Supermassive Black Hole Research Suggests That Dark Energy Is Accelerating
Dark energy, a force that is hypothesised to be omnipresent in space and responsible for the acceleration of the expansion of the universe, might be even more quizzical than previously thought. First proposed in the 1990s, the theory immediately gained traction, resulting in the Nobel Prize in physics for the three astrophysicists who discovered the concept.
A new study in Nature Astronomy challenges the view that dark energy is a constant, as previously thought. Researchers propose that quasars, super bright black holes have been varying since the birth of the universe 13.8 billion years ago.
“We observed quasars back to just a billion years after the Big Bang, and found that the universe’s expansion rate up to the present day was faster than we expected,” said study lead author Guido Risaliti of the University of Florence in Italy. “This could mean dark energy is getting stronger as the cosmos grows older.”
Quasars are rapidly growing black holes, supermassive and extremely luminous — the brightest objects in the universe. The light comes from the disks of material that swirls around quasars, generating ultraviolet light, some of which comes into contact with clouds of hot gas. This type of accretion results in UV radiation of X-ray levels: some quasars have luminosities that are thousands of times greater than that of the Milky Way galaxy.
Risaliti and study co-author Elisabetta Lusso of Durham University in the UK looked at the known correlations between the observed UV and X-ray radiation, using quasars as standard candles — previously limited to observations of supernova explosions — for the effect of dark energy dating back to within a billion years of the Big Bang. The researchers observed the correlation for approximately 1,600 quasars using NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton spacecraft to observe the quasars’ X-ray light, as well as the ground-based Sloan Digital Sky Survey for analysing the objects’ UV output of the quasars.
Universe expansion rate studies have generally relied on standard candles. Researchers calculated the distance to the objects the brightness of which was known, along with determining their speed based on the “redshifting” of their light. However, supernovas are significantly less luminous than quasars, and cannot be observed from a large distance. Using quasars as another standard candle provided researchers with information for a larger stretch of time to study the rate of the expansion of the universe.
“Since this is a new technique, we took extra steps to show that this method gives us reliable results,” Lusso said. “We showed that results from our technique match up with those from supernova measurements over the last 9 billion years, giving us confidence that our results are reliable at even earlier times.”
Some earlier observations of relatively nearby supernovas have found an accelerated expansion rate compared to the rate of the expansion of the universe. “Some scientists suggested that new physics might be needed to explain this discrepancy, including the possibility that dark energy is growing in strength,” Risaliti said. “Our new results agree with this suggestion.”
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