Dark energy is not yet an experimentally discovered form of matter that permeates the entire Universe we observe. It is she who is "responsible" for the fact that the Universe not only expands, but does so with acceleration.
It is not yet possible to “feel” this energy, but this does not mean that nothing can be said about it. It is possible, in particular, to estimate its amount by the influence that it has on the expansion of the Universe - its acceleration is the greater, the more dark energy in the Universe at this moment.
Astronomers determine the rate of expansion of the Universe and the change in this rate over time by supernovae. Their real luminosity is precisely known, therefore, by the brightness that can be observed from Earth, it is possible to accurately determine the distance from us to the supernova, and by the redshift, the speed with which this object was moving away from us at the moment of emission of the light visible today.
But this method has a serious limitation. It is suitable for studying the last nine billion years of the life of the universe. There are very few older supernovae in it. Meanwhile, the age of the universe is estimated at about 13.8 billion years. It would be extremely interesting to look into the beginning of her life.
The new technique uses ultraviolet (UV) and X-ray data to estimate distances to quasars.
A quasar is a huge black hole that intensively engulfs the surrounding matter. This matter glows, and very brightly. A typical quasar emits 1−2 orders of magnitude more energy than our entire galaxy. What is especially pleasant is that quasars appeared already at the dawn of the Universe.
Ultraviolet radiation is generated in the disk of matter surrounding the quasar. Some of the ultraviolet photons then collide with electrons in a cloud of hot gas above and below the disk, and these collisions can boost their energy to the level of X-rays. The brightness of a quasar in the UV and X-ray ranges is correlated: the more ultraviolet radiation was at the beginning, the greater the X-ray brightness will be.
Thus, we can calculate the true brightness of the quasar, and knowing it and the one we see, we can calculate the distance to it. After that, there is very little left: to compare the distance with the redshift of the object and draw a conclusion about the rate of removal of the quasar from us billions of years ago, when its light was emitted.
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The researchers collected data for 1,598 quasars, and estimated the rate of expansion of the universe at very early times. The results show that the amount of dark energy increases over time.
Since this is a new method, astronomers have taken additional steps to show that it gives reliable results. They showed that his results over the past nine billion years coincide with what was previously obtained from supernova data.
For details, see an article in Nature Astronomy. Its full preprint is available here.
Sergey Sysoev