A Cosmic Twofer: “Early” Dark Energy Could Explain Anomalous Galaxies and the Universe’s Disjointed Expansion Rate

Shortly after launch a couple years ago, JWST, NASA’s latest space telescope, stared deeper back into cosmic history than ever possible, seeking the universe’s earliest galaxies. What came into view gobsmacked researchers. While established models predicted primeval galaxies would be faint and fledgling, JWST revealed big, bright, developed galactic entities.

To help explain these paradigm-busting behemoths, scientists at the Massachusetts Institute of Technology’s Kavli Institute for Astrophysics and Space Research (MKI) have radically repurposed a seemingly unrelated theory. That theory was originally floated over a decade ago to address the “Hubble tension”—astronomer-speak for the yawning gap in measured rates of cosmic expansion in the distant, early universe versus the local, mature universe. The theory posits that a special kind of “dark energy” operated in the young universe, altering its expansion rate.

In a recent study, the MKI researchers have innovatively shown that this early-acting dark energy can also set the stage to yield JWST’s big galaxies. In this way, the MKI team has managed to hit two cosmic birds with one stone, potentially paving the way for dually resolving the precocious galaxy and Hubble tension conundrums.

“Our study shows that these two major puzzles that have really been befuddling us might be pointing towards the same solution,” says study co-author Rohan Naidu, a postdoctoral researcher at MKI.

“The early dark energy scenario becomes very interesting because it does give us a way to explain the craziest observations found by JWST, while also addressing the Hubble tension,” says lead author Xuejian (Jacob) Shen, also an MKI postdoc.

The idea of early dark energy derives from the discovery of the broader phenomenon dubbed dark energy, discovered in 1998. Two teams of researchers using JWST’s predecessor, the Hubble Space Telescope, independently reported then that the expansion rate of the universe has apparently accelerated over its roughly 14 billion years of existence. Scientists have since attributed this acceleration to a gravity-opposing energy field which, according to extensive research, stunningly comprises the bulk of the universe’s composition. Mapping how dark energy has possibly changed over the evolution of the universe, perhaps as an early-acting dark energy that spawned the Hubble tension, has served as a key objective in modern cosmology.

Neither Shen nor Naidu had been particularly versed in dark energy theories as they started contending with JWST’s eyebrow-raising galactic findings. Naidu worked on the front lines of the first observational studies from JWST that brought these confounding galaxies to light. After initial being disbelieved by the astronomical community, the galaxies have become increasingly entrenched.

“From the observational side, we've gotten much more detailed data on them, and it is now very clear that these galaxies are a bona fide class of galaxies,” says Naidu. “They exist in numbers that really are very uncomfortable for all the models that we had before JWST.”

Those models indicate that the galaxies unearthed by JWST should have taken billions of years to develop, rather than the mere five hundred million years after the Big Bang when JWST spied them. Shen approached this quandary from the theoretical end, seeking to devise plausible new models that, then run as computer simulations, could produce JWST’s whopper galaxies. In this endeavor, Shen and Naidu collaborated with Mark Vogelsberger, a fellow MKI member, as well as Sandro Tacchella, an astrophysicist at the Kavli Institute for Cosmology, Cambridge at the University of Cambridge.

The researchers wondered if the early dark energy theories crafted for addressing the Hubble tension could influence galactic formation models in such a way that big galaxies would have an easier time emerging in the cosmic dawn. Specifically, the researchers examined how the repulsive force of early dark energy would impact the rest of the universe’s contents.

Most of that remaining contents is a theoretical substance known as dark matter. As its name implies, dark matter is essentially invisible, and thought to interact with “normal” matter—the familiar kind composing people, planets, and stars—only via the force of gravity. Although dark matter has frustratingly eluded direct detection, its gravitational influence is writ large on the cosmos. Most prominently, dark matter appears to overwhelmingly account for the mass associated with galaxies. In cosmological models, dark matter serves as a sort of scaffolding for normal matter to glom onto in assembling galaxies.

Dark matter is thought to be one of the few, fundamental, critical ingredients in the cosmic stew for forming galaxies. Researchers generally feel that they have a solid understanding of how cosmic structures, including galaxies, form under its influence. Because the predictions from standard cosmological models do not agree with what researchers are now seeing galaxy-wise in the early universe, some additional ingredients—like early dark energy—may be needed to satisfactorily form galaxies.

When the MKI researchers went ahead and incorporated a Hubble tension-resolving early dark energy into their young universe models, the upshot was encouraging. The repulsive force increased clustering of dark matter into bigger dark matter “haloes”—the term for the collections of dark matter that concentrate matter into forming galaxies—thus yielding JWST-esque, upstart galaxies. “Bringing in early dark energy really changed what the models were showing us,” says Shen.

The researchers are excited because the early dark energy idea can be tested experimentally by continuing to learn more about the galactic populations and their properties at the edge of JWST’s incisive view. Such investigations will concomitantly put a solution to the Hubble tension through its paces as well.

“Because of early dark energy’s potential connection to the abundance of early, bright galaxies, we can actually now study it and constrain it using the growing data that we have about these galaxies at the cosmic frontier,” says Shen. “We'll keep pushing the frontier on detecting more early luminous galaxies, which can test if early dark energy really works as a solution for both the Hubble tension and these galaxies’ unexpected appearances.”

Narrowing down the possibilities and properties of early dark energy could also offer insight into its cousin, “regular” dark energy, helping to solve still another of the grandest mysteries in astrophysics and cosmology.

“With these distant galaxies, we can study beyond standard cosmological scenarios using unanticipated original data,” says Shen.

“This is why we build our big telescopes—so that we cannot just confirm that all our theories are correct, but more excitingly, we can figure out where our theories break down,” says Naidu. “The possibly elegant answer we offer in this study that would solve these two main challenges in one shot gives the community something concrete to target and motivates something potentially huge.”