Dark energy is apparently even more mysterious than astronomers thought. At first, scientists proposed the existence of this invisible force two decades ago to explain the surprising discovery that the expansion of the universe is accelerating .
The most widely used astrophysical model of the structure and evolution of the universe considers dark energy as a constant. In fact, many astronomers believe that it is the cosmological constant, which Albert Einstein postulated in 1917 as part of his theory of general relativity.
But a new study of huge super-bright black holes known as quasars suggests that dark energy could be thought of as the cosmological constant or any kind of constant: its power could have varied since the birth of the universe 13.8 billion years ago, according to scientists. authors in their study published in the journal Nature Astronomy.
“We observed quasars up to one billion years after the Big Bang, and we found that the expansion rate of the universe to this day was faster than we expected,” said Guido Risaliti, from the University of Florence (Italy) and study leader. ” This could mean that dark energy is getting stronger as the cosmos ages .”
Quasars are fast-growing supermassive black holes located at the hearts of galaxies. The incredible luminosity of quasars – which are the brightest objects in the universe – originates from the disks of material that revolve around black holes. These disks generate huge amounts of ultraviolet (UV) light, which can collide with electrons in nearby clouds of hot gas. Such interactions can increase UV radiation to X-ray levels, producing a powerful glow at multiple wavelengths of high-energy light.
The correlation between these two types of light can reveal the distance to a quasar. In the new study, experts examined this relationship in nearly 1,600 quasars. They used NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton spacecraft to observe X-ray light from quasars, and the Sloan Digital Sky Survey, on Earth, to analyze UV output from the objects.
The researchers found that many of the quasars were incredibly distant. The most distant, for example, shed enormous amounts of light into the cosmos only 1.1 billion years after the Big Bang.
Supernovae, while dramatic and powerful, are much less luminous than quasars and therefore cannot be observed from that far away. Thus, the new study offers researchers another standard candle, which can be used to assess the expansion of the universe over a longer period of time, much further than any of the supernovae used so far.
“Since this is a new technique, we took additional steps to show that this method gives us reliable results. We show that the results of our technique match those of supernova measurements over the past 9 billion years, which gives us confidence that our results are reliable even in earlier times “, clarifies Elisabeta Luso, co-author of the work.
The way to measure the Hubble constancy in the remote past of the universe is finally achieved
The new results are consistent with some previous observations of relatively close supernovae.
As it is, dark energy is getting stronger and stronger as the universe gets older as well.
“Some scientists have suggested in the past that new physics might be necessary to explain the discrepancies found so far, including the possibility that the strength of dark energy is increasing. And our results seem to confirm that suggestion,” the experts conclude.
Referencia: G. Risaliti et al. Cosmological constraints from the Hubble diagram of quasars at high redshifts, Nature Astronomy (2019). DOI: 10.1038/s41550-018-0657-z
