Dark matter may not exist at all, explains study

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The elusive dark energy, thought to constitute up to 68 percent of the universe, may not exist at all, scientists have claimed. Researchers believe that standard models of the universe fail to take account of its changing structure, but that once this is done the need for dark energy disappears.

From the 1920s, mapping the velocities of galaxies led scientists to conclude that the whole universe is expanding and that it began life as a vanishingly small point. In the second half of the twentieth century, astronomers found evidence for unseen ‘dark’ matter by observing that something extra was needed to explain the motion of stars within galaxies. Dark matter is now thought to make up 27 percent of the content of universe (in contrast ‘ordinary’ matter amounts to only five per cent).

Observations of the explosions of white dwarf stars in binary systems in the 1990s led scientists to the conclusion that a third component, dark energy, made up 68 percent of the cosmos, and is responsible for driving an acceleration in the expansion of the universe. In the new work, researchers led by PhD student Gabor Racz of Eotvos Lorand University in Hungary, question the existence of dark energy and suggest an alternative explanation.

They argue that conventional models of cosmology (the study of the origin and evolution of the universe), rely on approximations that ignore its structure, and where matter is assumed to have a uniform density. In practice, normal and dark matter fill the universe with a foam-like structure, where galaxies are located on the thin walls between bubbles and are grouped into superclusters. The insides of the bubbles are in contrast almost empty of both kinds of matter.

Using a computer simulation to model the effect of gravity on the distribution of millions of particles of dark matter, scientists reconstructed the evolution of the universe. Unlike conventional simulations with a smoothly expanding universe, taking the structure into account led to a model where different regions expand at different rates. The average expansion rate though is consistent with present observations, which suggest an overall acceleration.

“Our findings rely on a mathematical conjecture which permits the differential expansion of space, consistent with general relativity, and they show how the formation of complex structures of matter affects the expansion,” researchers said. “These issues were previously swept under the rug but taking them into account can explain the acceleration without the need for dark energy,” they said.

If this finding is upheld, it could have a significant impact on models of the universe and the direction of research in physics. For the past 20 years, astronomers and theoretical physicists have speculated on the nature of dark energy, but it remains an unsolved mystery. The research was published in the Monthly Notices of the Royal Astronomical Society.

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