Gravitational particle production, the cosmological tensions and fast radio bursts

Abstract

In [26] it had been found that gravitational particle production (to be more specific, gravitational vacuum polarization) results in an effective increase in the directly measured value of the Hubble constant $H_0$ while it does not affect the value of the Hubble constant derived from energy densities $\bar{H}_0$. It had also been pointed out that this may explain why the Hubble constant $H_0$ determined from direct measurements (such as in SN Ia measurements) and the Hubble constant determined from indirect measurements (such as in CMB calculations in the framework of $Λ$CDM) are different. In the present study, first I correct a misidentification in \cite{Erdem-Universe}, namely, $\hat{H}_0=\left(\frac{\bar{H}_0}{H_0}\right)\bar{H}_0$ (rather than $\bar{H}_0$) is the value of the Hubble constant measured in CMB and BAO measurements. Then I extend the analysis to the $σ_8$ tension, and to determination of the Hubble constant through observations of fast radio bursts. It is observed that inclusion of the effect of gravitational vacuum polarization essentially does not neither mitigate nor exacerbate the $σ_8$ tension (while it mitigates or relieves the Hubble tension). This result is significant in the light of the studies in literature that question existence of a true $σ_8$ tension. Moreover, the present framework predicts that the value of the Hubble constant measured in fast radio bursts is $\hat{H}_0$ as in CMB and BAO measurements. This may be checked with observations in future after more precise and conclusive measurements of $\hat{H}_0$, $\bar{H}_0$, $H_0$.