This Week’s Science News from SWTG

White Holes & Laser Cooling

Black Holes might turn into White Holes, and make up dark matter

White holes. Think of them as black holes played backwards. In Einstein’s equations, you can flip the arrow of time and get a solution that does the opposite of a black hole: nothing can go in or can stay inside, everything must come out. For half a century most physicists filed white holes as mathematical curiosity, but in recent years some have argued that they might be a general consequence of black holes. Not only this, but they might make up dark matter, and they could have observable consequences. Let’s have a look.

First, what exactly is a white hole? A black hole traps anything that crosses its event horizon. Mathematically, a white hole is the time-reversed case. It also has an event horizon, but the temporal direction is opposite to that of a black hole. So, nothing can fall in, everything must come out. It’s like that guy at the party who doesn’t stop talking. However, these mathematical white holes cannot be created, they would just have had to be around since the big bang. That doesn’t really make a lot of sense.

However, this situation changes if you take into account quantum gravity — that is, if you take into account that Einstein’s general relativity is missing quantum properties. Then, the story that white holes would have to be around since forever might not be correct. Instead, black holes might eventually all turn into white holes. And black holes, well, those can be created.

The idea is that when matter collapses, say, after a supernova explosion, it first forms a black hole horizon. But inside, the matter never reaches an infinite-density singularity. Instead, the collapse rebounds, creating something that very much resembles the mathematical white hole.

So, the plot thickens, which in physics means the equations get longer. This idea has been pursued, for example, by Carlo Rovelli and Francesca Vidotto, who say that this is exactly what happens in loop quantum gravity. The collapsing matter reaches a maximum density and then transitions into an expanding state. So, a white hole.

A paper that appeared just the other week supports this idea. The authors don’t use loop quantum gravity but instead unimodular gravity. Unimodular gravity is an alternative way of writing general relativity that just gives you back Einstein’s theory. However, if you look at the quantum properties, these are much easier for unimodular gravity. This is why many physicists think that maybe unimodular gravity is actually correct. Or at least more polite to the maths.