of the volume of the universe, the density of the crushed stuff inside them is so huge that they can change the average values of matter and pressure in the universe. And apparently this is enough to make the average pressure in the universe negative. That is, the universe is on net under tension, and GR math says that this makes the universe expand exponentially. 

The math also says that the mass of each “generic object of dark energy”, e.g., tension-filled black hole, is “cosmologically coupled”, so that it grows in proportion to the volume of the universe, with the average black hole mass per unit volume staying constant. (I’ll admit it is hard to see how exactly that works locally, but hopefully someone will soon show how, or prove it wrong.) This can help explain the otherwise puzzling enormous masses of many black holes. Being so tiny, it it hard for black holes to grow merely by plowing their way through sparse matter. 

There is now a literature testing the many predictions of this theory, giving the theory some wins and some apparent conflicts. Most recently, a good fit was found between predictions of expansion rates over time and the first year’s data from a 5 year measurement project. Of course the issue is far from settled, but I’d guess this theory has at least a 10% chance of working out. 

There’s also a literature on running heat engines using black holes as resources, for example shrinking the hole relative to a cooler background, or merging two black holes into one. If those methods work on tension-filled black holes then it seems that our descendants could have a long energetic future from running such engines. An exponentially expanding universe, with each black hole increasing in mass seems like a self-filling gas tank; can that really be right?

As the radius of a black hole is proportional to its mass, then when universe increases by eight in volume, its distances double, but each black hole increases by a factor of eight in both mass and radius. Thus the radius of black holes becomes an increasing fraction of the distance between them, until eventually they meet and merge. Does that put a limit on how long our descendants could use tension-filled black holes to run heat engines?