The universe is expanding, but how quickly this is happening and what role dark energy plays in this is controversial. Now one of the most comprehensive projects to measure cosmic expansion, the Dark Energy Survey (DES), has presented its final results. Astronomers measured the redshift and distance of almost 1,500 type 1a supernovae using a new, computer-aided method. The results are within the range of the standard cosmological model of Lambda Cold Dark Matter (ΛCDM). But there is a deviation that leaves it open whether the density of dark energy is really constant, as this model suggests.
Since the Big Bang, our universe has been expanding ever further - this was already postulated by the “father” of the Big Bang theory, Georges Lemaître. However, astronomers and cosmologists initially assumed that this expansion must slow down over time. It was only in 1998 that two research teams, while measuring the distance of distant supernovae, discovered that this cannot be true: the cosmic expansion is accelerating. This revolutionary and cosmologically significant discovery earned them the Nobel Prize in Physics in 2011. At the same time, it caused physics a need for explanation, because in order to enable accelerated expansion, there must be a form of energy in the cosmos that counteracts gravity and serves as a driving force for the expansion. However, what exactly this energy is like and how it behaves remains unclear to this day. According to common assumptions, this “dark energy” makes up around 70 percent of our universe. Dark energy also forms the basis for the current cosmological standard model of the so-called Lambda Cold Dark Matter (ΛCDM).
DES: Supernovae as a measure of cosmic expansion
The problem, however, is that it has not yet been clearly clarified whether the expansion and dark energy really behave as the ΛCDM suggests. One of the reasons for this is discrepancies in the measurement of the cosmic expansion rate. To clarify this, the Dark Energy Survey (DES) was initiated. This association of 400 researchers from around 25 research institutions and numerous countries aimed to measure dark energy and dark matter and the expansion of the cosmos with the highest possible accuracy. The basis for this is the explosions of white dwarfs in type 1a supernovae. These supernovae have a luminosity that can be easily standardized and are therefore particularly suitable for determining their distance and, using the red shift, the speed of their movement away from us - and thus the expansion of the cosmos.
Using the Dark Energy Camera on the four-meter telescope at the Cerro Tololo Inter-American Observatory in Chile, astronomers as part of the DES program spent six years surveying one-eighth of the entire sky and measuring the distances and redshifts of nearly 1,500 Type 1a supernovae mapped. “This is a massive expansion compared to the only 52 supernovae used 25 years ago,” says co-author Tamara Davis from the University of Queensland in Australia. The astronomers used four different methods to detect and measure the stellar explosions, including the one already used in 1998, but also a newly developed method that enables precise measurements not via the light spectrum, but via photometric measurements of the light curves. Learning algorithms helped with classification and evaluation.
Values close to target, but scope for new things
Now the astronomers in the DES collaboration have published their final results. They provide further evidence as to whether the cosmological standard model is correct or whether there are still processes and influences in the universe that have not yet been recognized and described. In this context, the question of one of the core predictions of the current theory is crucial: “As the universe expands, the density of matter decreases,” explains DES spokesman Rich Kron from the University of Chicago. “But according to the model, the density of dark energy is a constant. This means that their proportion must increase as the volume of the cosmos increases.” In physical terms, the parameter for the density of dark energy w specified by the ΛCDM model must therefore be 1.
But what did the DES measurements reveal? Based on the supernova measurements alone, the astronomers came to a value of w = -0.80 ± -0.18 - and therefore slightly less than -1. When the measurement results are combined with those of three other surveys, including the measurement of the cosmic background radiation with the Planck satellite, the values for the density of dark energy are in the range of -1 with less than two standard deviations. “w is not exactly equal to 1, but close enough to be consistent with the model,” explains Davis. But this slight discrepancy is exciting because it could also indicate hidden mechanisms. “The simplest model of dark energy, ΛCDM, does not fit perfectly. Although it is not so far away that we can refute it. But the results provide the first fascinating indications that dark energy could change over time. We may need a more complex explanation for what is happening.”
The results are not yet sufficient to show what this explanation might look like and whether the density or influence of dark energy actually changes over the course of the universe's evolution. Nevertheless, the results of the Dark Energy Survey now provide new, more precise data and limits for important cosmological parameters. “These data are now the gold standard of supernova cosmology and will remain so for some time,” says co-author Dillon Brout of Boston University.
Source: DES Collaboration, 243rd meeting of the American Astronomical Society, Astrophysical Journal (submitted), doi: 10.48550/arXiv.2401.02929