The Universe Might Not Be As Flat As Assumed

Map of the Cosmic Microwave Background. Plack Collaboration

Our model of the universe envisions a cosmos that is expanding faster and faster – one that it is essentially flat at scales much larger than superclusters of galaxies. This is an assumption backed by a lot of evidence, but a new study suggests that might be far from certain.

Imagining our universe as flat in three-dimensions doesn’t come easy since we are not used to picturing complicated geometries. In a flat universe, two photons that start moving parallel to each other will never meet. In a universe that is closed, they will eventually cross paths. To picture this, think of the meridian lines on the surface of the Earth. At the equator, they are 90 degrees to each other, but they meet at the poles. Another way to think about it is if you were to move in a straight line in a closed universe, you’d come back to where you started.

The original analysis of the data from the cosmic microwave background (CMB), the so-called echo of the Big Bang, strongly suggested the universe is flat. But models of this flat universe do not perfectly match what we observe. So three researchers performed a new analysis, finding that a closed universe is both consistent with the data and solves other tensions between models and observations. The findings are reported in Nature Astronomy.

Over the last few years, the standard model has been under fire when it comes to the measurement of the expansion rate of the universe. The CMB provides a certain value, while other methods produce a different one. The team believes that their approach actually makes this problem worse. A slightly curved universe would exacerbate the tension. For the team, this is not a defeat but instead a possible suggestion that there's more out there left to find.

"We may have missed something...i.e. that in reality the Universe is not closed but that there is some unknown mechanism that is mimicking it (and that is also producing the Hubble tension)," Professor Alessandro Melchiorri, from the University of Rome, told IFLScience.

Yet not everyone agrees with the new study, which uses publicly available data from the Planck Collaboration. Two members from the collaboration performed a new analysis on some of the data and found that the expected curvature from the Nature Astronomy work is less evident.

In the paper, the researchers do not claim that the standard model has to go. Instead, they suggest serious considerations for the origin of this mismatch between model and observations. The issue might be with the data itself, how it is analyzed, or instead with the model. More observations and analysis will hopefully bring forth a solution, but the possibility of a curved universe should not be discarded until then.

[H/T: Live Science]

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