Cosmic Background Explorer (COBE) Miission

The COBE satellite was developed by NASA’s Goddard Space Flight Center to measure the diffuse infrared and microwave radiation from the early universe to the limits set by our astrophysical environment. Credit: NASA

This result opens a new approach to reconciling cosmic microwave background and large-scale structure observations with high values of the Hubble constant H0: Find a cosmological model in which the scaling transformation can be realized without violating any measurements of quantities not protected by the symmetry. This work has opened a new path toward resolving what has proved to be a challenging problem. Further model building might bring consistency with the two constraints not yet satisfied: the inferred primordial abundances of deuterium and helium.

If the universe is somehow exploiting this symmetry researchers are led to an extremely interesting conclusion: that there exists a mirror universe very similar to ours but invisible to us except through gravitational impact on our world. Such “mirror world” dark sector would allow for an effective scaling of the gravitational free-fall rates while respecting the precisely measured mean photon density today.

“In practice, this scaling symmetry could only be realized by including a mirror world in the model — a parallel universe with new particles that are all copies of known particles,” said Cyr-Racine. “The mirror world idea first arose in the 1990s but has not previously been recognized 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 important problem in particle physics,” explains Cyr-Racine. “Our work allows us to link, for the first time, this large literature to an important problem in cosmology.”

“This scaling symmetry could only be achieved in practice by integrating a mirror world in the model — a parallel universe with new particles that are all duplicates of known particles,” Cyr-Racine explained. “The mirror world concept first surfaced in the 1990s, but it was not previously recognized as a potential Hubble constant solution.

“At first glance, this may appear absurd, yet such mirror worlds have a substantial physics literature in a completely different context since they can aid in the solution of significant problems in particle physics,” argues Cyr-Racine. “For the first time, our work connects this vast literature to an important cosmological challenge.”

COBE Satellite

An artist’s rendition of the COBE Satellite. Credit: Matthew Verdolivo, UC, Davis

Researchers are also asking if the Hubble constant gap could be caused in part by measurement errors, in addition to looking for missing ingredients in our present cosmological model. While this is still a possibility, it’s worth noting that the disparity has grown in importance as higher-quality data has been included in the analysis, implying that the data isn’t to blame.