  The paper aims to provide an explanation for the tiny value of the
cosmological constant and the low vacuum energy density to represent the dark
energy. To accomplish this, we will search for a fundamental principle of
symmetry in space-time by means of the elimination of the classical idea of
rest, by including an invariant minimum limit of speed in the subatomic world.
Such a minimum speed, unattainable by particles, represents a preferred
reference frame associated with a background field that breaks down the Lorentz
symmetry. The metric of the flat space-time shall include the presence of a
uniform vacuum energy density, which leads to a negative pressure at
cosmological length scales. Thus, the equation of state for the cosmological
constant [$p$(pressure)$=- \epsilon$ (energy density)] naturally emerges from
such a space-time with an energy barrier of a minimum speed. The tiny values of
the cosmological constant and the vacuum energy density will be successfully
obtained, being in agreement with the observational results of Perlmutter,
Schmidt and Riess.
