The Universe is not only expanding, but its expansion even seems to be speeding up. What is causing this acceleration, modestly concealed under the term dark energy?
Research conducted as part of the Dark Energy Survey (DES) offers unprecedented results. After six years of observations made with the DECam camera, installed on the Victor M. Blanco telescope, scientists have been able to analyze information concerning hundreds of millions of galaxies. This dataset covers one eighth of the celestial vault and probes epochs dating back billions of light-years.
(Main) the colliding galaxy clusters that form the Bullet Cluster, seen by DECam. (Inset) the Victor M. Blanco telescope, housing DECam.
Credit: CTIO/NOIRLab/DOE/NSF/AURA/ Matsopoulos
To understand this phenomenon, the team combined four distinct approaches. They incorporate the observation of Type Ia supernovae, weak gravitational lensing effects, the distribution of galaxies, as well as baryon acoustic oscillations. This combination of methods offers a more global view of cosmic history.
The collected measurements span the last six billion years. The researchers then compared their results with two major cosmological models: the standard LCDM model, in which dark energy is constant, and the extended wCDM model, where it can vary. The observations are consistent with both frameworks, while notably refining the constraints on the effects attributed to dark energy.
One parameter, however, shows a discrepancy: how matter clusters in the recent Universe. Theoretical expectations, calibrated on measurements of the primordial Universe, do not perfectly match current observations. This discrepancy even appears to be confirmed with the new data, thus raising new questions for cosmologists.
To refine these results, DES plans to merge its data with that of the future Vera C. Rubin Observatory. Its ten-year LSST survey program will observe billions of additional galaxies, providing an even more detailed picture of dark energy. The program director at the NSF specified that Rubin will allow new experiments on the nature of gravity.
Cosmic probes: how to observe expansion
To quantify the expansion of the Universe, astronomers use several complementary techniques. One of the best known relies on Type Ia supernovae. Their standard intrinsic luminosity makes them reliable distance markers, allowing the evolution of the expansion rate to be traced. Their study was, in fact, decisive in highlighting cosmic acceleration.
Another method lies in weak gravitational lensing. When light from a distant galaxy grazes a very massive object, like a galaxy cluster, its path is slightly bent. Examining these distortions allows estimation of the amount of matter, visible or not, located along the line of sight. This helps map the large-scale architecture of the cosmos.
The distribution of galaxies and baryon acoustic oscillations (BAO) are also instruments of choice. BAO are fossil traces of pressure waves that traveled through the primordial Universe, frozen about 380,000 years after the Big Bang. By measuring intergalactic distances, these oscillations serve as a standard to trace expansion.
The combination of these different probes, as exemplified by the work accomplished by the Dark Energy Survey, offers a multidimensional view of the Universe. Each technique has its strengths and weaknesses, but their combination allows cosmological models to be tested with ever greater accuracy.