  One of the fundamental challenges in string theory is to derive realistic
four-dimensional cosmological backgrounds from it despite strict consistency
conditions that constrain its possible low-energy backgrounds. In this work, we
focus on energy conditions as covariant and background-independent consistency
requirements in order to classify possible backgrounds coming from low-energy
string theory in two steps. Firstly, we show how supergravity actions obey many
relevant energy conditions under some reasonable assumptions. Remarkably, we
find that the energy conditions are satisfied even in the presence of objects
which individually violate them due to the tadpole cancellation condition.
Thereafter, we list a set of conditions for a higher-dimensional energy
condition to imply the corresponding lower-dimensional one, thereby
categorizing the allowed low-energy solutions. As for any no-go theorem, our
aim is to highlight the assumptions that must be circumvented for deriving
four-dimensional spacetimes that necessarily violate these energy conditions,
with emphasis on cosmological backgrounds.
