An international team of scientists is suggesting that the accelerating expansion of the Universe could be caused by a mystery substance called "quintessence" that permeates the cosmos.

The tentative hypothesis could offer tantalizing new clues about the nature of dark energy, the mysterious force that physicists believe is responsible for the increasing speed at which the universe is expanding.

But the researchers' conclusions — and their wide sweeping implications — have left some of their peers questioning the idea, as Nature reports.

The theory that dark energy is responsible for accelerating the expansion of the universe was first proposed back in 1998, when researchers found that the amount of the mysterious force was fixed per unit of volume of space as a "cosmological constant."

Not all scientists fully subscribe to the theory though, as Nature points out, arguing instead that dark energy is made out of a "fifth element," or what the researchers from the High Energy Accelerator Research Organization (KEK) in Japan and the Max Planck Institute for Astrophysics in Germany are now calling quintessence. As they frame it, quintessence is neither an inherent property of space determined by a constant nor a form of matter. If dark energy were indeed such a quintessence, that would mean its density would drop over time as the galaxy expands.

To test the theory, the researchers first posited that quintessence would have to affect light in certain ways as it spreads throughout the cosmos.

Back in 1998, according to Nature, a team of scientists led by Sean Carroll, a theoretical physicist at the California Institute of Technology in Pasadena, suggested it would technically be possible to find evidence of this happening.

They suggested that by looking at maps of the cosmic microwave background (CMB) — the relic electromagnetic radiation left over from the early stages of the universe — scientists could theoretically look for certain light signatures to prove the quintessence theory.

These light signatures would show electric fields of polarized light "wiggling" in specific directions, they suggested, rather than any which way.

And that's exactly what two cosmologists from KEK and the Max Planck Institute for Astrophysics in Germany claim they have done, as detailed in a paper published in the journal Physical Review Letters this week.

By looking at CMB data from the European Space Agency's Planck mission, they were able to pinpoint signs of quintessence using an entirely new technique.

But the discovery is far from set in stone yet — physicists are warning that the evidence isn't complete yet, as Nature reports.

If borne out by further research, though, the theory could have drastic consequences for our understanding of the universe. If dark energy were indeed quintessence, the expansion could actually slow down and eventually disappear entirely.

The process could even be reversed, causing the universe to crunch up like a soda can, according to Carroll, who was not involved in the study.

"We’re back to a situation where we have zero idea about how the Universe is going to end," Carroll told Nature.

READ MORE: Hints of twisted light offer clues to dark energy’s nature [Nature]

More on the expansion of the universe: To Measure the Universe’s Expansion, We Might Need New Physics

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