Space News Deep Dive: The Black Hole/Dark Energy Connection

Just recently the news came out about a new study that seems to firm up a theory about dark energy and black holes. Namely that, through complex and ridiculous physics, black holes cause dark energy. Dark energy being one of the biggest mysteries of the universe as well as being a great headline buzzword (as is “black holes” for that matter) the story got quite a bit of attention.

But what exactly is going on between these black holes and the expansion of the universe? What does it have to do with general relativity? And just what the heck is “cosmological coupling” anyway? I’m going to answer those questions to the best of my ability. But we’ll start with talking about dark energy itself.

Mysterious Forces

In a nutshell, dark energy is the outward force driving the universe to expand faster and faster over time. To explain that, let’s go all the way back to The Beginning, when the universe Big Banged and the force of that eruption sent everything flying outward. 

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For a long time astronomers (once they’d collectively gotten over the idea of a static and unchanging universe, which took until the 1920s or so) assumed that this outward motion would eventually slow down and probably even reverse. It would have to, by all sensible measures. After all, the outward force from the Big Bang would dissipate eventually and that would leave the gravity of all the matter in the universe as the dominant force. That gravity would, given enough time, pull everything back together in a Big Crunch, and that’s how the universe would end.

It wasn’t until 1998 that observations of Type 1a supernovae led to a paradigm-shifting discovery: the expansion of the universe wasn’t slowing down. It was accelerating. Something was counteracting the gravitational pull of, well, everything and pushing things farther and farther apart faster and faster.

Astronomers in 1998 had absolutely no idea what this gravity-counteracting force could be, so they gave it a name fitting its enigmatic nature: dark energy. And it turns out to be the dominant thing in the entire universe.

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If you’ve ever heard the statistic that everything we can see is just 5% of what’s out there, it’s not because 95% of the universe is beyond our field of view. It’s because 95% of the universe is invisible to us. Of that 95%, roughly 27% seems to be dark matter, some form of stuff that seems to have perfectly normal gravitational interactions and just be completely not visible in any form of light we know about. The remaining 68% or so of the universe is dark energy. 

And if you’ve ever seen that and wondered how the heck energy could make up 68% of the stuff in the universe, it’s because in physics mass and energy are interchangeable. Mass can become energy and vice versa. Mass, matter, stuff is just one way for energy to be. It’s what E=mc² means, if you’ve ever seen that famous Einsteinian equation. The E there is energy and the m is mass (the c is the speed of light). So any given amount of energy can exist instead as mass without breaking things. Remember this fact, it’s going to pop back up later.

So if you turned all the dark energy in the universe into mass and piled it all up with all the dark matter and normal matter, the dark energy mass would make up 68% of that pile. There’s a lot of it out there. Which makes it super frustrating that we don’t know what it is. But now we at least have a rough idea of where it may be coming from. And it all started with some oddly chonky black holes.

Growing Black Holes

In February 2023 a study dropped a bombshell into the field of cosmology. It looked at the supermassive black holes at the center of big, old elliptical galaxies. These are the kinds of black holes that don’t have a bunch of stuff falling into them anymore. Since supermassive black holes grow either by having stuff fall into them or by merging with another supermassive black hole, that means these worthy ancients shouldn’t be gaining mass. 

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But, as you can probably guess from the setup, they are. That means there needs to be another mechanism for them to be growing. That February 2023 study proposed just such an alternative mechanism, and this is where things are going to get…well…bizarre.

Let’s Get Weird

The 2023 study suggested that black holes can gain mass because they are “cosmologically coupled” to the universe. What this means is that as the universe is stretching it’s stretching the black holes along with it. When this first came out the best analogy I found was to think of these stretching black holes as a rubber band. A stretched rubber band has more energy than a non-stretched one (which is why you can fire them from your fingers). And, as we’ve already seen, mass and energy are related. So that stretched energy in the black hole manifests as extra mass.

Just how much extra mass gets gained depends on something called the coupling strength, represented by the letter k. Essentially you can think of it as the rubber band analogy again: stretching a stiffer rubber band will give you more energy than stretching a looser one by the same amount. A higher value for k is a stiffer rubber band. Looking at the massive old black holes at the hearts of elliptical galaxies and how much extra mass they seem to have gained over time, the 2023 study came up with a k value of about 3.11 for them. And that is interesting. 

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Often when we think of black holes, folks use the term “singularity”. The idea is that the mass of the black hole is contained in an infinitely small point, the singularity. It’s a fun idea, but it also kind of breaks the laws of physics. Most analogies for this one that I’ve heard compare it to dividing by zero. If you’ve ever tried to do that on a calculator or an Excel spreadsheet (am I showing my age here?) you’ll find that it Does Not Work. Singularities are like the physics equivalent of that.

That said, for a long while this was kind of how a lot of people thought of black holes. But here’s the thing: a black hole that was a singularity shouldn’t have a k value of 3.11. It should have a k of zero. So…suddenly this 2023 study was suggesting that singularities were not a thing. Mathematicians everywhere probably rejoiced (I don’t know, I wasn’t invited to the celebration parties). But if black holes don’t have singularities in them…what do they have?

Break Out the Hoover

Vacuum energy. Yeah, I had never heard of it prior to that day this study either. Here’s where I admit that I don’t fully understand the concept of vacuum energy, but it seems like it comes from squishing matter together to the greatest possible degree without actually forming a singularity. Okay, that definitely seems like the kind of thing that could happen within a black hole. And (so I read) the presence of vacuum energy means that there doesn’t need to be a singularity to explain a black hole. General relativity would definitely prefer that to be the case.

But, you might ask yourself, the k value being 3.11 means no singularities but does it necessarily follow that it’s vacuum energy lurking in these black holes instead? Well, it seems like a vacuum-energy black hole will show a k value of…3. And it would gain mass as a result of the expansion of the universe. Suddenly the vacuum energy model is looking pretty solid.

The Missing Piece

Okay, we’ve talked about how this model of things links growing black holes with the expansion of the universe, but we haven’t gotten around to actually linking the black holes to accelerating expansion, aka to dark energy. And this part is also a little hand-wavey, I confess.

Essentially it all comes down to the fact that some things need to be conserved in the universe, and these conservations can have effects that are not necessarily intuitive. For an example a little more familiar to us, let’s look at the Earth and the Moon. The tug of the Moon’s gravity on Earth’s oceans is very gradually slowing the spin of the Earth over time. But on its own this would reduce the amount of angular momentum in the Earth-Moon system, and angular momentum must be conserved. Physics says so. 

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As a result, as Earth slows down and loses angular momentum, the Moon is gaining it, which is causing it to slowly move farther away from the Earth. So technically Earth’s oceans are very slowly causing us to lose the Moon because angular momentum must be conserved. Ah, physics (don’t worry, the Sun will die before we fully lose the Moon. I’m sure that’s a comfort).

With our black holes, the thing being conserved is called the stress-energy tensor. It has to do with changes in energy and momentum. For our purposes it comes down to the fact that the increase in the mass of the black holes from the universe’s expansion causes a kind of rebound effect in order to conserve the stress-energy tensor. That rebound effect is a negative pressure pushing outward on the universe. That, essentially, is dark energy.

Back to Now

Okay, all of that is from 2023. And it was a major claim: the source of dark energy potentially identified. That’s the kind of claim that needs a lot of evidence to back it up, as delightful as the initial study was. So what is it that happened recently?

A paper was published using results from DESI, the Dark Energy Spectroscopic Instrument. Essentially DESI can help us figure out how fast the universe was expanding at different points in its history with incredible precision. It does this by studying millions of galaxies back into the distant reaches of the universe. If black holes are connected to dark energy, then changes in the amount of dark energy (aka changes in how fast the universe is accelerating) should correlate to changes in the formation and growth rates of black holes. 

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And that is what was recently announced. The two data points matched up nicely. The more black holes were being made in a given era, the more dark energy DESI detected. In the interest of good science literacy I should point out here that this does not prove that dark energy comes from black holes—it heavily suggests a strong relationship between dark energy and black holes.

That said, coupled with the data from 2023, it paints a pretty compelling picture. In that picture, we may still not be sure of the specifics of what dark energy actually is, nor all the hows and the whys of its connection to black holes, nor all the hows and whys of the connection between black holes and the universe. But it’s a picture in which we know where dark energy comes from.

In that picture, the expansion of the universe is a self-perpetuating feedback loop driven by the most powerful engines in its bounds, black holes. The universe expands which makes the black holes gain mass which makes dark energy which makes the universe expand faster. Onward and onward until the heat death of the universe, when even the black holes have dissolved into the silent emptiness.

You know what? I’ll take that over the Big Crunch.