  Typical cosmological models are based on the postulate that space is
homogeneous. Space however contains overdense regions in which matter is
concentrating, leaving underdense regions of almost void. The evolution of the
scale factor of the universe has been established from measurements on SNIa.
Since such events occur in regions were matter is present, we may expect that
most of the SNIa are located in overdense regions. This means that the
evolution of the scale factor has been established in a biased manner, by
considering only information coming from overdense regions, excluding the one
from the underdense regions. We develop a simple model to analyze the effect of
this bias, and show that it leads to the appearance of a new tensor in the
Einstein equation of general relativity, which can account for the apparent
acceleration of the expansion of the universe. We further show that this tensor
tends to be proportional to the FLRW metric tensor, and that the constant of
proportionality quantitatively corresponds to the measured cosmological
constant with a remarkable accuracy. We finally explain why these properties
remain valid for other techniques used in determining the dynamics of the
universe, such as the baryon acoustic oscillations.
