  The effect of gravitational particle production of scalar particles on the
total effective cosmic energy density (in the era after photon decoupling till
the present) is considered. The effect is significant for heavy particles. It
is found that gravitational particle production 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 in the calculation of the
number density of baryons at present time that is used to calculate
recombination redshift. This may explain why the Hubble constant determined by
local measurements and non-local measurements (such as CMB) are different. This
suggests that gravitational particle production may have a non-negligible
impact on the $H_0$ tension.
