Astronomers have used quasars from just after the Big Bang to estimate the expansion rate of the Universe.
They claim that it is actually expanding faster than predicted by the standard model of cosmology, a finding that could mean that the 'dark energy' in the Universe in not constant but growing stronger as the Universe grows older.
For astronomers, it is a tough job to determine the rate of expansion of the universe, also known as the Hubble Constant. Determining this value is important in order to accurately determine the age of the Universe, sizes of remote galaxies, or the expansion history of the Universe.
Past studies suggest that the value of the Hubble constant ranges from about 67 to 73 kilometres per second per megaparsec. In other words, it means that two points in space located at a distance of one megaparsec (3.26 million light-years) are moving away from each other at a speed between 67 and 73 kilometres per second.
Typically, scientists have used the method of 'standard candles' to determine the rate of expansion of the Universe. Standard candles refer to the objects with known luminosity, for example, Type Ia supernovae or cepheid variable stars. Scientists use the magnitude of their luminosity to determine their distances.
In the current study, Guido Risaliti of Università di Firenze, Italy, and Elisabeta Lusso of Durham University, UK, along with other team members, decided to investigate the expansion history of the Universe by using a well-known relation between the ultraviolet and X-ray brightness of quasars.
Quasars are the brightest objects in the Universe. They are the cores of galaxies with an active supermassive black hole, which consumes matter from the surroundings at intense rates. As the matter falls onto the black hole, it creates a swirling accretion disk, which radiates ultraviolet and visible light. This light also heats up nearby electrons to generate X-rays.
The team collected X-ray data for over 7,000 quasars from the European Space Agency's XMM-Newton archive, and combined the data with the ultraviolet observations from the Sloan Digital Sky Survey. They also used quasar data obtained through XMM-Newton in 2017 as well as with NASA's Swift X-ray and Chandra observatories.
The team brought the sample data down to about 1,600 quasars from the period just 1.1 billion to 2.3 billion years after the Big Bang. Finally, they estimated distance to these sources to finally calculate the expansion rate of the early Universe.
Researchers found that the Universe's expansion rate up to the present day was faster than expected, which could also mean that 'dark energy' is getting stronger as the Universe grows older.
"One of the possible solutions would be to invoke an evolving dark energy, with a density that increases as time goes by," says Guido.
Astronomers now plan to investigate more quasars in future to refine their results.
The findings of the study are published in the journal Nature Astronomy.
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