Dark energy may not be a cosmological constant, according to a new study of quasars. New scientific work suggests that this force may have changed in the 13.8 billion years since the Big Bang.
“We observed quasars just a billion years after the Big Bang and found that the expansion of the universe to this day has been faster than expected,” says lead author Guido Risaliti of the University of Florence. "This could mean that the older the space, the stronger the dark energy becomes."
Quasars are fast-growing supermassive black holes at the centers of galaxies. The incredible luminosity of quasars comes from disks of material orbiting black holes. These rapidly spinning discs produce massive amounts of ultraviolet radiation, some of which collides with electrons in nearby gas clouds. Such interactions can amplify ultraviolet radiation to X-ray levels, producing powerful light at multiple wavelengths of high-energy light.
Risality and his colleague Elisabetta Lusso of Durham University in England have determined that the relationship between the two types of light can indicate the distance to the quasar. They investigated this ratio in nearly 1600 quasars. They used the Chandra X-ray Observatory and the XMM-Newton spacecraft to observe the X-rays from the quasars, and the ground-based Sloan Digital Sky Survey for ultraviolet analysis.
Risality and Lusso found that many quasars are incredibly far away. The most distant of them, for example, threw huge amounts of light into space just 1.1 billion years after the Big Bang.
“Because this is a new technique, we have taken additional steps to show that this method gives reliable results,” says Lusso. "We have shown that the results obtained with our technology coincide with measurements of supernovae over the past nine billion years, which gave us confidence in the reliability of our results at an earlier time."
Thus, the new results are consistent with some early observations of relatively nearby supernovae. Previous work found an apparently accelerated expansion compared to the early universe.
“Some scientists have suggested that new physics may be needed to explain this difference, which could also explain the increase in dark energy,” concludes Risality. "The new results are consistent with this assumption."
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