Abstract
We discuss the recently suggested Ricci-Gauss-Bonnet holographic dark energy in Chern-Simons modified gravity. We have tested some general forms of the scale factor a(t), and used two physically reasonable forms which have been proved to be consistent with observations. Both solutions predict a sign flipping in the evolution of cosmic pressure which is positive during the early-time deceleration and negative during the late-time acceleration. This sign flipping in the evolution of cosmic pressure helps in explaining the cosmic deceleration-acceleration transition, and it has appeared in other cosmological models in different contexts. However, this work shows a pressure singularity which needs to be explained. The evolution of the equation of state parameter omega(t) shows the same asymptotic behavior for both solutions indicating a quintessence-dominated universe in the far future. We also note that omega(t) goes to negative values (leaving the decelerating dust-dominated era at omega = 0) at exactly the same time the pressure becomes negative. Again, there is another singularity in the behavior of omega(t) which happens at the same cosmic time of the pressure singularity.