According to new study, an unseen’mirror world’ of particles that interacts with our universe solely through gravity could be the key to solving the Hubble constant problem, which is currently a major puzzle in cosmology.
The Hubble constant is the universe’s current rate of expansion. The rate predicted by cosmology’s standard model is much slower than the rate discovered 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. 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 striving to solve 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 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 many of the cosmological observations have an intrinsic symmetry when the universe is rescaled as a whole.” This could explain why there appears to be a disagreement between different measurements of the rate of expansion of the Universe.”
This finding suggests a new 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 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 dismal “mirror world”
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.”
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.
