Scientists detect mysterious suppression in cosmic structure growth

A new study in published in Physical Review Letters analyzes the most complete set of galaxy clustering data to test the ΛCDM model, revealing discrepancies in the formation of cosmic structures in the universe, hinting at a new physics.

The ΛCDM model is the standard model of cosmology describing the universe’s evolution, expansion, and structure. It encompasses cold dark matter (CDM), normal matter and radiation, and the cosmological constant (Λ), which accounts for dark energy.

The model has been successful in explaining several cosmological observations, including the large-scale structure of the universe, the accelerating expansion of the universe, and the cosmic microwave background (CMB) radiation, which is the afterglow of the Big Bang.

Despite this, ΛCDM fails to account for phenomena like cosmic inflation, dark energy, and dark matter. Recent observations, like data from DESI (Dark Energy Survey Instrument), have suggested potential anomalies in ΛCDM.

The research team aimed to analyze whether these anomalies might be connected and could point to a specific new physical model.

The team consisted of Dr. Shi-Fan Chen, from the Institute for Advanced Study, New Jersey; Prof. Mikhail Ivanov, from the Massachusetts Institute of Technology; Dr. Oliver Philcox, from Columbia University; and Lukas Wenzl, a graduate student at Cornell.

Speaking of the motivation behind their work, Dr. Chen said, “Being able to predict anything about the universe is cool, but what’s especially neat is that we have many different observables from many surveys whose measurements we can model using one consistent effective theory.”

Connecting the cosmic dots

As mentioned, the ΛCDM model fails to account for certain phenomena based on recent observations.

These include the disagreement between direct and indirect measurements of the expansion rate of the universe (the Hubble tension), the disagreement between the direct and indirect measurements of matter clustering, i.e., structure growth (The σ₈ tension), and recent DESI data suggesting possible evidence for dynamical dark energy.

The research team’s approach is new because they want to see if the same underlying physics could explain these anomalies. To test the hypothesis, the researchers combined measurements from multiple sources to create a comprehensive data set.

This included the BOSS (Baryon Oscillation Spectroscopic Survey) DR12 dataset with northern and southern galactic caps, LOWZ (low-redshift galaxies) and CMASS (high-mass galaxies) samples covering different redshift ranges, and cross-correlation with Planck CMB gravitational lensing maps.

This data was analyzed in two settings, within the standard ΛCDM model and within a dynamical dark energy model to test DESI’s findings.

Dr. Philcox explained how they maintained high accuracy of the chosen data. “We really tried to pick consistent definitions for galaxy samples, throwing out parts of the available data with accidental mistakes in the selection criteria, at the expense of our statistical constraints even when past analyses have used these data.”

“In addition, we performed many tests on the cross-correlations with CMB lensing as part of previous papers to make sure that no obvious systematics were there.”

A universe growing too slowly?

The ΛCDM analysis revealed a slightly lower growth rate of cosmic structures than predicted, showing a significant disagreement (4.5σ tension) with Planck’s results.

Additionally, it confirmed existing values for matter density, the Hubble constant, and structure growth.

In the dynamical dark energy analysis, the team found no strong evidence for dynamical dark energy, suggesting that dark energy behaves like a cosmological constant. The suppression of structure growth observed is similar to those predicted by the ΛCDM analysis.

Finally, the value of the Hubble constant aligns with Planck data but disagrees with direct, local measurements.

Prof. Ivanov explained, “We found that the structure formation in the late universe where the effects of dark energy are most pronounced, at least as measured by galaxies in the BOSS survey, seems substantially suppressed compared to expectations from the early universe and the CMB.”

“This is true even when we allow the expansion history to deviate from the standard cosmological constant form of dark energy.”

New physics or errors in data

According to the team, the odds of suppressed structure growth being a random chance is 1 in 300,000, which strongly suggests that something unexplained is happening in the form of unknown systematics in the data or new physics.

The findings also provide the strongest evidence to date for the σ₈ tension and show that dynamical dark energy cannot resolve it.

Wenzl explained, “In addition to new observational techniques and systematics tests mentioned, if this signal survives, it will be interesting to see what kinds of new physics can help resolve the tension with the CMB.”

“For example, it would be very cool if non-standard dark matter candidates like axionic dark matter or dark matter that interacts with itself or baryons in some way, which would alter the formation of structure, can explain the signal.”

The study’s findings challenge our understanding of cosmic structure formation and, more importantly, one of the most fundamental models in cosmology.

Data from upcoming galaxy surveys will provide clarity on these discrepancies, and if we need a fundamental change in our understanding of large-scale structures in the universe.

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