Scientists just got the clearest picture of the dark universe yet: 'Now the dream has come true'

Scientists have been gifted with a clearer picture of the expansion of the universe and dark energy, the mysterious force driving the acceleration of this expansion, than ever before. This comes courtesy of the analysis of six years' worth of data collected by the Dark Energy Camera (DECam) mounted on the U.S. National Science Foundation Víctor M. Blanco 4-meter telescope.

The data analysed consists of 758 nights of observations of one-eighth of the sky conducted by the Dark Energy Survey (DES) Collaboration between 2013 and 2019, during the deep, wide-area survey of the sky conducted using the 570-megapixel DECam, which recorded information from 669 million galaxies located billions of light-years from Earth.

This analysis represents the first time the four separate methods of studying dark energy have been united as one. The results doubled the strength of the constraints on the effect of dark energy, an essential step toward discovering the true nature of this mysterious force that dominates the universe.

"These results from DES shine new light on our understanding of the universe and its expansion," Regina Rameika, Associate Director for the Office of High Energy Physics in the Department of Energy’s Office of Science, said in a statement. "They demonstrate how long-term investment in research and combining multiple types of analysis can provide insight into some of the universe’s biggest mysteries."

An expanding problem

The first hints of dark energy were uncovered in 1998 when two separate teams of astronomers observed distant supernovas, finding that the further away they were, the faster they were receding away from Earth. That not only confirmed that the universe is expanding as Edwin Hubble suggested a century ago, but shockingly revealed that this expansion is accelerating. Dark energy is the placeholder name given to whatever is driving this acceleration. In the 28 years since that discovery, scientists have determined that dark energy accounts for around 68% of the total energy and matter budget of the cosmos. It has also been discovered that dark energy hasn't always dominated the 13.8 billion-year-old universe in this way; its effect only "kicked in" and overwhelmed the attractive force of gravity at large scales between 3 and 7 billion years ago. These findings have only emphasised the need to understand what dark energy is.

This new analysis considered Type-Ia supernovas, the same type used to first discover dark energy, in addition to three other probes of cosmic structure and expansion. Those other phenomena are so-called weak gravitational lensing, a phenomenon that occurs when light from a background source passes an object of great mass and is curved; the clustering of galaxies; and so-called baryon acoustic oscillations, fluctuations of density in the early universe caused by pressure waves frozen into space around 380,000 years after the Big Bang. "It is an incredible feeling to see these results based on all the data, and with all four probes that DES had planned," DES Collaboration member Yuanyuan Zhang, of NOIRLab, said. "This was something I would have only dared to dream about when DES started collecting data, and now the dream has come true."

Using the data provided by DECam and the techniques described above, the DES team reconstructed matter distribution over the past 6 billion years of cosmic history. They then compared these results against two of the prevailing models of the universe. These are the standard model of cosmology, also known as the Lambda Cold Dark Matter (LCDM) model, in which dark energy is stable over time; and the extended model (wCDM), in which dark energy is allowed to evolve over time.

The DES results conformed well to the LCDM, but also fit nicely with the wCDM.

But there is one parameter that these new results found to be off in comparison to both of these cosmic models: how matter in the modern universe is predicted to cluster based upon measurements of the early universe. These findings not only confirmed that modern galaxies don't cluster as either the LCDM or the wCDM predicts, but the difference between observations and theory became even more pronounced.

The next step for DES will be to combine DECam data with observations of around 20 billion galaxies from the recently completed Vera C. Rubin Observatory when it begins its decade-long Legacy Survey of Space and Time (LSST).

This should present an even clearer picture of the history of the universe and the nature of dark energy.

"DES has been transformative, and the Vera C. Rubin Observatory will take us even further," Chris Davis, National Science Foundation Program Director, said. "Rubin's unprecedented survey of the southern sky will enable new tests of gravity and shed light on dark energy."

The team's research has been submitted to the journal Physical Review D and is available on the paper repository site arXiv.