If someone asked you what the shape of the universe was, what would you say?
Most research into the matter has concluded its flat, but there's new evidence suggesting it's actually curved.
When scientists say 'flat', they mean if you left Earth and travelled in a perfectly straight line, you'd keep going forever and never see the Earth again - in two dimensions, think of a flat piece of paper that extends forever in every direction.
While in a 'curved', or 'round' universe you'd eventually end up back where you started - much like a 'straight' line across the two-dimensional surface of a ball or a globe always ends up where it began, except in a much-harder-to-visualise three dimensions.
Photons of light travelling in parallel in a flat universe would never meet, but in a curved universe they would - like how lines of longitude on a globe all travel in perfect straight lines across the Earth's surface from north to south, and meet at the poles.
Scientists found an anomaly by looking at data from the cosmic microwave background (CMB), faint radiation left over from the Big Bang, according to a new study published in Nature Astronomy.
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Some of the CMB light had been had pulled off its original course - "gravitationally lensed" so much, the only explanation they could find is there's more matter in the universe than previously believed. But the amount of matter required to account for the lensing would be enough to have the universe "close" in on itself.
But this finding contradicts much of what scientists think they already know about the universe. The study's authors themselves called it a "cosmological crisis" and the two theories - curved and flat - "mutually inconsistent".
"Future measurements are needed to clarify whether the observed discordances are due to undetected systematics, or to new physics or simply are a statistical fluctuation."
There's a 0.2 percent chance it's all a statistical fluke, the paper says.
There have already been discrepancies found in different measurements of the universe's expansion, for example, which would be thrown into further disarray if it turned out the universe wasn't flat; and one of the study's authors told LiveScience calculations of how the Big Bang unfolded would also have to be redone.