  We study unimodular gravity in the context of cosmology, particularly some
interesting consequences that might be able to describe the background
cosmology and the late cosmic acceleration. We focus our attention on the
hypothesis of \textit{non conservation of the energy momentum tensor}. This
characteristic has an interesting outcome: we can obtain a modified Friedmann
equation along with the acceleration equation and also new fluid equations
related to a third order derivative of the scale factor, known in cosmography
as the jerk parameter. As a consequence of this theory, it seems that radiation
and the cosmological constant are intimately related, in agreement with what
some authors have called the third coincidence problem. Their connection is the
parameter $z_{ini}$, which has a value of $11.29$ and coincide with the
reionization epoch. As a result, we are able to explain the late acceleration
as a natural consequence of the equations, associating the new fluid to
radiation and, thus, eliminating the need for another component (i.e. dark
energy). Finally, we interpret the results and discuss the pros and cons of
using the cosmological constant under the hypothesis of non conservation of the
energy momentum tensor in the unimodular gravity scenario.
