The latest findings about early black holes nicely patch up a gap in our cosmological thinking. This is a great opportunity to explain how the terms we've used to describe the problem might not have helped along the way.
The physics of our universe can be splendidly straightforward at times, even if it takes physicists a while to get there. That’s the current major takeaway from a paper published in Astrophysical Journal Letters on February 15, authored by 17 astronomers in collaboration across nine countries, led by the University of Hawai’i. You’re going to hear the paper talked about in terms of “dark energy” on mainstream media, and we’ll get there, but the more interesting story lies in how term use can often get in the way of understanding the cosmos.
This latest paper builds on work some of the same authors published in 2021, which first advocated for thinking about the physics of black holes in relation to the expansion of the universe. This was because we were finding black holes with far higher masses than expected, and our usual explanations for their size, like mergers between them, weren’t cutting it. In 2021, Croker et al, at the University of Hawai’i, measured gravitational waves to highlight the mass gap in the current explanatory model, and they further outlined why other possible explanations, like stellar mass loss and other solar explosion interference, weren’t sufficient fixes for the problem.
This led to the term “cosmological coupling”: simply a fancy shorthand for the idea that the growth rate of black holes can only be fully understood when we factor in the expansion of the universe.
Which should be obvious in hindsight, right? How can anyone measure cosmic phenomenon properly if we’re not taking into account their environmental context?
Well, yes and no. We’ve known since 1929, with Edwin Hubble’s observations, that the universe is expanding, so surely its momentum would have some impact. But how was it expanding? Under what operating principles? That, we didn’t know yet.
In 1933, Fritz Zwicky coined the term “dark matter” to explain a curious local example of cosmic expansion. The galaxies in the Coma Cluster were moving apart too fast for their evident mass; there had to be a different source of mass, a mass we couldn’t see because it didn’t interact much with the electromagnetic field (which is what allows us to read light signatures, and other waveforms along the EM spectrum).
This is an important concept, because we now know that the majority of the universe seems to be made up of unseen mass, with everything we’ve observed through traditional metrics only covering 5 to 27 percent of the cosmos. Dark matter particles just aren’t interacting with the universe in the ways we usually use to measure our world. For the average human, this is equivalent to needing goggles to read heat signatures. We’ve been spending the last century trying to find the right “goggles”, so that we can see the rest of the mass in our universe, too.
The hunt for better goggles
Mostly, we’ve been able to identify the existence of dark matter by inference, through what effects it does have on visible stuff. In the 1970s, Vera Rubin provided our first strong evidence for the existence of dark matter, while researching the orbital speed of stars around black holes at the center of their galaxies. “Dark” matter, matter we can’t see by traditional means, had to explain why stars weren’t rotating the way gravity tells us they should around black holes.
Then, in 1998, there was a bit of a race in astrophysics around Hubble’s original problem of an expanding universe. Even though Hubble had found evidence of universal expansion, we hadn’t yet figured out the rate at which this expansion was slowing. Because obviously it had to be slowing, right? If it obeyed the laws of gravity? And what a career win this would be, if the Harvard and Berkeley teams could figure out the magnitude of deceleration, or the rate of “cosmic breaking”. If we could figure out by how much universe had been slowing down since the Big Bang, we could figure out the total density of the cosmos.
But it wasn’t slowing. Quite the opposite: universal expansion from that initial explosion was accelerating. This was a discovery that surprised we gravity-bound Earth-dwellers, because it meant that something was working against gravity on a cosmic scale. Something was pulling all known matter further apart.
So what was it? Cosmologist Michael Turner coined the phrase “dark energy” in 1998, which made some sense inasmuch as it described an element of our universe that didn’t appear to interact significantly with the electromagnetic field. Like dark matter, it was “dark” to us.
But this was quite misleading, too, because dark energy is just another way of describing this cosmic phenomenon of accelerated expansion since the Big Bang. That’s it.
Accelerated expansion told us that there was something more our models needed to account for, but renaming it “dark energy” might have distracted us into thinking of the problem as only having mysterious particles as the solution.
This is why, in recent years, you might have heard bandied about the idea that “dark energy” had been debunked: because many in the scientific community have been pushing back on this particular, holistic way of talking about the concept of accelerated expansion. And at times, yes, researchers have been leaning on select data sets to try to disprove aspects of dark energy theory, but never with significant replicability or applicability to our core problem:
Namely, that for the last quarter century, we’ve known that the universe is accelerating as it expands, and we had no idea why.
Enter the latest, much simpler explanation of what’s going on.
Black hole sums, won’t you come
When thirteenth century Thomas Aquinas wrote his “five ways”, one of his arguments for the existence of a god relied on the idea that there had been nothing, and then there was something. This was not what all philosophers had assumed. Rather, Aristotle, living in the 300s B.C., was part of a tradition that believed the universe to be eternal: no beginning, no end, and thus no reason to imagine an even more complex conscious entity just to explain the emergence of “stuff”.
But everyday humans better understand the concept of “nothing” (for instance, I had a paycheck, and then I had skyrocketing expenses under inflation, and now I have…), so it’s not surprising that even when we did start to understand the vacuum of outer space, we colloquially treated it as “empty”.
Obviously it wasn’t, though, or light from the sun and other stars would never reach us, but this misconception carries on even today, and especially in the way we talk about the Big Bang. For the scientifically literate (or semi-literate, like myself), the Big Bang took place in an underlying field of quantum mechanical interactions, and even today it’s possible for scientists to “let there be light” by switching virtual particles into real particles in the supposed vacuum of outer space.
With that in mind, we come to “vacuum energy”, which is used to describe a special case of zero-point energy: itself a fancy way of naming the lowest energy state in any given system. Vacuum energy generally describes the lowest energy state of a system with no physical particles: in other words, when it’s just potentials fluctuating all over the place (because even at absolute zero, vibrational motion persists).
Vacuum energy is also one way that astrophysicists have been trying to account for what’s been colloquially known as dark energy (i.e. the accelerated expansion of our universe). Since the 1960s, zero-point energy been discussed theoretically, when scientists realized that the special case of zero-point energy in a vacuum might be the cosmological constant that Einstein had originally added to his calculations (then scrapped), to try to balance out the effects of gravity back when people thought the universe was static.
Why were the two concepts related? How could vacuum energy offer a counter-force to gravity? Because all zero-point energy has some pretty weird properties: it has negative pressure, and thus causes the space where it exists to expand.
And yet, so-called vacuum energy (or spaces in the universe where these weird properties play out) is still something. And something has mass.
Which leads us to the last amusing problem caused by our naming traditions, when it comes to helping everyday people understand the wonders of our cosmos as our cosmologists are unpacking them, too.
E = mc2 might be the world’s most famous equation, but it’s also one of the most misunderstood, because it represents energy (“E”) in visual opposition to mass (“m”): the same, but… also different, right? In colloquial discourse, everyday science journalism, and far too many early physics classes, this often means treating energy and mass as binary opposites, instead of relational properties. In consequence, when we have to communicate the idea of vacuum energy explaining the existence of more mass in the universe, we’re setting ourselves up for failure.
Energy can be a massless form of a particle, in that calculating a waveform’s mass in special circumstances can return the result of “0”. But energy is also moving, which means it’s being directed by forces. And anything acted on by forces, such as gravity, or when you compress a spring in your hands, gains heft. It gains mass.
(Under Earth’s gravity, we call this relational mass “weight”. Everywhere else in the universe, your mass is just in a complicated gravitational relationship with other masses. Feel free to update your New Year’s resolutions accordingly!)
Back to the latest news
With all this in mind, then, what these latest observations out of the University of Hawai’i et al tell us is that black holes have been gaining mass by increasing in vacuum energy, these pockets of lowest possible energy states created by the surrounding compression of particle matter under a black hole’s overwhelming gravitational pull. Gravity itself is in some ways creating its own counter-force, by pulling matter tight and leaving pockets of not-empty space, which have both their own mass and their own, special tendency to expand.
Even better? According to this team’s calculations, the amount of mass created by these manifestations of vacuum energy, which have been growing in lockstep with the visible mass of black holes over cosmological time, might just explain the curious phenomenon of accelerated expansion, a.k.a. “dark energy”, entirely.
And the best part of this concept, if further research corroborates these findings?
We might finally be able to work on sharpening our vocabulary around quantum and astrophysics, so that everyday folks can also revel in the fascinating findings of cosmology, and not treat it as just another unfathomable mystery in a sea of human stories. One day soon, if fortunate, an accurate representation of our universe might actually be accessible to us all.