Cosmic Survey Suggests Evolution of Dark Energy

A comprehensive survey of the cosmos is uncovering fresh insights into one of the most enigmatic aspects of the universe: dark energy. Intriguingly, when juxtaposed with other observations, the data suggest that dark energy, conventionally believed to maintain a consistent density over time, might actually undergo evolution alongside the cosmos.

This outcome has sparked significant interest within the cosmology community, as expressed by physicist Daniel Scolnic of Duke University, who was not involved in the recent study.

Dark energy, an elusive phenomenon responsible for the accelerating expansion of the universe, remains poorly understood despite constituting the majority of the universe’s contents. To delve deeper into this conundrum, the Dark Energy Spectroscopic Instrument (DESI) has generated the most extensive 3-D map of the universe to date, as detailed in a series of ten papers released on today, April 4 on the DESI website and during presentations at a meeting of the American Physical Society in Sacramento, California. By analyzing the spatial distributions of galaxies and other celestial objects on this map, scientists can elucidate the historical trajectory of cosmic expansion.

While the data align with the conventional cosmological framework, they also allow for the prospect that the equation of state of dark energy, which describes its pressure in relation to its density, may vary over time. Such a scenario would necessitate a reevaluation of scientists’ understanding of the universe’s evolution.

The study scrutinizes the inaugural year of data from the project, pinpointing the locations of 6.4 million galaxies and quasars, the intensely luminous cores of active galaxies. This mapping enables scientists to estimate the rate of universal expansion by leveraging a convenient size reference inherent in the universe. Early universe sound waves established density patterns, known as baryon acoustic oscillations, which persist as galaxies form, resulting in preferred distances between galaxies. These distances serve as a cosmic ruler, stretched by the universe’s expansion, and measuring variations in its size across different cosmic eras elucidates the universe’s expansion history.

Scientists divided the universe into seven epochs spanning up to 11 billion years ago to measure its expansion over time. Recent time segments corresponded to distinct types of galaxies, while for earlier epochs, distant quasars and measurements of hydrogen gas absorption of their light were utilized due to the faintness of galaxy light at those distances.

The DESI researchers compared their data with the standard cosmological theory, lambda CDM, which posits a constant density for dark energy over time. While the data aligned well with this model, researchers also explored a theory allowing for the variable behavior of dark energy, particularly its equation of state.

The DESI data alone couldn’t definitively ascertain the evolution of dark energy, prompting the team to combine their data with supernova studies to refine the estimate. This combined data indicated a preference for evolving dark energy over a static model, with the significance of this effect surpassing 3 sigma in certain analyses, hinting at potential new physics.

Further data collection, including five more years of observations cataloging millions more galaxies and quasars, will provide clarity on these preliminary findings. DESI physicists aim to unravel the nature of dark energy and investigate deviations from the standard lambda CDM model.