A Possible Origin of Dark Energy

We discuss the possibility that the existence of dark energy may be due to the presence of a spin zero field $\varphi (x)$, either elementary or composite. In the presence of other matter field, the transformation $\varphi (x)\to \varphi (x)+$ constant can generate a negative pressure, like the cosmological constant. In this picture, our universe can be thought as a very large bag, similar to the much smaller MIT bag model for a single nucleon.