A mirror world is a common trope in fantasy and fiction, but it may also be the answer to one of Space’s biggest mysteries today. A group of scientists behind a new research paper suggest that a “mirror world” of particles that remains unseen from us may be the answer to the Hubble Constant problem. The Hubble constant problem refers to the discrepancy in the theoretical value of the rate of expansion in the universe and the actual rate of expansion as observed by measurements. The issue remains to reconcile the two without upending the entire cosmological model as it stands today. As doing so would ruin the agreements with the current scientific models and the observed phenomenon in Space like the cosmic microwave background.

“Basically, we point out that a lot of the observations we do in cosmology have an inherent symmetry under rescaling the universe as a whole. This might provide a way to understand why there appears to be a discrepancy between different measurements of the Universe’s expansion rate,” said lead researchers Francis-Yan Cyr-Racine from the University of New Mexico, and Fei Ge and Lloyd Knox at the University of California.

Their observations were published in the paper titled Symmetry of Cosmological Observables, a Mirror World Dark Sector, and the Hubble Constant, which was released recently in Physical Review Letters.

“The mirror world idea first arose in the 1990s but has not previously been recognised as a potential solution to the Hubble constant problem. This might seem crazy at face value, but such mirror worlds have a large physics literature in a completely different context since they can help solve an important problem in particle physics,” said Cyr-Racine. “Our work allows us to link, for the first time, this large literature to an important problem in cosmology.”

Apart from the mirror world idea, scientists have also considered the possibility of measurement errors to be behind the discrepancy. But as measurement tools have gotten better, the deviation between the theoretical and observed value has only increased, leading many to believe that measurement errors are not the reason behind the discrepancy.