Physicists suggest that an enigmatic force called early dark energy may help address two significant mysteries in cosmology and enhance our insights into the development of the early universe.
A groundbreaking study from MIT physicists puts forth the idea that an elusive force termed early dark energy could provide solutions to two critical issues in cosmology and bridge some significant gaps in our comprehension of how the early universe unfolded.
A key issue under consideration is the “Hubble tension,” which highlights the discrepancies in values measured for the rate of the universe’s expansion. The second issue involves the discovery of various bright galaxies that appeared in a period when the universe should have been less densely populated.
The MIT team suggests that both of these issues could find resolution if the early universe featured an additional, brief ingredient: early dark energy. This mysterious form of energy is believed by physicists to be responsible for the current expansion of the universe. Early dark energy is a similar, hypothetical concept that may have had a transient presence, affecting the universe’s expansion during its initial moments before vanishing completely.
Some researchers have theorized that early dark energy could possibly resolve the Hubble tension, allowing for an acceleration of cosmic expansion that would reconcile the discrepancies in measurement.
The MIT group has also discovered that early dark energy could clarify the puzzling abundance of bright galaxies identified in the early universe. Their recent study, published in the Monthly Notices of the Royal Astronomical Society, involved modeling how galaxies formed during the universe’s first few hundred million years. By adding a dark energy component to just that earliest phase, they observed a significant increase in the number of galaxies that emerged from the initial conditions, aligning with astronomers’ findings.
“We have these two major unresolved issues,” states co-author Rohan Naidu, a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research. “Our findings suggest that early dark energy provides a remarkably straightforward and elegant answer to both of these pressing cosmological problems.”
The study includes contributions from lead author and Kavli postdoc Xuejian (Jacob) Shen, along with MIT physics professor Mark Vogelsberger, Michael Boylan-Kolchin from the University of Texas at Austin, and Sandro Tacchella at the University of Cambridge.
City Lights Effect
Standard cosmological and galaxy formation theories indicate that it should have taken considerable time—billions of years—for the primordial gas to gather into galaxies as substantial and luminous as the Milky Way.
However, in 2023, NASA’s James Webb Space Telescope (JWST) made an astonishing discovery. With an unprecedented ability to observe the universe’s early stages, the telescope revealed a surprising number of bright galaxies comparable in size to today’s Milky Way within just the first 500 million years, a mere 3 percent of the current age of the universe.
“The bright galaxies that the JWST detected resemble seeing clusters of lights around major cities, in stark contrast to predictions that suggest a sparse lighting scenario like that found in more rural areas, such as Yellowstone National Park,” recounts Shen. “The clustering of lights observed is unexpected at such an early stage.”
For physicists, these observations suggest either a fundamental flaw in the existing physical models or a crucial missing element from early cosmic history. The MIT team focused on the latter possibility, considering whether early dark energy might be that missing ingredient.
Researchers theorize that early dark energy acts as a form of antigravitational force, operational only during the early universe. This force negates gravity’s inward pull and accelerates the universe’s early expansion, potentially resolving the existing measurement discrepancies. Early dark energy is therefore viewed as a strong candidate to address the Hubble tension.
Framework for Galaxies
The MIT researchers investigated whether early dark energy might also elucidate the unexpected volume of large, bright galaxies identified by JWST. In their recent work, they deliberated on how early dark energy could shape the primordial structure of the universe, leading to the formation of the first galaxies. Their focus was on the development of dark matter halos—regions with concentrated gravitational forces where matter starts to gather.
“We consider dark matter halos to be the unseen framework of the universe,” explains Shen. “These structures emerge first, providing the basis for galaxy formation. Consequently, we anticipate that the quantity of bright galaxies will correspond with the number of significant dark matter halos present.”
The team created an empirical model for early galaxy formation, which predicts the abundance, luminosity, and size of galaxies that should form in the universe’s early stages based on various “cosmological parameters.” These parameters are fundamental elements or mathematical expressions that outline the evolution of the universe.
Researchers have established that there are at least six essential cosmological parameters, including the Hubble constant, which describes the universe’s expansion rate. Other parameters account for density variations in the universe’s primordial state immediately following the Big Bang, from which dark matter halos can form.
The MIT team posited that if early dark energy indeed influences the universe’s growth rate early on, thereby addressing the Hubble tension, it could also adjust the balance of other cosmological parameters, resulting in a higher number of early bright galaxies. To explore this, they integrated a model of early dark energy, which also resolves the Hubble tension, into an empirical framework for galaxy formation to observe how the earliest dark matter organizations develop and give rise to initial galaxies.
“Our findings demonstrate that the foundational structure of the early universe is subtly modified, increasing the amplitude of fluctuations, leading to larger halos and brighter galaxies appearing at earlier points than predicted by standard models,” explains Naidu. “This indicates a more abundant and densely clustered early universe.”
“Initially, I wouldn’t have anticipated that the abundance of bright early galaxies observed by JWST would link directly to early dark energy, but their finding that EDE influences cosmological parameters to enhance early-galaxy formation is intriguing,” remarks Marc Kamionkowski, a theoretical physics professor at Johns Hopkins University, who was not associated with the study. “Further efforts will be essential to solidify the connection between early galaxies and EDE, but regardless of the outcome, this approach is clever and promising.”
“We illustrated the potential significance of early dark energy as a combined answer for two major dilemmas in cosmology. This may serve as evidence of its existence if subsequent observational data from JWST supports these findings,” concludes Vogelsberger. “In the future, we plan to integrate this knowledge into comprehensive cosmological simulations to generate more precise predictions.”
This research received partial support from NASA and the National Science Foundation.
