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 φ(χ),either elementary or composite. In the presence of other matter field, the transformation φ(χ)→φ(χ)＋constant can generate a negative pressure, like the cosmological constant. In this picture, our universe can be thought asa very large bag, similar to the much smaller MIT bag model for a single nucleon.