Dark energy, the enigmatic force propelling the speeding expansion of the cosmos, may not truly exist, researchers assert. Their investigations have cast doubt on one of the fundamental principles of contemporary cosmology.
In a recent research paper, published on Dec. 19, 2024 in the journal Monthly Notices of the Royal Astronomical Society, the scientists examined data from the Pantheon+ survey—the most extensive dataset of type Ia supernovae, whose uniform brightness enables astronomers to accurately gauge distances throughout the universe. Their findings indicate that what we interpret as acceleration could be an illusion stemming from the large-scale architecture of the universe.
Investigating the universe with type Ia supernovae
Type Ia supernovae, the violent end of white dwarf stars, have long been one of the most potent instruments in cosmology. These celestial occurrences transpire when a white dwarf absorbs sufficient mass from a companion star, initiating a thermonuclear detonation. Due to the consistent peak brightness produced by type Ia supernovae, measuring their luminosity from Earth can reveal their distance.
“Type Ia supernovae are incredibly advantageous in astronomy as they function as standard candles for measuring immense distances in the cosmos,” study co-author Zachary Lane, a researcher at the University of Canterbury in New Zealand, communicated via email to Live Science.
By merging this distance data with the redshift of the supernovae—the elongation of light towards redder wavelengths due to the universe’s expansion—researchers have mapped the universe’s development over time. Years ago, scientists utilized this technique to demonstrate that the universe’s expansion was accelerating, a finding that led to the hypothesis of dark energy—a perplexing, unseen force believed to permeate space and drive this acceleration.
Pantheon+ dataset
The Pantheon+ dataset represents the most extensive and precise compilation of type Ia supernovae ever gathered. Covering decades of observations from both ground-based and space telescopes, it consists of data on 1,500 supernovae throughout space-time.
“At the time of this research, the Pantheon+ Type Ia Supernovae spectroscopic dataset was the largest and cleanest collection of purely Type Ia supernovae,” Lane stated.
The precision and magnitude of the dataset create an invaluable resource for evaluating cosmological theories. Its comprehensive records of brightness and redshift yield unmatched insights into the evolution of the universe, serving as a crucial testing arena for alternative hypotheses to the conventional cosmological model.
Questioning dark energy
While the concept of dark energy accounts for much of the observed acceleration in the cosmos, it has always been enshrouded in mystery. Dark energy has never been directly observed, nor has its origin been theoretically clarified, leading some researchers to investigate alternative explanations.
The recent study targets a crucial presumption of the standard model: that the universe is homogeneous and isotropic on large scales, meaning that it appears identical in all directions and from all perspectives.
This premise underpins the necessity for dark energy to elucidate the universe’s expansion. However, Lane and his collaborators explored an alternative proposition known as the timescape model, which posits that the observable acceleration may be a byproduct of cosmic structures like voids—vast, nearly empty expanses of space in between galaxy clusters.
“The standard cosmological model is founded on the assumption that the universe is uniform and devoid of features on large scales and that cosmic structures do not materially influence the evolution of the universe,” Lane explained. “Timescape discards these assumptions and determines that the apparent acceleration of the universe is the consequence of the interaction between cosmic structures.”
Due to their sparse matter content and weaker gravitational influence, voids expand more rapidly than denser regions of the universe, such as galaxy clusters. As proposed by the timescape model, the prevalence of these voids in the cosmic setting could elucidate the perceived acceleration without invoking dark energy.
Evidence supporting timescape
The research team examined the Pantheon+ dataset and discovered that their findings correlated exceptionally well with the timescape model—and in certain instances, even surpassed the standard cosmological model.
“When evaluating all supernovae, including those very near to us in the Milky Way, which may be influenced by local structures, we find a very strong preference for the Timescape model,” Lane stated. By excluding supernovae in the proximate universe to adjust for local discrepancies, the evidence remained consistent, in harmony with observations from the Dark Energy Survey (DES).
These findings challenge the essentiality of dark energy. “Consistently discovering moderate or stronger support for a cosmological model absent dark energy using one of the most historically notable observational methods presents an exhilarating opportunity for the future of cosmology,” Lane remarked.
The path forward
Though the results are intriguing, Lane emphasized that additional investigation is required to bolster the case for timescape. “While other factors must be considered for this to gain more traction within the cosmological community, it represents a promising preliminary test,” he noted.
In the future, the team intends to integrate the Pantheon+ dataset with data from the Dark Energy Survey and baryon acoustic oscillations—patterns in the arrangement of galaxies that can serve as another cosmic ruler. The astronomers are also undertaking simulations to understand how voids expand within the framework of general relativity and examining how these phenomena relate to galaxy formation and evolution.
“Our research group is investigating several extensions to our current work, aiming to challenge fundamental aspects of cosmology,” Lane stated. “A robust competing framework will continue to enhance the future of cosmology and our present comprehension of the challenges confronting the field.”