The Hubble constant is the current rate of universe expansion. The standard model of cosmology predicts that this rate is much slower than the rate found by our most accurate measurements here on Earth.
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.
The question that scholars like Francis-Yan Cyr-Racine, assistant professor in the Department of Physics and Astronomy at The University of New Mexico, Fei Ge, and Lloyd Knox at the University of California, Davis, have been seeking to answer is whether such a cosmic scenario exists.
Cosmology, according to NASA, is the scientific study of the universe’s large-scale properties. Cosmologists investigate topics such as dark matter and dark energy, as well as whether there is only one universe or a multiverse. Cosmology encompasses the entire cosmos, from conception to death, and is full of mysteries and intrigue.
Now, Cyr-Racine, Ge, and Knox have identified a previously overlooked mathematical feature 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 new study’s findings suggest a novel way to reconcile cosmic microwave background and large-scale structure measurements with high Hubble constant H0 values: Find a cosmological model in which the scaling transformation may be implemented without causing any measurements of values that are not protected by the symmetry to be violated. This effort has paved the way for a novel approach to resolving a difficult challenge. Further model development could provide uniformity to the two remaining constraints: the inferred primordial deuterium and helium abundances.
Researchers are driven 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 unseen to us except through gravitational impact on our world. Such a “mirror world” dark sector would allow for effective scaling of gravitational free-fall speeds while maintaining the precise mean photon density now reported.
“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,” added 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,” explained Cyr-Racine. “Our work allows us to link, for the first time, this large literature to an important problem in cosmology.”
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.
“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,” added Cyr-Racine. “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.”
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