  We study a general class of effective backgrounds that break diffeomorphism
invariance and their potential roles in cosmology. Specifically, we examine
both explicit and spontaneous background fields which display distinct
transformation properties and are characterized with different dynamics. For
explicit breaking, we focus on the $t$-sector of the minimal gravitational
Standard-Model Extension (SME) and for spontaneous breaking on a vector field
called the bumblebee model. In both cases, we derive the modified Friedmann
equations and find a configuration of the background fields that preserve
isotropy and homogeneity. We show that the explicit $t$-sector admits phases of
accelerated expansion of the universe with standard matter.
