We live inside of something that we call a universe, but we are not quite sure what it is, where it came from, what its future holds, or if there’s more universes like (or unlike) it. Now, scientists have cast doubt on yet another fundamental feature of the universe, its shape, in a new paper from Nature Astronomy that warns of a “cosmological crisis.”
Most observational evidence favors a model of a flat universe, in which light travels across the cosmos in a straight line. But a team led by Eleonora Di Valentino, a cosmologist at the University of Manchester, identified a key piece of data that suggests space may be curved into a closed cosmic sphere, meaning that a beam of light would eventually loop around the entire cosmos to return to its point of origin.
The team’s results are based on new data released from the Planck mission, a European Space Agency (ESA) satellite that operated from 2009 to 2013.
“Here, we show that, if indeed credible, the Planck preference for a closed Universe introduces a new problem for modern cosmology,” the researchers said in the paper. They added that the observations “should not be discarded merely as a statistical fluctuation, but must be seriously investigated, since at face value, they point towards a drastic rethinking of the current cosmological concordance model.”
The Planck spacecraft was designed to map the oldest light in the universe, called the cosmic microwave background (CMB). Dating back more than 13 billion years, this light is the radiative afterglow of the Big Bang, which is thought of as the start point of the cosmos.
In the newly released data, Planck captured a phenomenon called gravitational lensing. This lensing effect occurs when the gravitational fields of objects bend, distort, and even magnify light from sources located behind them from our perspective on Earth.
The lensing amplitudes of the CMB have been measured by previous observatories such as the Wilkinson Microwave Anisotropy Probe, the South Pole Telescope, and the Atacama Cosmology Telescope.
But Alessandro Melchiorri, a cosmologist at the University of Rome who co-authored the new paper, said those experiments had far less precision than Planck. “Planck improves these measurements thanks to the better angular resolution and better overall sensitivity of the detectors,” Melchiorri said in an email.
This is where it gets weird: According to Planck’s measurements, the CMB is being gravitationally lensed much more than expected. One possible explanation to account for this observation is that there is a curvature woven into the fabric of spacetime.
“A closed Universe can provide a physical explanation for this effect, with the Planck cosmic microwave background spectra now preferring a positive curvature at more than the 99% confidence level,” the team said in the study.
This is a provocative result that complexifies the established idea of the universe’s shape, but the authors emphasize that it will take much more research to confirm whether the Planck data presents a terminal challenge to the flat cosmos.
“In principle, the next CMB experiments such as the Simon's Observatory [in Chile] should answer the question,” Melchiorri said, though he said that would not necessarily be enough.
“Personally, I think we need a new CMB mission like Planck with improved detectors,” he added, such as a concept mission called CORE, which ESA rejected. “In reality, we may end up waiting several years before having a definitive answer.”
In the meantime, the flat universe remains the leading model of our cosmic surroundings. But given that we have barely begun to understand this weird expanding object that we live in, more surprises are sure to come.