Primitive dark energy could solve cosmology’s two biggest puzzles

A new study by MIT physicists suggests that a mysterious force known as primitive dark energy could solve two of cosmology’s biggest puzzles and fill some major gaps in our understanding of the evolution of the early universe.

One of the puzzles in question is the “Hubble tension,” which refers to an inconsistency in measurements of the speed of the universe’s expansion. The other concerns observations of many bright, old galaxies that existed at a time when the early universe should have been much less populated.

The MIT team found that both of these puzzles could be solved if the early universe had an additional, fleeting ingredient: primitive dark energy. Dark energy is an unknown form of energy that physicists believe is driving the universe’s current expansion.

Primitive dark energy is a similar hypothetical phenomenon that may have only made a brief appearance, influencing the expansion of the universe in its early moments before disappearing completely.

Some physicists have suspected that primitive dark energy might be the key to solving the Hubble tension, because this mysterious force could accelerate the initial expansion of the universe by an amount that would solve the measurement mismatch problem.

MIT researchers have now discovered that primitive dark energy could also explain the baffling number of bright galaxies that astronomers have observed in the early universe. In their new study, published in the Monthly Notices of the Royal Astronomical SocietyThe team modeled the formation of galaxies during the first few hundred million years of the universe.

When they incorporated a dark energy component into just this oldest time slice, they found that the number of galaxies that emerged from the primordial environment increased to match astronomers’ observations.

“We’re faced with two open puzzles,” says Rohan Naidu, a co-author of the study and a postdoctoral fellow at MIT’s Kavli Institute for Astrophysics and Space Research. “We found that primitive dark energy is a very elegant, low-tech solution to two of the most pressing problems in cosmology.”

Co-authors of the study include Kavli’s senior author and postdoctoral fellow Xuejian (Jacob) Shen and MIT physics professor Mark Vogelsberger, as well as Michael Boylan-Kolchin of the University of Texas at Austin and Sandro Tacchella of the University of Cambridge.

Lights of the big city

According to classical models of cosmology and galaxy formation, the Universe would have taken a long time to spin the first galaxies. It would have taken billions of years for the primordial gas to gather into galaxies as large and bright as the Milky Way.

But in 2023, NASA’s James Webb Space Telescope (JWST) made a surprising observation. With the ability to look farther than any other observatory to date, the telescope discovered a surprising number of bright galaxies as large as the modern Milky Way during the first 500 million years, when the universe was only 3% of its current age.

“The bright galaxies observed by JWST would be comparable to a cluster of lights around large cities, whereas theory predicts something similar to the light around more rural areas like Yellowstone National Park,” Shen says. “And we don’t expect such a cluster of light this early.”

For physicists, these observations imply either that there is something fundamentally wrong with the physics underlying the models, or that there is a missing ingredient in the early universe that scientists have not accounted for. The MIT team investigated the latter possibility and looked at whether the missing ingredient might be early dark energy.

Physicists have suggested that primitive dark energy is a kind of antigravitational force that kicks in very early on. This force would counteract gravitational attraction and accelerate the early expansion of the universe in a way that would solve the measurement mismatch problem. Primitive dark energy is therefore considered the most likely solution to the Hubble tension.

Galaxy Skeleton

The MIT team investigated whether early dark energy could also explain the unexpected population of large, bright galaxies detected by JWST. In their new study, the physicists looked at how early dark energy might affect the early structure of the universe that gave rise to the first galaxies. They focused on the formation of dark matter halos, regions of space where gravity is stronger and matter begins to accumulate.

“We think of dark matter halos as the invisible skeleton of the universe,” Shen says. “Dark matter structures form first, and then galaxies form inside these structures. So we expect the number of bright galaxies to be proportional to the number of large dark matter halos.”

The team developed an empirical framework for early galaxy formation that predicts the number, brightness, and size of galaxies that should have formed in the early universe, based on measurements of “cosmological parameters.” Cosmological parameters are the basic ingredients, or mathematical terms, that describe the evolution of the universe.

Physicists have determined that there are at least six main cosmological parameters, including the Hubble constant, a term that describes the expansion rate of the universe. Other parameters describe density fluctuations in the primordial soup, immediately after the Big Bang, from which dark matter halos eventually form.

The MIT team reasoned that if early dark energy affects the early expansion rate of the universe in a way that resolves the Hubble tension, then it could affect the balance of other cosmological parameters in a way that could increase the number of bright galaxies that appear in the early stages.

To test their theory, they incorporated a primitive dark energy model (the same one that solves the Hubble tension) into an empirical framework of galaxy formation to see how early dark matter structures evolve and give rise to the first galaxies.

“What we’re showing is that the skeletal structure of the early universe is altered in subtle ways, with increased amplitudes of fluctuations, larger halos, and brighter galaxies that formed at older times than in our more classical models,” Naidu says. “That means things were more abundant and more clustered together in the early universe.”

“I wouldn’t have expected the abundance of early bright galaxies in JWST to have anything to do with early dark energy, but their observation that the EDE pushes the cosmological parameters in a direction that increases the abundance of early galaxies is interesting,” says Marc Kamionkowski, a professor of theoretical physics at Johns Hopkins University who was not involved in the study.

“I think more work is needed to establish a link between the early galaxies and the EDE, but whatever the outcome, it’s a smart move – and I hope it’s fruitful.”

“We have demonstrated the potential of primitive dark energy as a unified solution to two major problems facing cosmology. This could be a proof of its existence if the observational results from JWST are further consolidated,” Vogelsberger concludes.

“In the future, we can integrate this into large cosmological simulations to see what detailed predictions we get.”

This article is republished with kind permission from MIT News (web.mit.edu/newsoffice/), a popular site covering the latest research, innovation, and teaching at MIT.