Ghostly 'mirror world' might be cause of cosmic controversy

According to new study, an unseen'mirror world' of particles that interacts with our universe solely through gravity could be the key to solving a crucial cosmological issue known as the Hubble constant problem.

The Hubble constant is the current rate of universe expansion. The rate predicted by the mainstream model of cosmology is much slower than the rate measured by our most precise local measurements. Many cosmologists have attempted to resolve this difference by altering our present cosmological paradigm. The problem is to do so without jeopardizing the consistency of standard model predictions with many other cosmological phenomena, such as the cosmic microwave background.

Researchers such as Francis-Yan Cyr-Racine, assistant professor in the Department of Physics and Astronomy at The University of New Mexico, Fei Ge, and Lloyd Knox of the University of California, Davis, have been attempting to answer the question of whether such a cosmic scenario occurs.

Cosmology, according to NASA, is the study of the universe's large-scale properties. Cosmologists look at topics like dark matter and dark energy, as well as whether there is only one universe or a multiverse. Cosmology is the study of the entire cosmos, from conception to death, and it is full of secrets and intrigue.

Now, Cyr-Racine, Ge, and Knox have identified a previously overlooked mathematical characteristic of cosmological models that, in theory, might allow for a quicker expansion rate without little affecting the mainstream cosmology model's most accurately proven other predictions. Most dimensionless cosmic observables are substantially invariant when gravitational free-fall rates and photon-electron scattering rates are scaled uniformly.

"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."

The study was recently published in Physical Review Letters under the title "Symmetry of Cosmological Observables, a Mirror World Dark Sector, and the Hubble Constant".

This finding paves the way for a novel way to reconcile observations of the cosmic microwave background and large-scale structure with high Hubble constant H0 values: Find a cosmological model in which the scaling transformation may be implemented without causing any measurements of values not protected by the symmetry to be violated. This research has paved the way for a novel approach to solve a difficult problem. The inferred primordial abundances of deuterium and helium, which have yet to be satisfied, could be resolved with more model building.

Researchers are led to an extremely interesting conclusion if the universe is somehow leveraging this symmetry: that there exists a mirror universe that is remarkably identical to ours but is invisible to us save through gravitational interaction with our world. Such a "mirror world" dark sector would allow for effective scaling of gravitational free-fall speeds while maintaining the current mean photon density.

"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," Cyr-Racine added. The mirror world concept was first proposed in the 1990s, but it was not previously recognized as a potential Hubble constant solution.

"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," argues Cyr-Racine. "Our work allows us to link, for the first time, this large literature to an important problem in cosmology."

Researchers are also investigating if the Hubble constant mismatch 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 size as higher-quality data has been included in the analysis, implying that the data isn't to blame.

"It went from two and a half Sigma, to three, and three and a half to four Sigma. By now, we are pretty much at the five-Sigma level," Cyr-Racine stated. "That's the key number which makes this a real problem because you have two measurements of the same thing, which if you have a consistent picture of the universe should just be completely consistent with each other, but they differ by a very statistically significant amount."

"That's the premise here and we've been thinking about what could be causing that and why are these measurements discrepant? So that's a big problem for cosmology. We just don't seem to understand what the universe is doing today."