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Dark energy may vary over time

Astronomers have found evidence that the invisible force called dark energy, which is generally considered to be permanent, may be stronger over time. If this result were to be confirmed, astronomers could force them to re-examine their basic understanding of the history and structure of the universe.

A new study uses X-ray X-ray and XMM-Newton X-ray observation data along with ultraviolet (UV) data from the Sloan Digital Sky Survey (SDSS).

First discovered 20 years ago by measuring distances to exploding stars called supernovae, dark energy is the type of force or energy that penetrates the entire universe and causes the expansion of the universe to accelerate. This represents about 70% of the composition of the universe.

In the "consensus model" currently used in most studies of the history and structure of the universe, dark energy is interpreted as a "cosmological constant". This means that energy is associated with empty space and is constant throughout space and time.

The basis for the latest result is the development of a new method for determining the distances from quasars, fast-growing black holes in a distant universe that shines very clearly. This method, which used data on about 1600 quasars, allows astronomers to determine the quadruple distances that are farther away from the Earth than the observed supernovae.

Using these quadruple distances, Guido Risalti of the University of Florence in Italy and Elisabeth Lusso of Durham University in the United Kingdom have expanded the expansion velocity calculations to longer distances and hence earlier times in space. XMM-Newton uncovered quasars until the universe reached only 2.3 billion years, and Chandra and XMM detected quasars back to 1.1 to 2 billion years ago. (The current age of the Universe itself is 13.8 billion years old ..

As stated in the latest issue of Nature Astronomy, they found that the extent of the extension is different from the model's match predictions.

"We have observed quasars back only a billion years after the Big Bang and we have found that the rate of expansion of the universe to this day was faster than we expected," says Risaliti. "It could mean dark energy becomes stronger as the universe gets old."

The new technique uses data from these quads to measure their distances.

In quasars, the disc creates a black hole around the mass of UV radiation. Some of the UV lights collide with electrons in the hot gas cloud above and below the disk, and these collisions can increase the energy of UV radiation to X-ray energy. This interaction causes a correlation between the amount of observed UV and X-rays. The distance from the quasi depends on this correlation.

Risaliti and Lusso have compiled UV data from SDSS and Chandra and XMM X-ray data from 1,598 quasars to derive the relationship between UV and X-ray flows and the distance to quasars. Then they used this information to study the speed of expansion of the universe back in very early times. They have found evidence that the amount of dark energy is growing with time.

"As this is a new technique, we have taken further steps to show that this method gives us reliable results," says Lusso. "We have shown that the results of our technology are consistent with the results of supernova measurements over the past 9 billion years, which makes us confident that our results are reliable even in earlier periods."

Scientists have also carefully devoted themselves to selecting their quasars, minimizing statistical errors, and avoiding systematic errors that could depend on the distance from Earth to the object.

If this result is confirmed, it means that dark energy is not a cosmological constant. It could also help resolve the ongoing mismatch between the Hubble constant measurement of space expansion – based on local indicators and measurements based on the cosmic microwave background (CMB).

With supernova observations, astronomers have previously reported that the universe is now expanding faster than expected from its trajectory that appeared shortly after the big bang when it was produced by CMB.

"Some scientists believe that new physics might be needed to explain this mismatch, including the possibility that dark energy is growing in strength," says Risaliti. "Our new results agree with this proposal."

To further test these results, Risaliti and Lusso plan to use a large sample of Chandra quasar observations over a wide range of distances and apply the same technique.

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