A mechanism to generate varying speed of light via Higgs-dilaton coupling: Theory and cosmological applications

We allow the Higgs field $\Phi$ to interact with a dilaton field $\chi$ of the background spacetime via the coupling $\chi^2\,\Phi^\dagger\Phi$. Upon spontaneous gauge symmetry breaking, the Higgs VEV becomes proportional to $\chi$. While traditionally this linkage is employed to make the Planck mass and particle masses dependent on $\chi$, we present an $\textit alternative$ mechanism: the Higgs VEV will be used to construct Planck's constant $\hbar$ and speed of light $c$. Specifically, each open set vicinity of a given point $x^*$ on the spacetime manifold is equipped with a replica of the Glashow-Weinberg-Salam action operating with its own effective values of $\hbar_*$ and $c_*$ per $\hbar_*\propto\chi^{-1/2}(x^*)$ and $c_*\propto\chi^{1/2}(x^*)$, causing these ``fundamental constants'' to vary alongside the dynamical field $\chi$. Moreover, in each open set around $x^*$, the prevailing value $\chi(x^*)$ determines the length and time scales for physical processes occurring in this region as $l\propto\chi^{-1}(x^*)$ and $\tau\propto\chi^{-3/2}(x^*)$. This leads to an $\textit anisotropic$ relation $\tau^{-1}\propto l^{-3/2}$ between the rate of clocks and the length of rods, resulting in a distinct set of novel physical phenomena. For late-time cosmology, the variation of $c$ along the trajectory of light waves from distant supernovae towards the Earth-based observer necessitates modifications to the Lema\^itre redshift relation and the Hubble law. These modifications are capable of: (1) Accounting for the Pantheon Catalog of SNeIa $\textit{through a declining speed of light in an expanding Einstein--de Sitter universe}$, thus avoiding the need for dark energy; (2) Revitalizing Blanchard-Douspis-Rowan-Robinson-Sarkar's CMB power spectrum analysis that bypassed dark energy [A&A 412, 35 (2003)]; and (3) Resolving the $H_0$ tension without requiring a dynamical dark energy component.