In a groundbreaking study published in Physical Review Letters, a team of cosmologists led by Dr. Shi-Fan Chen from the Institute for Advanced Study, New Jersey, delved into the most comprehensive galaxy clustering dataset to date. Their analysis critically tests the ΛCDM model—currently the leading framework that governs cosmological theories about the universe’s evolution—and finds emerging discrepancies that might point to the need for new physics. This study, conducted with collaborators Prof. Mikhail Ivanov (Massachusetts Institute of Technology), Dr. Oliver Philcox (Columbia University), and Lukas Wenzl (Cornell University), opens a new frontier for understanding the formation and growth of cosmic structures. The researchers suggest that anomalies could exist that might fundamentally alter our view of the cosmos.
The ΛCDM model, short for Lambda Cold Dark Matter, is the prevailing cosmological model used to explain the universe’s evolution. It incorporates three main elements: cold dark matter (CDM), dark energy represented by the cosmological constant (Λ), and the regular matter we observe in the universe, alongside radiation. As it stands, the ΛCDM model has had great success in accounting for a variety of cosmic phenomena such as the large-scale structure of the universe, the cosmic microwave background radiation (CMB), and the accelerating expansion of the universe. However, there are phenomena that it cannot fully explain or account for, such as cosmic inflation, the nature of dark matter, and the behavior of dark energy.
The research team’s investigation stems from concerns raised by recent observational data, particularly from the Dark Energy Survey Instrument (DESI), which has pointed to discrepancies within the ΛCDM model. The peculiarities arise in areas like the Hubble tension, the σ8 tension, and some intriguing data from DESI, suggesting unexpected behaviors of dark energy. These findings invite the possibility that something fundamental in our understanding of cosmology may need to be revised.
Speaking about the motivation for their study, Dr. Chen noted the intriguing challenge of using a unified physical theory to predict various cosmic phenomena across diverse datasets. The researchers sought to test if the anomalies reported in these cosmic measurements might all stem from the same underlying physical issue or model. With this goal, they combined data from numerous observational surveys, a critical step in uncovering any potential contradictions between cosmological models and data.
The data analyzed by the team comes from multiple sources. They considered the BOSS (Baryon Oscillation Spectroscopic Survey) DR12 dataset, which includes data from galaxies in both the northern and southern galactic caps, as well as samples of low-redshift galaxies (LOWZ) and high-mass galaxies (CMASS) from various redshift ranges. Additionally, they incorporated cross-correlation data from the Planck mission’s cosmic microwave background (CMB) gravitational lensing maps. Combining these observations into one dataset enabled the team to perform a multifaceted analysis of cosmic structures, testing the predictions made by ΛCDM as well as an alternate model that incorporates dynamical dark energy—a hypothesis that has recently gained interest due to the latest DESI findings.
One of the most striking results the team found was a significant disagreement between the expected and observed rates of structure formation in the universe. In particular, their ΛCDM analysis showed a discrepancy of about 4.5 standard deviations (σ), a finding they referred to as “4.5σ tension.” Essentially, their models predicted that cosmic structures such as galaxies and clusters of galaxies would grow more rapidly than the observations indicated, challenging the ΛCDM model’s capacity to explain structure growth at large scales.
Dr. Oliver Philcox further elaborated on the importance of high data accuracy in this endeavor. “We took great care in selecting galaxy samples and scrutinizing potential errors or systematics in the dataset,” Philcox explained. “Past analyses sometimes included poorly defined galaxy samples, and we made sure our data selections were both accurate and consistent to minimize any statistical errors that could affect the outcome.” The researchers’ effort to produce high-fidelity data allowed them to clearly interpret the discrepancies that arose in their analysis and ensured that they were addressing robust cosmic data, even in the face of complex datasets.
For the team, another key outcome of the research was how the dynamical dark energy model—which adapts the cosmological constant in order to incorporate a more time-dependent behavior of dark energy—did not resolve the discrepancies regarding the growth of cosmic structures. They noted that, although dynamical dark energy was a promising explanation, no definitive evidence could be found in their dataset. More importantly, it seemed unlikely that even this model would successfully explain the suppression of structure formation that the team observed.
In the case of the Hubble constant, the study revealed interesting contradictions. While the team found that measurements based on CMB data (Planck’s results) aligned closely with those derived from their ΛCDM model, their analysis yielded values that were inconsistent with more direct, local measurements of the Hubble constant. This long-standing issue, known as the “Hubble tension,” suggests that there may be an as-yet undiscovered component in the physical models explaining the expansion rate of the universe.
Prof. Ivanov explained their finding that in the late universe, where dark energy’s effects should be most prominent, structure formation appeared markedly suppressed compared to what ΛCDM would predict. He noted, “Our analysis of late-time structure growth, specifically using galaxies observed in the BOSS survey, presents a suppressed rate of structure formation, which stands in stark contrast to the expectations based on early universe measurements like those from the CMB.”
The results published by the team highlight a significant issue: the observed suppression of structure growth cannot be dismissed as statistical noise, with the team calculating that the likelihood of this effect being a random occurrence stands at only 1 in 300,000. This reinforces their belief that the discrepancy may indicate something deeper—whether it be a failure of the ΛCDM model to accurately describe all aspects of the universe, or evidence of new physics waiting to be discovered.
Lukas Wenzl offered some speculation regarding possible avenues for exploration: “The idea that different forms of dark matter, such as axionic dark matter or other forms that interact differently from the conventional cold dark matter, could be altering structure formation is a compelling direction to explore. If these new dark matter models interact in novel ways with baryons, the resulting changes in cosmic structure growth could very well explain some of the signals we are observing.”
The study thus raises important questions about our understanding of the universe, highlighting that there could be unknown factors influencing the growth and behavior of cosmic structures. The implications of this study are broad. If these findings survive further scrutiny and confirmation from additional data, they may suggest that the current ΛCDM model, while highly successful in many respects, does not offer the full explanation for cosmic structure formation and expansion. As a result, this could point to the need for substantial revisions in fundamental cosmological theories or even the introduction of entirely new models that account for observed anomalies.
Indeed, these findings contribute to an increasing body of research suggesting that our comprehension of the universe’s early and late evolution remains incomplete. The data from future galaxy surveys and cosmic probes will likely offer further insights into these discrepancies, and possibly provide the necessary tools for a paradigm shift in how we conceive of the universe’s large-scale structure and the forces that drive its expansion. For now, one thing is clear: cosmology’s most fundamental models are still being tested and refined, and the universe might have more secrets to reveal than we ever thought.
Reference: Shi-Fan Chen et al, Suppression without Thawing: Constraining Structure Formation and Dark Energy with Galaxy Clustering, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.231001. On arXiv: DOI: 10.48550/arxiv.2406.13388