According to a new study, an invisible “mirror universe” of particles that interact with our world exclusively through gravity may hold the key to solving a significant dilemma in cosmology today—the Hubble constant problem.
The Hubble constant is the present rate of expansion of the cosmos. This pace’s prediction from cosmology’s standard model is much slower than the rate reported by our most accurate local observations. Many cosmologists have attempted to resolve this difference by changing our present cosmological paradigm. The difficulty is to do so without jeopardizing the consistency between standard model predictions and other cosmological phenomena, like the cosmic microwave background. Researchers have been trying to address the issue of whether such a cosmic situation occurs.
Cyr-Racine, Ge, and Knox have identified a hitherto unknown mathematical feature of cosmological models that, in theory, might allow for a higher expansion rate without barely altering the most carefully proven other predictions of the conventional cosmological model. They discovered that uniform scaling of gravitational free-fall rates and photon-electron scattering rates results in almost invariant most dimensionless cosmic observables.
This finding offers a new avenue for reconciling measurements of the cosmic microwave background and large-scale structure with high values of the Hubble constant H0: Find a cosmological model in which the scaling transformation may be performed without causing any measurements of values not protected by the symmetry to be violated. This effort has given way to a new approach to tackling a difficult challenge. Further model development might offer consistency with the two unsatisfied constraints: the inferred primordial abundances of deuterium and helium.
If the universe exploits this symmetry somehow, researchers are driven to an incredibly intriguing conclusion: that there exists a mirror universe quite identical to ours but unseen to us save for gravitational influence on our world. Such a “mirror world” dark sector would enable successful scaling of gravitational free-fall speeds while maintaining today’s accurately measured mean photon density.