Analysis, that introduces from relativity theory, originals relationships between radiation energy and its space coordinate. The latter, even for spontaneous process, shows changes of such magnitude, which could be correlated with the duality configuration photon-wavelength. Parametric up-conversion, in which two photons generate one of half wavelength, may allow Cosmic Microwave Background (CMB) photons to shrink within their locus space-time configuration (Casimir Effect). CMB Parametric up- and down-conversion could allow CMB wavelength fluctuations in vacuum. These, could be difficult to detect because may occur within the range of its black body spectra (between 5×10^-1 cm. to 2×10^-1 cm.). The relic CMB photons, by number (411 cm 3 ), and by their very low energy (2.71 K), means a large space-time dimension may be, therefore, not only dimensioning the vacuum, but also generating its fluctuations.