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Dark energy may change over time

Astronomers have found evidence that an invisible force, called dark energy, which, as is commonly believed, is constant, can increase over time. If this result is confirmed, it can force astronomers to reconsider their fundamental understanding of the history and structure of the universe.

The new study uses X-ray data from the X-ray Observatory Chandra NASA and XMM-Newton ESA, as well as ultraviolet (UV) data obtained in Sloan-Digital Sky Survey (SDSS).

First discovered about 20 years ago by measuring distances to exploding stars, called supernovae, dark energy is an estimated type of force, or energy, that permeates all space and causes the expansion of the Universe to accelerate. It accounts for about 70% of the composition of the universe.

In the “reconciliation 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 it is energy associated with empty space, and constant in all space and time.

The essence of the latter result is the development of a new method for determining distances to quasars, fast-growing black holes in a distant Universe, which shine extremely brightly. This method, which used data on about 1600 quasars, allows astronomers to determine distances to quasars that are much farther from Earth than the observed supernovae.

Using these quasar distances, Guido Risalti from the University of Florence in Italy and Elizabeth Lussault from the University of Durham in Great Britain expanded the calculations of the rate of expansion of the Universe over long distances and, therefore, earlier times in the evolution of the Universe. XMM-Newton discovered quasars at a time when the universe was only 2.3 billion years old, and Chandra and XMM discovered quasars at the age of 1.1 to 2.3 billion years. (At present, the accepted age of the universe itself is 13.8 billion years ..

As reported in the latest release of Nature Astronomy, they found that the expansion rate is different from the predictions of the matching model.

“We observed quasars just a billion years after the Big Bang and found that the rate of expansion of the universe to our day was faster than we expected,” says Risaly. "This may mean that dark energy becomes stronger with the age of the cosmos."

The new technique uses the ultraviolet and x-ray data from these quasars to estimate their distances.

In quasars, a matter disc around a black hole produces ultraviolet light. Part of the ultraviolet light collides with electrons in a cloud of hot gas above and below the disk, and these collisions can increase the energy of the ultraviolet to the energies of x-rays. This interaction causes a correlation between the amount of observed ultraviolet and x-rays. The distance to the quasar depends on this correlation.

Picturesaly and Lousseau collected SDSS-based ultraviolet data and X-ray data from Chandra and XMM for 1,598 quasars to link the ultraviolet and x-ray fluxes and the distances to the quasars. They then used this information to study the rate of expansion of the Universe at very early times. They found evidence that the amount of dark energy increases with time.

“Since this is a new method, we have taken additional steps to show that this method gives us reliable results,” says Lusso. "We have shown that the results of our methodology coincide with the results of measurements of supernovae over the past 9 billion years, which gives us confidence that our results are reliable even in earlier times."

The researchers also carefully monitored how their quasars were chosen to minimize statistical errors and avoid systematic errors that may depend on the distance from the Earth to the object.

If confirmed, this result will mean that dark energy is not a cosmological constant. It can also help to eliminate the existing discrepancy between the measurement of the Hubble constant – the rate of expansion of the Universe – based on local indicators and measurements based on the cosmic microwave background (CMB).

Using supernovae observations, astronomers had previously reported that the Universe seems to be expanding faster than expected from its trajectory, noticed shortly after the Big Bang, when the CMB was obtained.

“Some scientists have suggested that new physics may be required to explain this discrepancy, including the likelihood that dark energy is growing in strength,” says Risaly. "Our new results are consistent with this proposal."

In order to further verify these results, Risaly and Lusso plan to use a large sample of Chandra’s observations of quasars over a wide range of distances and use the same technique.

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