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Dark Energy Might Be Stealing the Glue Holding the Universe Together

If recent measurements are correct, it may not be so farfetched.
​Image: ​YouTube

Dark energy is eating dark matter. Bit by bit, the supreme attractive force of the universe, gravitationally speaking, is being superseded by the supreme repulsive force. So, not only is the universe's outward expansion accelerating, the cosmos is losing the very glue that holds it together.

This is the theory, at least, being advanced in a new paper in Physical Review Letters describing a rather ominous-sounding "dark sector" interaction.

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First, to recap: The fate of the universe, existence itself, is as a cold, dark void. It's kind of like the fate of you and me, except that the universe won't go on to fertilize grass and "be star stuff." It will just be nothing, and maybe that's actually comforting.

It will happen because the universe is expanding. It's doing more than that actually; it's expanding faster and faster, bizarrely gaining pressure as its volume of space increases. The universe is like an anti-balloon, which is one of the more recent and more certain discoveries in cosmology. The cause is generically referred to as dark energy, the outward pressure of a void.

Dark energy is usually thought of as vacuum energy or zero-point energy. Nature really does abhor a vacuum and it comes equipped with quantum mechanical properties that supply empty space (what's left when we take everything from some region) with a steady fizz of virtual particles, or real bits of energy that pop in and out of existence within a vacuum because, really, the whole idea of emptiness and vacuums are just some shit we made up. As the amount of empty space increases, dark energy increases too.

Observed astrophysical data suggest the universe is indeed accelerating. Image: hyperphysics

This dark energy fizz manifests as an outward pressure, a repulsive force, just like air pushing on the inside of a balloon. Dark matter, meanwhile, is an attractive force. It's made of real matter (probably). Because it is, you know, actual stuff, dark matter attracts gravitationally, and this attraction acts as a scaffolding of sorts that allows for the formation of galaxies and, eventually, planets.

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As the physicists behind the new paper, led by University of Rome physicist Valentina Salvatelli, explain, these two darknesses are subtly coupled, with dark matter steadily becoming dark energy.

Salvatelli and his team didn't pull this idea out of thin air. It arises from inconsistencies in measurements based on the Cosmic Microwave Background, the dim leftover glimmer of the post-Big Bang universe, from which the growth rates behind large-scale structures of the universe can be inferred. The observed rate of expansion is too slow, suggesting that dark matter may be dwindling, possibly being converted to dark energy.

"The latest CMB data are in tension with local measurements of the Hubble expansion rate from supernovae Ia and other cosmological observables," Salvatteli and team write, "which point towards a lower growth rate of large-scale structure. [We] show that energy transfer from dark matter to the vacuum can resolve the tension between the CMB and RSD measurements of the growth of large-scale structures, making it consistent to combine these two datasets."

To test out the theory, the physicists divided the history of the universe into four distinct time periods, or bins, noting that their dark matter/dark energy coupling mechanism only begins manifesting in the latter two. So, this dark matter decomposition has only been going on for around 8 billion years—that is, if Salvatteli and his team's theory is correct. Further measurements might easily exclude it from the field of possible explanations.

"Other researchers have said that this tension can be resolved if the dark-matter particle is a sterile neutrino," Catherine Heymans, a physicist at the University of Edinburgh, told Physics World. "Others still are looking at different modified-gravity theories to explain the result. More data and further meticulous analysis of those data and the systematics that might be associated with them are the way to find out if this fascinating theory could be true."

A preprint, open-access version of the study is available at arXiv.