Five years ago, the Nobel Prize in Physics was awarded to three astronomers for their discovery in the late 1990s. They found that the universe is expanding faster and faster. The scientists' conclusions were based on analyzes of type Ia supernovae - spectacular thermonuclear explosions of dying stars - that were observed by the Hubble Space Telescope and ground-based telescopes. All of this has led to widespread acceptance of the idea that the universe is filled with a mysterious substance, dark energy, that accelerates expansion.
And now a group of scientists led by Professor Subir Sarkan from the Department of Physics at Oxford University has expressed doubts about this standard cosmological concept. Using a vastly expanded dataset - a catalog of 740 Type Ia supernovae more than 10 times the size of the original sample - the scientists found that the expansion data may be less accurate than previously thought. The data corresponds to a constant expansion rate.
The study was published in Scientific Reports in the journal Nature.
Professor Sarkar, who also works at the Niels Bohr Institute in Copenhagen, said: “The discovery of the accelerating expansion of the universe earned the Nobel Prize, the Gruber Prize and the Breakthrough Prize in Fundamental Physics. This led to widespread adoption of the idea that the universe is dominated by "dark energy" that behaves like a cosmological constant - and now it is the "standard model" of cosmology.
However, there is now a much larger database of supernovae on the basis of which rigorous and detailed statistical analyzes can be carried out. We analyzed the latest catalog of 740 Type Ia supernovae - ten times the original sample - and found that evidence of accelerating expansion is at best, as physicists say, “3 sigma.” This is far from the 5 sigma required by the standard for a discovery to be of fundamental importance.
A similar example in this context would be the recent assumption of the existence of a new 750 GeV particle based on data from the Large Hadron Collider at CERN. It originally had high significance - 3.9 and 3.4 sigma in December last year - and more than 500 theoretical papers have been written. But in August, it was announced that new data showed that the significance had dropped to less than 1 sigma. Everything turned out to be a statistical fluctuation and there is no particle."
There are other data that should support the idea of an accelerating expansion of the Universe, for example, information about the cosmic microwave background - the faint afterglow of the Big Bang - obtained by the Planck satellite. However, Professor Sarkar says that “all these tests are indirect, carried out within the framework of the assumed model, and dark energy does not directly affect the CMB. In fact, there may be a weak Sachs-Wolfe effect, but there has been no clear confirmation of this yet.
“It is possible that we have been misled, and the apparent manifestation of dark energy is a consequence of the analysis of data within the framework of a simplified theoretical model - which was accepted as a fact in the 1930s, long before the appearance of normal data. A more complex theoretical framework, taking into account that the universe is not entirely homogeneous and that its material contents may not behave like an ideal gas - two key assumptions of standard cosmology - can explain all observations without the need to include dark energy. And regarding the energy of the vacuum, we have absolutely no understanding of it in the fundamental theory.
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“Naturally, a lot of work will have to be done to convince the physicist community of this, but our work must demonstrate that the key pillar of the standard cosmological model is very fragile. Hopefully this will encourage us to better analyze cosmological data as well as develop other cosmological models.”
ILYA KHEL