Proportions—think of the golden ratio and its kindred—are the bedrock of our reality, found in everything from music to the structure of the cosmos. Discerning them is the source of our empirical knowledge of the world. Leveraging proportions also allows the hidden to be seen, in ways that can seem almost magical. On Earth, paleontologists can estimate the size of a long-extinct dinosaur, for instance, by digging just its thigh bone from the ground—because the part is proportional to the whole. Across the sky, astronomers have found and exploited many similar relationships—ranging from the architectures of planetary systems to the clustering of galaxies—to gain a deeper understanding of the universe.
Sometimes, however, the greatest insights proportions may offer arise from discord rather than harmony. One fresh example comes from the James Webb Space Telescope (JWST), which has just revealed an unexpected disproportion in the cosmos, one that may enlighten us about the birth of the first black holes.
In recent decades, observations have shown that supermassive black holes, millions or billions of times more massive than our sun, reside at the centers of large galaxies, including the Milky Way. In the nearby universe, an elegant ratio reigns: the mass of each central black hole is around 0.1 percent, or one thousandth, of the starry mass of each host galaxy.
That’s remarkable. Imagine a piano where musical pitches represent mass: high pitch means high mass, low pitch indicates low mass. In the soft light of the cosmic auditorium, a high-pitched note introduces the appearance of a colossal galaxy! Then, a low-pitched note: a black hole materializes! For the Milky Way and the vast majority of the big galaxies that surround it out to a few billion light-years, the 1,000-fold difference between the black hole and galactic pitches would be about 10 octaves. That’s a range greater than what a piano can produce. Surprisingly, JWST’s studies of the universe much farther away from us—and, given light’s finite speed, much deeper back in time—show that black holes and galaxies are tuning in to an entirely different harmony. Galaxies in those remote, ancient epochs contain black holes 10 to 100 times more massive than the ones found in similar galaxies in today’s universe. In our pitch analogy, they are separated by as little as three octaves—suggesting a cosmic sonata easily performed on a piano.
As astrophysicists say, these faraway black holes are “overmassive” with respect to their hosts compared to those we find in galaxies around us today. Many JWST observations, for example in the JADES and CEERS surveys, now support this conclusion. We are witnessing a population of infant black holes overgrowing their nurseries and flourishing faster than expected in the distant, early universe. The current record holder for the farthest, earliest black hole ever seen, GN-z11, fits this surprising trend, and astronomers have even spotted another far-distant, early-universe black hole that may be as massive as its galactic host.
Why do black holes and their galactic mansions differ in relative mass, depending on whether they reside in the faraway or nearby universe? JWST’s data suggest that black holes are initially similar in mass to their hosts. Then, over billions of years, the black hole and its galaxy tune their pitches, eventually reaching that beautifully discordant 10-octave-spanning melody we witness in local galaxies. The fact that black holes and galaxies can “communicate” at all is astonishing. Despite their colossal power, black holes are minuscule in comparison to their galactic homes. If you imagine the event horizon of a typical supermassive black hole being the length of an ant, then its home galaxy’s radius would barely fit in the space between New York City and Los Angeles. That is, these “ants” are somehow influencing a surrounding region equivalent to the continental U.S. Scientists know the “communication channel” between these disparate realms involves the immense energy central black holes release, which effectively regulates the rate at which the galaxies form new stars and sets the 1-to-1,000 black hole/galaxy mass ratio. But many crucial details remain unknown about this chaotic and enigmatic mechanism.
Why does this ratio not rule in the early universe? Why was the music so different back then? The implications of these questions are rippling through the very foundations of astrophysics as, for the first time with JWST, we observe the early moments of black hole formation and co-evolution with galaxies. Some recent studies argue that these observations provide the best evidence yet that some black holes were born massive. That is, unlike most black holes, which are relatively featherweight remnants left behind by dying massive stars, these must have formed from the direct collapse of giant gas clouds that filled the early universe, leading to leviathans that, from the start, are far more massive than any star. If true, this increasingly well-informed speculation would finally answer one of astrophysicists’ classic “chicken-or-egg” problems: Did the central supermassive black holes form first, or did their host galaxies? So far, JWST’s observations suggest that a heavy black hole came first, with galactic star formation kicking off later.
To conclude, why are we discovering this surprising property of early-universe galactic systems now? The answer is that we have only just gained the necessary “electronic eyes” to see them: JWST excels at seeing cosmic objects that are not only far away but also faint. It can uniquely study more youthful black holes breathtakingly close to the cosmic dawn of time, ones weighing millions instead of billions of solar masses like the gargantuan heavyweights other telescopes have for generations routinely observed. The smaller the black hole, the fainter the light it radiates away. The “lighter supermassive” black holes that JWST now detects cannot outshine their host galaxy, as was the case for the larger ones researchers have become more familiar with. Hence, for these smaller, younger objects, we can see the starlight belonging to their host galaxies while also measuring their mass. JWST’s unprecedented power has brought us to the cusp of solving one of astrophysics’ most profound mysteries—the origins of galaxies and their supermassive black holes—by simply allowing us, as Dante wrote, to see, once more, the stars.
This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.