Dark Energy is a mysterious substance responsible for the accelerated expansion of the universe. Since its discovery two decades ago, researchers have been trying to work out its properties. One assumption has been that dark energy is constant, but new evidence suggests that this might not be the case.
The expansion of the universe has been measured using the Cosmic Microwave Background (CMB), the so-called “echo of the Big Bang”, around 13 billion years ago, and by using a certain type of supernovae in the “more local” universe, which can trace the expansion up to 8 billion years ago. In a new study published in Nature Astronomy, researchers have used quasar observations as “standard candle” cosmic milestones to measure how the universe has changed over time. Quasars, which can measure up to 12 billion years ago, help fill a gap in the previous observations and from their data, the researchers got a hint that things are not what they had assumed.
"When we combine the quasar sample, which spans almost 12 billion years of cosmic history, with the more local sample of type-Ia supernovas, covering only the past eight billion years or so, we find similar results in the overlapping epochs," co-lead author Dr Elisabeta Lusso, from the University of Durham, said in a statement. "However, in the earlier phases that we can only probe with quasars, we find a discrepancy between the observed evolution of the Universe and what we would predict based on the standard cosmological model."
The result indicates a possible tension in the standard model of cosmology, the system that includes dark energy as well as dark and regular matter. So the researchers used a model where dark energy is allowed to change over the ages of the universe, and if this mysterious substance is allowed to increase it fits better with the data.
This alternative model has another winning point. It might solve another disagreement in cosmology, this one related to the expansion rate of the universe. Different values have been found for that rate depending on if it's estimated using supernovae or the CMB. A recent study using quasars independently showed that this disagreement is unlikely to be a fluke of the data.
"This model is quite interesting because it might solve two puzzles at once, but the jury is definitely not out yet and we'll have to look at many more models in great detail before we can solve this cosmic conundrum," added co-lead author Professor Guido Risaliti, from the Università di Firenze.
Additional clues will come from upcoming missions, including ESA’s Euclid which is scheduled for 2022. Observing standard candles in the early universe will hopefully put constraints on the most correct model of the cosmos.